More enforceable security policies

More Enforceable Security Policies

Lujo Bauer,Jarred Ligatti and David Walker

Department of Computer Science

Princeton University

Princeton,NJ08544

Tech Report TR-649-02

June17,2002

Abstract

We analyze the space of security policies that can be enforced by mon-itoring programs at runtime.Our program monitors are automata that

examine the sequence of program actions and transform the sequence

when it deviates from the speci?ed policy.The simplest such automaton

truncates the action sequence by terminating a program.Such automata

are commonly known as security automata,and they enforce Schneider’s

EM class of security policies.We de?ne automata with more powerful

transformational abilities,including the ability to insert a sequence of ac-

tions into the event stream and to suppress actions in the event stream

without terminating the program.We give a set-theoretic characteriza-

tion of the policies these new automata are able to enforce and show that

they are a superset of the EM policies.

1Introduction

When designing a secure,extensible system such as an operating system that allows applications to download code into the kernel or a database that allows users to submit their own optimized queries,we must ask two important ques-tions.

1.What sorts of security policies can and should we demand of our system?

2.What mechanisms should we implement to enforce these policies? Neither of these questions can be answered e?ectively without understanding the space of enforceable security policies and the power of various enforcement mechanisms.

Recently,Schneider[Sch00]attacked this question by de?ning EM,a sub-set of safety properties[Lam85,AS87]that has a general-purpose enforcement

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mechanism-a security automaton that interposes itself between the program and the machine on which the program runs.It examines the sequence of security-relevant program actions one at a time and if the automaton recognizes an action that will violate its policy,it terminates the program.The mechanism is very general since decisions about whether or not to terminate the program can depend upon the entire history of the program execution.However,since the automaton is only able to recognize bad sequences of actions and then ter-minate the program,it can only enforce safety properties.

In this paper,we re-examine the question of which security policies can be enforced at runtime by monitoring program actions.Following Schneider,we use automata theory as the basis for our analysis of enforceable security policies. However,we take the novel approach that these automata are transformers on the program action stream,rather than simple recognizers.This viewpoint leads us to de?ne two new enforcement mechanisms:an insertion automaton that is able to insert a sequence of actions into the program action stream,and a suppression automaton that suppresses certain program actions rather than terminating the program outright.When joined,the insertion automaton and suppression automaton become an edit automaton.We characterize the class of security policies that can be enforced by each sort of automata and provide examples of important security policies that lie in the new classes and outside the class EM.

Schneider is cognizant that the power of his automata is limited by the fact that they can only terminate programs and may not modify them.However,to the best of our knowledge,neither he nor anyone else has formally investigated the power of a broader class of runtime enforcement mechanisms that explic-itly manipulate the program action stream.Erlingsson and Schneider[UES99] have implemented inline reference monitors,which allow arbitrary code to be executed in response to a violation of the security policy,and have demon-strated their e?ectiveness on a range of security policies of di?erent levels of abstraction from the Software Fault Isolation policy for the Pentium IA32ar-chitecture to the Java stack inspection policy for Sun’s JVM[UES00].Evans and Twyman[ET99]have implemented a very general enforcement mechanism for Java that allows system designers to write arbitrary code to enforce security policies.Such mechanisms may be more powerful than those that we propose here;these mechanisms,however,have no formal semantics,and there has been no analysis of the classes of policies that they enforce.Other researchers have investigated optimization techniques for security automata[CF00,Thi01],cer-ti?cation of programs instrumented with security checks[Wal00]and the use of run-time monitoring and checking in distributed[SS98]and real-time sys-tems[KVBA+99].

Overview The remainder of the paper begins with a review of Alpern and Schneider’s framework for understanding the behavior of software systems[AS87, Sch00](Section2)and an explanation of the EM class of security policies and security automata(Section2.3).In Section3we describe our new enforcement

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mechanisms–insertion automata,suppression automata and edit automata. For each mechanism,we analyze the class of security policies that the mecha-nism is able to enforce and provide practical examples of policies that fall in that class.In Section4we provide the syntax and operational semantics of a sim-ple security policy language that admits editing operations on the instruction stream.In Section5we discuss some unanswered questions and our continuing research.Section6concludes the paper with a taxonomy of security policies.

2Security Policies and Enforcement Mechanisms

In this section,we explain our model of software systems and how they execute, which is based on the work of Alpern and Schneider[AS87,Sch00].We de?ne what it means to be a security policy and give de?nitions for safety,liveness and EM policies.We give a new presentation of Schneider’s security automata and their semantics that emphasizes our view of these machines as sequence transformers rather than property recognizers.Finally,we provide de?nitions of what it means for an automaton to enforce a property precisely and conserva-tively,and also what it means for one automaton to be a more e?ective enforcer than another automaton for a particular property.

2.1Systems,Executions and Policies

We specify software systems at a high level of abstraction.A system S=(A,Σ) is speci?ed via a set of program actions A(also referred to as events or program operations)and a set of possible executionsΣ.An executionσis simply a?nite sequence of actions a1,a2,...,a n.Previous authors have considered in?nite executions as well as?nite ones.We restrict ourselves to?nite,but arbitrarily long executions to simplify our analysis.We use the metavariablesσandτto range over?nite sequences.

The symbol·denotes the empty sequence.We use the notationσ[i]to denote the i th action in the sequence(beginning the count at0).The notation σ[..i]denotes the subsequence ofσinvolving the actionsσ[0]throughσ[i],and σ[i+1..]denotes the subsequence ofσinvolving all other actions.We use the notationτ;σto denote the concatenation of two sequences.Whenτis a pre?x ofσwe writeτ?σ.

In this work,it will be important to distinguish between uniform systems and nonuniform systems.(A,Σ)is a uniform system ifΣ=A where A is the set of all?nite sequences of symbols from A.Conversely,(A,Σ)is a nonuniform system ifΣ?A .Uniform systems arise naturally when a program is completely unconstrained;unconstrained programs may execute operations in any order.However,an e?ective security system will often combine static program analysis and preprocessing with run-time security monitoring.Such is the case in Java virtual machines,for example,which combine type checking with stack inspection.Program analysis and preprocessing can give rise to nonuniform systems.In this paper,we are not concerned with how nonuniform

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systems may be generated,be it by model checking programs,control or data?ow analysis,program instrumentation,type checking,or proof-carrying code;we care only that they exist.

A security policy is a predicate P on sets of executions.A set of executions Σsatis?es a policy P if and only if P(Σ).Most common extensional program properties fall under this de?nition of security policy,including the following.?Access Control policies specify that no execution may operate on certain resources such as?les or sockets,or invoke certain system operations.?Availability policies specify that if a program acquires a resource during an execution,then it must release that resource at some(arbitrary)later point in the execution.

?Bounded Availability policies specify that if a program acquires a resource during an execution,then it must release that resource by some?xed point later in the execution.For example,the resource must be released in at most ten steps or after some system invariant holds.We call the condition that demands release of the resource the bound for the policy.

?An Information Flow policy concerning inputs s1and outputs s2might specify that if s2=f(s1)in one execution(for some function f)then there must exist another execution in which s2=f(s1).

2.2Security Properties

Alpern and Schneider[AS87]distinguish between properties and more general policies as follows.A security policy P is deemed to be a(computable)property when the policy has the following form.

P(Σ)=?σ∈Σ.?P(σ)(Property)

where?P is a computable predicate on A .

Hence,a property is de?ned exclusively in terms of individual executions.

A property may not specify a relationship between possible executions of the https://www.wendangku.net/doc/d36517614.html,rmation?ow,for example,which can only be speci?ed as a con-dition on a set of possible executions of a program,is not a property.The other example policies provided in the previous section are all security properties.

We implicitly assume that the empty sequence is contained in any property. For all the properties we are interested in it will always be okay not to run the program in question.From a technical perspective,this decision allows us to avoid repeatedly considering the empty sequence as a special case in future de?nitions of enforceable properties.

Given some set of actions A,a predicate?P over A induces the security property P(Σ)=?σ∈Σ.?P(σ).We often use the symbol?P interchangeably as a predicate over execution sequences and as the induced property.Normally, the context will make clear which meaning we intend.

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Safety Properties The safety properties are properties that specify that “nothing bad happens.”We can make this de?nition precise as follows.?P is a safety property if and only if for allσ∈Σ,

??P(σ)??σ ∈Σ.(σ?σ ???P(σ ))(Safety)

Informally,this de?nition states that once a bad action has taken place(thereby excluding the execution from the property)there is no extension of that exe-cution that can remedy the situation.For example,access-control policies are safety properties since once the restricted resource has been accessed the policy is broken.There is no way to“un-access”the resource and?x the situation afterward.

Liveness Properties A liveness property,in contrast to a safety property,is a property in which nothing exceptionally bad can happen in any?nite amount of time.Any?nite sequence of actions can always be extended so that it lies within the property.Formally,?P is a liveness property if and only if,

?σ∈Σ.?σ ∈Σ.(σ?σ ∧?P(σ ))(Liveness)

Availability is a liveness property.If the program has acquired a resource,we can always extend its execution so that it releases the resource in the next step.

Other Properties Surprisingly,Alpern and Schneider[AS87]show that any property can be decomposed into the conjunction of a safety property and a liveness property.Bounded availability is a property that combines safety and liveness.For example,suppose our bounded-availability policy states that every resource that is acquired must be released and must be released at most ten steps after it is acquired.This property contains an element of safety because there is a bad thing that may occur(e.g.,taking11steps without releasing the resource).It is not purely a safety property because there are sequences that are not in the property(e.g.,we have taken eight steps without releasing the resource)that may be extended to sequences that are in the property(e.g.,we release the resource on the ninth step).

2.3EM

Recently,Schneider[Sch00]de?ned a new class of security properties called EM. Informally,EM is the class of properties that can be enforced by a monitor that runs in parallel with a target program.Whenever the target program wishes to execute a security-relevant operation,the monitor?rst checks its policy to determine whether or not that operation is allowed.If the operation is allowed, the target program continues operation,and the monitor does not change the program’s behavior in any way.If the operation is not allowed,the monitor terminates execution of the program.Schneider showed that every EM property

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satis?es(Safety)and hence EM is a subset of the safety properties.In addition, Schneider considered monitors for in?nite sequences and he showed that such monitors can only enforce policies that obey the following continuity property.

?σ∈Σ.??P(σ)??i.??P(σ[..i])(Continuity)

Continuity states that any(in?nite)execution that is not in the EM policy must have some?nite pre?x that is also not in the policy.

Security Automata Any EM policy can be enforced by a security automaton A,which is a deterministic?nite or in?nite state machine(Q,q0,δ)that is speci?ed with respect to some system(A,Σ).Q speci?es the possible automaton states and q0is the initial state.The partial functionδ:A×Q→Q speci?es the transition function for the automaton.

Our presentation of the operational semantics of security automata deviates from the presentation given by Alpern and Schneider because we view these machines as sequence transformers rather than simple sequence recognizers.We specify the execution of a security automaton A on a sequence of program actions σusing a labeled operational semantics.

The basic single-step judgment has the form(σ,q)τ?→A(σ ,q )whereσand q denote the input program action sequence and current automaton state;σ and q denote the action sequence and state after the automaton processes a single input symbol;andτdenotes the sequence of actions that the automaton allows to occur(either the?rst action in the input sequence or,in the case that this action is“bad,”no actions at all).We may also refer to the sequenceτas the observable actions or the automaton output.The input sequenceσis not considered observable to the outside world.

(σ,q)τ?→A(σ ,q )

(σ,q)a?→A(σ ,q )(A-Step) ifσ=a;σ

andδ(a,q)=q

(σ,q)·?→A(·,q)(A-Stop) otherwise

We extend the single-step semantics to a multi-step semantics through the fol-lowing rules.

(σ,q)τ=?A(σ ,q )

(σ,q)τ1?→A(σ ,q )(σ ,q )τ2=?A(σ ,q )

result in an action sequence that continues to obeys the policy.Henceforth, when we use the term enforces without quali?cation(precisely,conservatively) we mean enforces precisely.

We say that automaton A1enforces a property?P more precisely or more e?ectively than another automaton A2when either

1.A1accepts more sequences than A2,or

2.The two automata accept the same sequences,but the average edit dis-

tance1between inputs and outputs for A1is less than that for A2.

3Beyond EM

Given our novel view of security automata as sequence transformers,it is a short step to de?ne new sorts of automata that have greater transformational capabil-ities.In this section,we describe insertion automata,suppression automata and their conjunction,edit automata.In each case,we characterize the properties they can enforce precisely.

3.1Insertion Automata

An insertion automaton I is a?nite or in?nite state machine(Q,q0,δ,γ)that is de?ned with respect to some system of executions S=(A,Σ).Q is the set of all possible machine states and q0is a distinguished starting state for the machine.The partial functionδ:A×Q→Q speci?es the transition function as before.The new element is a partial functionγthat speci?es the insertion of a number of actions into the program’s action sequence.We call this the insertion function and it has type A×Q→ A×Q.In order to maintain the determinacy of the automaton,we require that the domain of the insertion function is disjoint from the domain of the transition function.

We specify the execution of an insertion automaton as before.The single-step relation is de?ned below.

(σ,q)a?→I(σ ,q )(I-Step) ifσ=a;σ

andδ(a,q)=q

(σ,q)τ?→I(σ,q )(I-Ins) ifσ=a;σ

andγ(a,q)=τ,q

(σ,q)·?→I(·,q)(I-Stop) otherwise

We can extend this single-step semantics to a multi-step semantics as before. Enforceable Properties We will examine the power of insertion automata both on uniform systems and on nonuniform systems.

Theorem1(Uniform I-Enforcement)

If S is a uniform system and insertion automaton I precisely enforces?P on S then?P obeys(Safety).

Proof:Assume(anticipating a contradiction)that an insertion automaton I enforces some property?P that does not satisfy(Safety).By the de?nition of safety,there exists a sequenceτsuch that??P(τ)and an extensionσsuch that?P(τ;σ).Without loss of generality,consider the action of I when it has seen an input stream consisting of all but the last symbol inτ.Now,when I is confronted with the last symbol of an input sequence with pre?xτ,the automaton can do one of three things(corresponding to each of the possible operational rules).

?Case(I-Step):I accepts this symbol and waits for the next.Unfortu-nately,the input sequence that is being processed may be exactlyτ.In this case,the automaton fails to enforce?P since??P(τ).

?Case(I-Ins):I inserts some sequence.By taking this action the au-tomaton gives up on enforcing the property precisely.The input sequence might beτ;σ,an input that obeys the property,and hence the automaton unnecessarily edited the program action stream.

?Case(I-Stop):As in case(I-Ins),the automaton gives up on precise enforcement.

Hence,no matter what the automaton might try to do,it cannot enforce?P precisely and we have our contradiction.

If we consider nonuniform systems then the insertion automaton can enforce non-safety properties.For example,reconsider the scenario in the proof above, but this time in a carefully chosen nonuniform system S .In S ,the last action of every sequence is the special stop symbol,and stop appears nowhere else in S . Now,assuming that the sequenceτdoes not end in stop(and??P(τ;stop)),our insertion automaton has a safe course of action.After seeingτ,our automaton waits for the next symbol(which must exist,since we asserted the last symbol of τis not stop).If the next symbol is stop,it insertsσand stops,thereby enforcing the policy.On the other hand,if the program itself continues to produceσ,the automaton need do nothing.

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It is normally a simple matter to instrument programs so that they conform to the nonuniform system discussed above.The instrumentation process would insert a stop event before the program exits.Moreover,to avoid the scenario in which a non-terminating program sits in a tight loop and never commits any further security-relevant actions,we could ensure that after some time period, the automaton receives a timeout signal which also acts as a stop event.

Bounded-availability properties,which are not EM properties,have the same form as the policy considered above,and as a result,an insertion automaton can enforce many bounded-availability properties on non-uniform systems.In general,the automaton monitors the program as it acquires and releases re-sources.Upon detecting the bound,the automaton inserts actions that release the resources in question.It also releases the resources in question if it detects termination via a stop event or timeout.

We characterize the properties that can be enforced by an insertion automa-ton as follows.

Theorem2(Nonuniform I-Enforcement)

A property?P on the system S=(A,Σ)can be enforced by some insertion automaton if and only if there exists a functionγp such that for all executions σ∈A ,if??P(σ)then

1.?σ ∈Σ.σ?σ ???P(σ ),or

2.σ∈Σand?P(σ;γp(σ))

Proof:(If Direction)We can construct an insertion automaton that precisely enforces any of the properties?P stated above.The automaton de?nition follows.?States:q∈A ∪{end}(the sequence of actions seen so far,or end if the automaton will stop on the next step)

?Start state:q0=·(the empty sequence)

?Transition function(δ):

Consider processing the action a.

If our current state q is end then stop(i.e.,δandγare unde?ned and hence the rule(I-Stop)applies).

Otherwise our current state q isσand we proceed as follows.

–If?P(σ;a)then we emit the action a and continue in stateσ;a.

–If??P(σ;a)and?P(σ)and?σ ∈Σ.σ;a?σ ???P(σ )then we

simply stop.

–If??P(σ;a)andσ;a∈Σand?P(σ;a;γp(σ;a)).then emit a and

continue in stateσ;a

?Insertion function(γ):Consider processing the action a in state q=σ.

–If??P(σ;a)and??P(σ)and?σ ∈Σ.σ;a?σ ???P(σ )then insert

γp(σ)and continue in state end.

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Ifσis the input so far,the automaton maintains the following invariant Inv p(q).

?If q=end then the automaton has emittedσ;γp(σ)and?P(σ;γp(σ))and the next action is a and?σ ∈Σ.σ;a?σ ???P(σ ).?Otherwise,q=σandσhas been emitted and either?P(σ)or(??P(σ)and σ∈Σand?P(σ;γp(σ))).

The automaton can initially establish Inv p(q0)since our de?nition of a prop-erty assumes?P(·)for all properties.A simple inductive argument on the length of the inputσsu?ces to show that the invariant is maintained for all inputs.

Given this invariant,it is straightforward to show that the automaton pro-cesses every inputσ∈Σproperly and precisely enforces?P.There are two cases.

?Case?P(σ):

Consider any pre?xσ[..i].By induction on i,we show the automaton acceptsσ[..i]without stopping,inserting any actions or moving to the state end.

If?P(σ[..i])then the automaton accepts this pre?x and continues.

If??P(σ[..i])then sinceσ[..i]?σ(and?P(σ)),it must be the case that σ[..i]∈Σand?P(σ[..i];γp(σ[..i])).Hence,the automaton accepts this pre?x and continues.

?Case??P(σ):

Inv p and the automaton de?nition imply that whenever the automaton halts(because of lack of input or because it stops intentionally),?P(σo) whereσo is the sequence of symbols that have been output.Hence,the automaton processes this input properly as well.

(Only-If Direction)De?neγp(σ)to be someτsuch that?P(σ;τ)(and unde-?ned if no suchτexists).We consider any arbitraryσ∈A such that??P(σ) and show that one of the following must hold.

1.?σ ∈Σ.σ?σ ???P(σ ),or

2.σ∈Σand?P(σ;γp(σ))

Consider any case in which1.does not hold.Then,?σ ∈Σ.σ?σ ∧?P(σ ),so by de?nition ofγp,?P(σ;γp(σ)).All that remains to show that2.holds in any case in which1.does not is thatσ∈Σ(given that some insertion automaton I enforces?P,??P(σ),σ?σ ,and?P(σ )).

By the?rst part of the de?nition of precise enforcement,I must not make any edits when suppliedσ as input.This excludes the use of rules I-Ins and I-Stop,so I only steps via rule I-Step on inputσ .At any point during I’s processing ofσ(whereσ?σ ),exactly these same I-Step transitions must be made because the input sequence may later be extended toσ .Therefore,

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(σ,q0)σ=?I(·,q )for some q .Assume(for the sake of obtaining a contra-diction)thatσ∈Σ.Then,by the second part of the de?nition of precise enforcement and the fact that(σ,q0)σ=?I(·,q ),we have?P(σ),which violates the assumption that??P(σ).Thus,σ∈Σas required,and case2.above must hold whenever1.does not.

Limitations Like the security automaton,the insertion automaton is limited by the fact that it may not be able to be able to exert su?cient controls over a system.More precisely,it may not be possible for the automaton to synthesize certain events and inject them into the action stream.For example,an automa-ton may not have access to a principal’s private key.As a result,the automaton may have di?culty enforcing a fair exchange policy that requires two computa-tional agents to exchange cryptographically signed documents.Upon receiving a signed document from one agent,the insertion automaton may not be able to force the other agent to sign the second document and it cannot forge the private key to perform the necessary cryptographic operations itself.

3.2Suppression Automata

A suppression automaton S is a state machine(Q,q0,δ,ω)that is de?ned with respect to some system of executions S=(A,Σ).As before,Q is the set of all possible machine states,q0is a distinguished starting state for the machine and the partial functionδspeci?es the transition function.The partial function ω:A×Q→{?,+}has the same domain asδand indicates whether or not the action in question is to be suppressed(?)or emitted(+).

(σ,q)a?→S(σ ,q )(S-StepA) ifσ=a;σ

andδ(a,q)=q

andω(a,q)=+

(σ,q)·?→S(σ ,q )(S-StepS) ifσ=a;σ

andδ(a,q)=q

andω(a,q)=?

(σ,q)·?→S(·,q)(S-Stop) otherwise

We extend the single-step relation to a multi-step relation using the re?ex-ivity and transitivity rules from above.

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Enforceable Properties In a uniform system,suppression automata can only enforce safety properties.

Theorem3(Uniform S-Enforcement)

If S is a uniform system and suppression automaton S precisely enforces?P on S then?P obeys(Safety).

Proof(sketch):The argument is similar to the argument for insertion au-tomata given in the previous section.If we are attempting to enforce a property ?P,we cannot allow any sequenceτsuch that??P(τ),even though there may be an extensionσsuch that?P(τ;σ).Any step of S when processing the?nal symbol ofτwould result in either acceptingτ(despite the fact that??P(τ))or giving up on precise enforcement altogether.

In a nonuniform system,suppression automata can once again enforce non-EM properties.For example,consider the following system S.

A={aq,use,rel}

Σ={aq;rel,

aq;use;rel,

aq;use;use;rel}

The symbols aq,use,rel denote acquisition,use and release of a resource. The set of executions includes zero,one,or two uses of the resource.Such a scenario might arise were we to publish a policy that programs can use the resource at most two times.After publishing such a policy,we might?nd a bug in our implementation that makes it impossible for us to handle the load we were predicting.Naturally we would want to tighten the security policy as soon as possible,but we might not be able to change the policy we have published. Fortunately,we can use a suppression automaton to suppress extra uses and dynamically change the policy from a two-use policy to a one-use policy.Notice that an ordinary security automaton is not su?cient to make this change because it can only terminate execution.2After terminating a two-use application,it would be unable to insert the release necessary to satisfy the policy.

We can also compare the power of suppression automata with insertion au-tomata.A suppression automaton cannot enforce the bounded-availability pol-icy described in the previous section because it cannot insert release events that are necessary if the program halts prematurely.That is,although the suppres-sion automaton could suppress all non-release actions upon reaching the bound (waiting for the release action to appear),the program may halt without releas-ing,leaving the resource unreleased.Note also that the suppression automaton

cannot simply suppress resource acquisitions and uses because this would mod-ify sequences that actually do satisfy the policy,contrary to the de?nition of precise enforcement.Hence,insertion automata can enforce some properties that suppression automata cannot.

For any suppression automaton,we can construct an insertion automaton that enforces the same property.The construction proceeds as follows.While the suppression automaton acts as a simple security automaton,the insertion automaton can clearly simulate it.When the suppression automaton decides to suppress an action a,it does so because there exists some extensionσof the input already processed(τ)such that?P(τ;σ)but??P(τ;a;σ).Hence,when the suppression automaton suppresses a(giving up on precisely enforcing any sequence withσ;a as a pre?x),the insertion automaton merely insertsσand terminates(also giving up on precise enforcement of sequences withσ;a as a pre-?x).Of course,in practice,ifσis uncomputable or only intractably computable fromτ,suppression automata are useful.

There are also many scenarios in which suppression automata are more pre-cise enforcers than insertion automata.In particular,in situations such as the one described above in which we publish one policy but later need to restrict it due to changing system requirements or policy bugs,we can use suppression automata to suppress resource requests that are no longer allowed.Each sup-pression results in a new program action stream with an edit distance increased by1,whereas the insertion automaton may produce an output with an arbitrary edit distance from the input.

Before we can characterize the properties that can be enforced by a sup-pression automaton,we must generalize our suppression functions so they act over sequences of symbols.Given a set of actions A,a computable function ω :A →A is a suppression function if it satis?es the following conditions.

1.ω (·)=·

2.ω (σ;a)=ω (σ);a,or

ω (σ;a)=ω (σ)

A suppression automaton can enforce the following properties.

Theorem4(Nonuniform S-Enforcement)

A property?P on the system S=(A,Σ)is enforceable by a suppression au-tomaton if and only if there exists a suppression functionω such that for all sequencesσ∈A ,

?if?P(σ)thenω (σ)=σ,and

?if??P(σ)then

1.?σ ∈Σ.σ?σ ???P(σ ),or

2.σ∈Σand?σ ∈Σ.σ?σ ??P(ω (σ ))

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Proof:(If Direction)As in the previous section,given one of the properties ?P described above,we can construct a suppression automaton that enforces it.

?States:q∈A ×{+,?}(the sequence of actions seen so far paired with+(?)to indicate that no actions have(at least one action has)been suppressed so far)

?Start state:q0= ·,+

?Transition function(for simplicity,we combineδandω):

Consider processing the action a.

If the current state q is σ,+ then

–If?P(σ;a),then we emit the action a and continue in state σ;a,+ .

–If??P(σ;a)and?σ ∈Σ.σ;a?σ ???P(σ )then we simply halt.

–Otherwise(i.e.,??P(σ;a)and?σ ∈Σ.σ;a?σ ∧?P(σ )),

?ifω (σ;a)=ω (σ)we suppress a and continue in state σ;a,? .

?and?nally,ifω (σ;a)=ω (σ);a we emit a and continue in state

σ;a,+ .

Otherwise our current state q is σ,? .

–If?P(ω (σ))then stop.

–Otherwise(i.e.,??P(ω (σ))),

?ifω (σ;a)=ω (σ)then suppress a and continue in state σ;a,? .

?and?nally,ifω (σ;a)=ω (σ);a then emit a and continue in

state σ;a,? .

Ifσis the input so far,the automaton maintains the following invariant Inv p(q).

?If q= σ,+ then

1.ω (σ)has been emitted.

2.(?P(σ)andω (σ)=σ)or(??P(σ)andω (σ)=σandσ∈Σand

?σ ∈Σ.σ?σ ??P(ω (σ ))).

?If q= σ,? then

1.ω (σ)has been emitted.

2.?P(ω (σ))or(??P(ω (σ))and?σ ∈Σ.σ?σ ??P(ω (σ ))and

σ∈Σ).

The automaton can initially establish Inv p(q0)since our de?nition of a prop-erty assumes?P(·)for all properties andω (·)=·for all suppression functions.

A simple inductive argument on the length of the inputσsu?ces to show that the invariant is maintained for all inputs.

15

Given this invariant,it is straightforward to show that the automaton pro-cesses every inputσ∈Σproperly and precisely enforces?P.There are two cases.

?Case?P(σ):

This case is similar to the analogous case for insertion automata.We prove the automaton accepts the input without stopping or suppressing any actions by induction on the length of the sequence.

?Case??P(σ):

As before,Inv p implies the automaton always stops in the state in which the automaton outputσo satis?es the property.This implies we process σproperly.

(Only-If Direction)De?neω (σ)to be whatever sequence is emitted by the suppression automaton S on inputσ.We?rst show that this is indeed a suppres-sion function.Clearly,ω (·)=·.When processing some action a after having already processed any sequenceσ,the automaton may step via S-StepA,S-StepS,or S-Stop.In the S-StepA case,the automaton emits whatever has been emitted in processingσ(by de?nition,this isω (σ)),followed by a.In the other cases,the automaton emits onlyω (σ).Hence,ω is a valid suppression function.

Now consider any arbitraryσ∈A .By the de?nition of precise enforcement, automaton S does not modify any sequenceσsuch that?P(σ),so we have satis?ed the requirement that if?P(σ)thenω (σ)=σ.All that remains is to show that if??P(σ)then

1.?σ ∈Σ.σ?σ ???P(σ ),or

2.σ∈Σand?σ ∈Σ.σ?σ ??P(ω (σ ))

Consider any case where??P(σ)and1.does not hold(i.e.,?σ ∈Σ.σ?σ ∧?P(σ )).Analogous to the case for insertion automata,because?P(σ ),S may only processσ with S-StepA and therefore only processesσ(which may later be extended toσ )with S-StepA.Hence,(σ,q0)σ=?S(·,q )for some q . Since S enforces?P and??P(σ),it must be the case thatσ∈Σ.

Finally,by the de?nition ofω and the second part of the de?nition of precise enforcement,?σ ∈Σ.?P(ω (σ )),implying in case2.above that?σ ∈Σ.σ?σ ??P(ω (σ )).Therefore,2.indeed holds whenever1.does not.

Limitations Similarly to its relatives,a suppression automaton is limited by the fact that some events may not be suppressible.For example,the program may have a direct connection to some output device and the automaton may be unable to interpose itself between the device and the program.It might also be the case that the program is unable to continue proper execution if an action is suppressed.For instance,the action in question might be an input operation.

16

3.3Edit Automata

We form an edit automaton E by combining the insertion automaton with the suppression automaton.Our machine is now described by a5-tuple with the form(Q,q0,δ,γ,ω).The operational semantics are derived from the composition of the operational rules from the two previous automata.

(σ,q)a?→E(σ ,q )(E-StepA) ifσ=a;σ

andδ(a,q)=q

andω(a,q)=+

(σ,q)·?→E(σ ,q )(E-StepS) ifσ=a;σ

andδ(a,q)=q

andω(a,q)=?

(σ,q)τ?→E(σ,q )(E-Ins) ifσ=a;σ

andγ(a,q)=τ,q

(σ,q)·?→E(·,q)(E-Stop) otherwise

We again extend this single-step semantics to a multi-step semantics with the rules for re?exivity and transitivity.

Enforceable Properties As with insertion and suppression automata,edit automata are only capable of enforcing safety properties in uniform systems. Theorem5(Uniform E-Enforcement)

If S is a uniform system and edit automaton E precisely enforces?P on S then ?P obeys(Safety).

Proof(sketch):The argument is similar to that given in the proofs of Uni-form I-and S-Enforcement.When confronted with processing the?nal action ofτsuch that??P(τ)but?P(τ;σ)for someσ,E must either acceptτ(despite the fact that??P(τ))or give up on precise enforcement altogether.

The following theorem provides the formal basis for the intuition given above that insertion automata are strictly more powerful than suppression automata. Because insertion automata enforce a superset of properties enforceable by sup-pression automata,edit automata(which are a composition of insertion and suppression automata)precisely enforce exactly those properties that are pre-cisely enforceable by insertion automata.

17

Theorem6(Nonuniform E-Enforcement)

A property?P on the system S=(A,Σ)can be enforced by some edit automaton if and only if there exists a functionγp such that for all executionsσ∈A ,if ??P(σ)then

1.?σ ∈Σ.σ?σ ???P(σ ),or

2.σ∈Σand?P(σ;γp(σ))

Proof:(If Direction)By the Nonuniform I-Enforcement theorem,given any functionγp and property?P satisfying the requirements stated above,we may build an insertion automaton I=(Q,q0,δ,γ)to enforce?P.Then,we can construct an edit automaton E=(Q,q0,δ,γ,ω),whereωis de?ned to be+over all of its domain(the same domain asδ).Clearly,E and I enforce the same property because whenever I steps via I-Step,I-Ins,or I-Stop,E respectively steps via E-StepA,E-Ins,or E-Stop,while emitting exactly the same actions as I.Because I enforces?P,so too must E.

(Only-If Direction)This direction proceeds exactly as the Only-If Direction of the proof of Nonuniform I-Enforcement.

Although edit automata are no more powerful precise enforcers than inser-tion automata,we can very e?ectively enforce a wide variety of security policies conservatively with edit automata.We describe a particularly important appli-cation,the implementation of transactions policies,in the following section.

3.4An Example:Transactions

To demonstrate the power of our edit automata,we show how to implement the monitoring of transactions.The desired properties of atomic transactions [EN94],commonly referred to as the ACID properties,are atomicity(either the entire transaction is executed or no part of it is executed),consistency preser-vation(upon completion of the transaction the system must be in a consistent state),isolation(the e?ects of a transaction should not be visible to other con-currently executing transactions until the?rst transaction is committed),and durability or permanence(the e?ects of a committed transaction cannot be undone by a future failed transaction).

The?rst property,atomicity,can be modeled using an edit automaton by suppressing input actions from the start of the transaction.If the transaction completes successfully,the entire sequence of actions is emitted atomically to the output stream;otherwise it is discarded.Consistency preservation can be enforced by simply verifying that the sequence to be emitted leaves the system in a consistent state.The durability or permanence of a committed transaction is ensured by the fact that committing a transaction is modeled by outputting the corresponding sequence of actions to the output stream.Once an action has been written to the output stream it can no longer be touched by the automaton;

18

furthermore,failed transactions output nothing.We only model the actions of a single agent in this example and therefore ignore issues of isolation.

Figure1:An edit automaton to enforce the market policy conservatively.

To make our example more concrete,we will model a simple market system with two main actions,take(n)and pay(n),which represent acquisition of n apples and the corresponding payment.We let a range over other actions that might occur in the system(such as window-shop or browse).Our policy is that every time an agent takes n apples it must pay for those apples.Payments may come before acquisition or vice versa.The automaton conservatively en-forces atomicity of this transaction by emitting take(n);pay(n)only when the transaction completes.If payment is made?rst,the automaton allows clients to perform other actions such as browse before paying(the pay-take transaction appears atomically after all such intermediary actions).On the other hand,if apples are taken and not paid for immediately,we issue a warning and abort the transaction.Consistency is ensured by remembering the number of apples taken or the size of the prepayment in the state of the machine.Once acquisition and payment occur,the sale is?nal and there are no refunds(durability).

Figure1displays the edit automaton that conservatively enforces our market policy.The nodes in the picture represent the automaton states and the arcs represent the transitions.When a predicate above an arc is true,the transition will be taken.The sequence below an arc represents the actions that are emitted. Hence,an arc with no symbols below it is a suppression transition.An arc with multiple symbols below it is an insertion transition.

4A Simple Policy Language

In this section we describe the syntax and semantics of a simple language for specifying edit automata.

19

4.1Syntax

A program in our language consists of the declaration of automaton state and transition function.The state is described by some number of state variables s1,...,s n,which are assigned some initial values v1,...,v n.We leave the value space unspeci?ed.Normally,it would include all values found in a full-?edged programming language.

We specify two transition functions in order to di?erentiate between internal events,which are events or program actions under the control of the security automaton,and external events,which the security automaton cannot suppress (or synthesize and insert).We use the metavariable i in to denote internal events and i ex to denote external events.Each transition function is a sequence of nested if-then-else statements.The guard of each statement consists of boolean predicates over the current state and the most recently read instruction.Guards can be combined using standard boolean algebra.We assume some set of atomic predicates p and a suitable evaluation function for these predicates.If,when evaluating a transition function,none of the guards evaluate to true then the automaton halts.

The main body of the transition function consists of assignments to state variables and commands to emit a sequence of instructions.We assume some set of functions f that compute values to be stored in a state variable or instructions to be emitted by the automaton.Each sequence of commands ends with a command to continue evaluation in the next state without consuming the current input symbol(next),to continue in the next state and to consume the input (consume;next),or to halt(halt).

The following BNF grammar describes the language syntax.

P::=let

states:

s1=v1

..

.

s n=v n

transitions:

λi ex.T

λi in.T

in

run

end T::=if B then S else T|halt B::=p(s1,...,s n,i)

|B1∧B2|B1∨B2|?B S::=s k=G;S

|emit(G);S

|next

|consume;next

|halt

G::=f(s1,...,s n,i)

4.2Operational Semantics

In order to specify the execution of our program,we need to de?ne the system con?gurations that arise during computation.A system con?guration M is a 4-tuple that contains the instruction stream being read by the automaton,a

20

morethan的多种用法

more than的多种用法 简简单单的“more than”，用法可多呢！ 下面是些好例子： ●A. “More than＋名词”表示“多于……”、“非但……尤其是”如： 1)Modern science is more than a large amount of information. 2)Jason is more than a lecturer; he is a writer, too. ●B. “More than＋数词”含“以上”或“不止”之意，如： 3) I have known David for more than 20 years. 4)Let's carry out the test with more than the sample copy. ●C. “More than＋形容词”等于“很”或“非常”的意思，如： 5)In doing scientific experiments, one must be more than careful with the instruments. 6)I assure you I am more than glad to help you. ●D. 在“More...than...”中，肯定“more”后面的而否定“than”后面的，约等于“是……而不是……”如： 7)The difference between pure linguistics and applied linguistics is more apparent than real. 8)This book seems to be more a manual than a text. 9)Catherine is more diligent than intelli-gent. 10)Hearing the loud noise, the boy was more surprised than frightened. ●E. “More than”或“More...than...”＋含“can”的分句时表示“否定意”，如： 11)That's more than I can do. 12) Don't bite off more than you can chew. 13)In delivering his lecture, Jason makes sure not to include more things than the students can understan d. ●F. “No more...than...”表示“不……；不如……”，如： 14) I can no more do that than anyone else.

氨糖品牌排行榜

氨糖品牌排行榜，氨糖品牌 氨糖是在欧洲医学界唯一认可的对骨关节疾病具有治疗作用的营养保健品。氨糖疗法即通过外源性的补充氨糖来对因冶疗骨关节疾病的一种全新的方法，具有针对性、多效性、绿色安全、巩固性佳等特点，没有副作用及药物依赖性，国际标准的成年人每日氨糖食用量为1500mg。目前国内市场上氨糖品牌比较多，比较混乱，那么国内氨糖品牌都有哪些呢？ 以下是根据氨糖品牌的成分及市场、疗效等综合做的评比，供大家参考。 一、软骨活素 软骨活素是北京康力基生物科技有限公司历时5年悉心研究并与中国药科大学联合研发的特高含量硫酸氨基葡萄糖片，在2010年获得国家食品药品监督管理局批准上市是中国第一个把氨糖开发为保健品牌。每片软骨活素含有硫酸氨基葡萄糖1500mg，符合国际标准的成年人每日氨糖摄取量，因此，只需每天一片软骨活素，就能远离关节疾病。 二、艾兰得关节活素 江苏艾兰得营养品有限公司是该产品的销售公司，真正的生产的公司是江苏江山制药有限公司。艾兰得这个产品市场营销及市场管理做的比较好，由于明星唐国强代言，产品知名度也相对提升了不少。但其氨糖成分是盐酸氨基葡萄糖（氨基葡萄糖盐酸盐），治疗效果上和硫酸氨基葡萄糖还是不能相提并论。 三、骨之力 生产厂家：武汉海奥圣医疗科技有限公司。改公司的实力一般，属于硫酸氨基葡萄糖，纯度也相对较高，但产品具有少量的钾钠氯离子。公司主要通过会销的销售模式，与老年人共同，产品销量还算不错。 四、维骨力 生产厂家：美国NICE USA INC., LOS ANGELES, CA, USA。中国的市场很大，难免有国外的洋品牌进入。此产品是硫酸氨基葡糖糖，从疗效上来说要优于市面上的盐酸氨基葡糖糖。纯度相对较高。但还是含有少量的钾钠氯离子。 五、骨润通 生产厂家：广东亿超生物科技有限公司。硫酸氨基葡萄糖的一种，此公司也算后起之秀，市场也在慢慢扩展。但其氨糖成分纯度方面，有待进一步考证。 六、布鲁拜耳关节活素片 生产厂家：烟台布鲁拜尔生物制药有限公司。据说这个公司是德国的一家公司投资的，技术也来自德国，产品也主要销售到德国。属于硫酸氨基葡萄糖，纯度达到95%以上。 七、葡力氨糖（邦节） 生产厂家：济南强生生物科技有限公司。拥有中国唯一的氨糖外用专利。该产品在90年代非常畅销，但由于此氨糖属于盐酸氨基葡萄糖，相对于硫酸氨基葡糖糖的疗效还是有一定的差距。不过，该公司也有所长进，新产品邦节软骨素在原有的基础上增加了软骨素治疗效果有一定的提升（工艺属于盐酸氨基葡萄糖）。 八、博盈中农牌橄榄氨糖胶囊

more...than用法

more than... 与more ...than ... more than 与more …than 短语在英语中使用得十分广泛。其用法和意义并不简单，一不留神就可能用错。因为more than 与more …than 除了具备其基本用法外，还有些特殊用法。理解和翻译时要特别小心，不能一看到more than就简单地按照字面上的意思将其理解为“比……更……”或“超过”等，而是要根据上下文找出more than 短语的确切含义，只有这样才不至于贻笑大方。 more than 相当于一个形容词或副词短语，在句中作定语或状语，修饰其后的动词、副词、数词或名词等。 一. more than 的用法 1. more than 后跟数词，相当于over，翻译成“……多、超过……”等，强调某物数量上超出某一范围。 More than 1,500 people were killed in the earthquake. 地震中有一千五百多人遇难。 I have collected more than 3 hundred stamps so far. 到目前为止，我已收集了300多张邮票。 I’ve known him for more than 20 years. 我认识他已经二十多年了。 Their college enrolled more than five hundred new students this year. 他们学院今年招收了五百多名新生。 2. more than 后跟名词或动名词，相当于over, not just, not only，表示“不只是、不仅仅是”等。例如： She is more than a teacher to us, she is our friend. 他不只是教师，她还是我们的朋友。 Hibernation is more than sleep. 冬眠不仅仅是睡眠。 Wood is used for more than building. 木头不仅仅用于建筑。 Blood is much more than the simple fluid it seems to be. 血液不仅仅是一种外表似乎简单的液体。 My trip to Beijing is more than sightseeing. 我去北京不仅仅是旅游观光。 I like autumn more than summer. 我喜欢秋天胜过夏天。

morethan的详细用法

一、 more than 1. more than 可放在数词之前，意为“超过；不止；以上”，用于此意义时可与 over 互换使用。例如： Altogether more than 70 percent of the surface of our planet is covered by water. 整体说来，我们这个星球表面有 70% 以上都为水所覆盖。 2. more than 可放在名词之前，表示“不只是；不仅仅”。例如： Hibernation is more than sleep. 冬眠不仅仅是睡眠。 Bamboo is used for more than building. 竹子不只是用于建筑。 3. more than 用于形容词或副词前，作“非常；十分”解，与 very 同义。例如： She is more than careful in doing things. 她做事非常细心。 In class, he listens more than attentively. 在课堂上，他听讲十分认真。 4. more than 之后接含有 can 的从句时，常表示否定意义。此时，从句中的谓语动词必须是及物动词，并且与句子的主语呈现逻辑上的动宾关系。例如： This secret is more than we can let out. 这个秘密我们是不能泄露的。（主语 secret 是 let out 的逻辑宾语） 比较：less than 后面接形容词、副词时，意为“不；很少；不到”，具有否定意义。例如： a) We were busy and less than delighted to have company that day. 那天我们很忙，不高兴有客人来。 b) The young man is less than twenty years old. 这个年轻人不到 20 岁。 二、more … than 1. more … than 意为“与其……不如……”；“是……而不是……”，常可与“ rather than ”或“ not so much … as”互换使用。例如： He is more like a spear than anything else. = He is like a spear rather than anything else. = He is not so much like anything else as like a spear.

维骨力

维骨力真能保骨本？ 葡萄糖胺＝维骨力？ 「维骨力」（Viartril-s）是意大利Rotta药厂的专利名称，主要成分是葡萄糖胺硫酸盐（glucosamine sulfate），适用于退化性膝关节炎。 由于名气太大了，所以一般民众把所有葡萄糖胺（glucosamine）为主要成分的产品，都叫做「维骨力」。 葡萄糖胺因为制造方法不同，而有葡萄糖胺硫酸盐、葡萄糖胺盐酸盐（glucosamine HCl）、及单纯葡萄糖胺（glucosamine free base）的区别。 狭义的维骨力单指以葡萄糖胺硫酸盐为主成分的产品，目前在国内列为处方用药，2003年健保局支付此类药品的费用约2亿3千万元。 其它二种型态的葡萄糖胺在一般药妆店就可以买到，被视为营养食品，例如：阿钙、速疗骨、骨力等。 因为考虑维骨力相关产品的安全性高，疗效争议大，卫生署在2004年2月公告「葡萄糖胺类产品管理类别草案」指出，相关产品「如不宣称疗效，得不以药品列管」（列为食品），而目前列为处方用药的产品，若未能提出疗效的证明，将变更为指示用药，而且适应症只限于退化性关节炎。 人体葡萄糖胺可修护软骨 根据统计，退化性关节炎的发生率，50岁以上为20～30％、70岁以上为70％，而且女性的发生率远高于男性。 退化性关节炎的病因是由于软骨结构的磨损，表面变得粗糙、厚度变薄，使得关节两端与骨下的负担加重、关节间距变窄，进而骨刺形成，造成关节变形、活动及负重疼痛、行动不便等症状，严重者甚至得实行手术治疗（例如：置换人工关节）。 葡萄糖胺是人体可自行合成的物质，存在于软骨与其它结缔组织中。它可以刺激软骨细胞生产胶原蛋白（collagen）及蛋白多醣（proteoglycan），修护受损的软骨组织，使软骨吸收足够的润滑液，维持骨关节的健康。 但是随着人体的老化，葡萄糖胺的合成速度逐渐赶不上分解的速度，影响关节内细胞的新陈代谢，使得关节出现僵硬、发炎及疼痛难耐的症状。 希望藉由外来葡萄糖胺的摄取，补充软骨所需之不足。但是，人工合成的葡萄糖胺，是否会被人体所吸收而发挥效果？ 国外研究证明有效，国内实例有些无效 过去一些临床观察的研究结果，叙述了维骨力对于初期退化性膝关节炎有某些程度的疗效。 比利时一项为期三年的大规模研究结果，发表在2001年的《刺胳针》（The Lancet）期刊，证实了每日服用1500毫克的维骨力，不仅有助于缓解膝盖的退化性关节炎症状，而且可以帮助修护软骨组织，延缓膝关节间距变窄。 同一个研究团队发表了另一篇文章，刊登在2004年的《更年期》（Menopause）期刊，结果指出长期服用维骨力不仅可以延缓膝关节间距变窄，甚至可能加宽（平均0.003mm）。2004年10月美国风湿学院（ACR）发表的最新研究报告也表示，持续服用葡萄糖胺硫酸盐，可以缓解退化性膝关节炎的疼痛症状，也可以改善骨关节的结构，预防人工膝关节置换的比率达73％。 此外还有一些国际知名的期刊，例如《内科医学》（Archives of Internal Medicine）也曾发表过肯定维骨力疗效的文章。

have got的详细用法回顾.doc

Module 4 ＆5需要掌握的重点语法和词组： 复习：have的用法及否定句、一般疑问句的变法。 ①have的意思是：________，它的单数形式是：_______。have是_______词。 例如：我有许多好朋友。_____ _____ many good friends.他有许多好朋友。_____ _____ many good friends. ②把下列两道题改为否定句： 1、I have many good friends：_____________________________ 总结：have 的句子改为否定句要_______________________________________________________________ 2、He has a dog：_________________________ 总结：has的句子改为否定句要_____________________________________ 同样的道理：请将下列两道题改为一般疑问句： I have many good friends：_____________________________ 总结：______________________________________ He has a dog：_________________________ 总结：________________________________________________________ 练习： 一、用have的正确形式填空： 1、He_____two brothers. 2、I_____a beautiful picture. 3、Betty_____ a lovely dog. 4、They_____some friends here. 二、请将下列的句子改为否定句和一般疑问句。 1-3题改为否定句：1、He has two brothers. ___________________________________________________ 2、I have a beautiful picture. ___________________________________________________ 3、Betty has some friends here. ___________________________________________________ 4-6题改为一般疑问句： 4、They have a good teacher. _________________________________肯定回答：_________________ 5、I have some cards. __________________________________________否定回答：____________________ 6、Tony has a sister. __________________________________________否定回答：____________________ 三、请用所给词的适当形式填空。 1、I _________ （have）a brother，but I_________ （not have）a sister. 2、He _________ （have）a beautiful pen. _________ you_________（have）a pen？ 3、Lingling _________ （have）an English dictionary. 4、_________ Tony_________（have）a car？ have got 的用法及否定句、一般疑问句的变法。 ①have got 表示_________________________________ 例：我有一只猫。I have got a cat. have got 的第三人称单数形式是：____________________________ ②have got可以缩写为：_______________ 例如：I have got a cat = ________________________ has got 可以缩写为：_______________ 例如：He has got a cat = ________________________ 练习:请用has got或have got填空。 1、I a bike. 2、He a bike. 3、You a bike.

英语单词,语法more than 结构用法小结

more than 结构用法小结 英语中more than 的用法比较复杂，它除了用于比较结构外，还可以与名词、形容词、动词或从句等连用，表达不同的含义。下面是more than 结构的一些常见用法 1. more than 结构后跟名词表示“不只是”；“不仅仅”等。例如： (1) However, we must consider more than the beginning of the motion. 然而，我们必须考虑的不只是运动的初始阶段。 (2) Peace is more than the absence of war. 和平不只是意味着没有战争。 2．more than 用来修饰形容词、分词和动词，表示所修饰的词份量不重或含义不够，而加以说明，译成汉语可为“非常”，相当于“very”或“much”。例如： (1) He is more than selfish. 他非常自私。 (2) He is more than happy about it. 他对此事极为高兴。 (3) I am sure conditions over there will more than satisfy your requirements. 我相信那边的条件会极大地满足你的要求。 3．more than 后接单数名词，谓语动词用单数。例如： (1) More than one person has been concerned in this. 这里涉及的不止是一个人。 (2) More than one member protested against the proposal. 不止一个成员反对这个建议。 4. more A than B 结构。用来比较两种说法的正确程度，即前一种说法（A项）比后一种说法（B项）要正确一些，表示“与其……不如……”。在这个句型里，more 后不能用形容词或副词的比较级形式，而要用原级形式，此外还可用名词、代词、动词、介词短语等。例如：(1) He is more good than bad. 与其说他坏不如说他好。 (2) It is more a poem than a picture. 与其说这是一幅画，不如说这是一首诗。 5. more than 或more... than 后接从句，可表示否定意义。例如： (1) The beauty of Hangzhou is more than words can describe. 杭州景色之美是说言所不能描述的。 (2) That is more than I can tell you,sir. 这一点我是不能告诉你的，先生。 （注意：在这种句型中，more than 后常接含有can 或could 的从句，表示“……不能”。）对于下列表示否定意义的句子，有的语法学家认为在than 后面省略了when，有的认为省掉了it，也有的认为than 用作关系代词，因而并无省略。这类句子在理解上并不困难，结构上变化也不大，从实用出发，可将其视为一种句型来学习。例如： a. You spent more money than was intended to be spent. 你花掉的钱比原计划的要多。（原计划要花的钱没有这么多。） b. This is more money than is needed. 这笔钱比需要的多。（需要的钱没有这么多。） c. We often advise him not to drink more wine than is good for his health.

movefree说明书

产品成分介绍： 1、Uniflex Propretary Extract A 保护关节及其连接组织免受自由基的引起的退化和侵害。 透明质酸Joint Fluid(Hyaluronic Acid)一种天然多糖，外观透明、具黏性的胶状物质，在人体填充在细胞与胶原纤维的空间中，且覆盖在某些结缔组织上。透明质酸不仅有保持皮肤弹性功能，还能锁住大量水分子，对组织具有保湿润滑作用，也是作为眼球水晶体、关节润滑液等物质的构成成分。 其分解的速度，于是发生体内及关节缺乏葡萄糖胺的现象，进而影响关节内软骨

细胞的新陈代谢，导致供给软骨组织的营养物质越来越少，使软骨组织厚度变薄，关节表面发生不平滑、粗糙、老化，以致骨质增生。同时关节间缺乏润滑和缓冲，导致老化的关节面之间相互摩擦，产生疼痛，引起日常所见的一系列骨关节病、渐进性软骨退化等症状和体征。补充葡萄糖胺，即可补充关节润滑液，提供受损后关节恢复和软骨组织健康所必须的材料，恢复退化软骨的生理功能并促进软骨细胞生成，修复退化关节损伤。同时还可增加骨组织对钙的固定作用。避免钙质流失，增加骨密度。/ 得，其主要机体为软骨糖胺，硫化软骨素。它们是修复及软骨细胞再生的必要物质，能防止老化，腹膜黏着，动脉硬化，关节炎，风湿症及骨质疏松，减轻关节疼痛。也是构成软骨和结缔组织的主要成份，是形成软骨细胞最主要的营养素之一，它可使软骨细胞持有足够的水份，达到缓冲、润滑和支撑关节的作用。 4、MSM是生物可利用硫的营养来源。关节需要硫的参与来形成软骨构件（比如硫酸软骨素）。动物研究表明，从口服MSM中摄取的硫可以进入软骨、骨和关节结构中。 5 的吸收；2它可以帮助骨骼保持钙的水平。

MOVE-FREE-维骨力-中文说明书翻译

中文名-维骨力 维骨力的主要成份为氨基葡萄糖(Glucosamine)，它是人体内合成的物质，是形成软骨细胞的重要营养素，是健康关节软骨的天然组织成份。随着年龄的增长，人体内的氨基葡萄糖的缺乏越来越严重，关节软骨不断退化和磨损。美国、欧洲和日本的大量医学研究表明：氨基葡萄糖可以帮助修复和维护软骨，并能刺激软骨细胞的生长。 氨基葡萄糖 帮助形成健康关节软骨 中老年人 200粒/瓶 每日2粒，1次或分多次服用-为膳食补充剂，不能代替药物 加关节腔的软骨发育，以及增加关节腔的润滑液.它可以合成蛋白多糖来刺激软骨细胞生长，促进形成关节软骨组织，达到预防关节炎的发生。 根据统计，全台湾50岁以上的人口，每2人就有1人患有退化性关节炎；由于无法行动，又伴随疼痛等不适症状，严重者甚至会不良于行、必须进行人工关节置换手术，台湾中老年人为了抢救关节健康，纷纷寻求各种改善关节健康的方法，具有舒缓症状效果的葡萄糖胺产品，也成为许多医师及第一、二期退化性关节炎患者的第一选择。 膝关节疼痛，骨关节炎；降高血脂；辅助对抗糖尿病； 针对胰岛素抵抗综合征[X综合征]；帮助用于肥胖症； 缓解扭伤、拉伤；缓解憩室病；帮助伤口愈合；帮助低血糖症。 硫酸软骨素这是市场上常见的一种缓解疼痛的药物。 养公司研究与发展部经理巴索科说：“硫酸软骨素的这个作用十分有效，因为在软骨中是没有血液供应的。 组织间物质交换均有重要作用，具有调节细胞生长、发育和增殖的能力。另外，硫酸软骨素可去除大量吸收滑膜中的Na+，促进骨质增长，修复受损的关节，增加骨关节腔中软骨密度。硫酸软骨素还具有抑制NK细胞活性的作用，可防止ADCC对组织的损伤，有利于组织的修复。 也可使软骨细胞保有足够的水分，从而能达到缓冲震动及润滑的作用。 骨关节炎；、帮助伤口愈合(内外用兼并)；、对抗高胆固醇； 缓解扭伤、拉伤；、帮助肾结石；、预防**发作；、预防动脉硬化。（甲磺酰甲烷）； 关节对于硫反应很敏感，甲磺酰甲烷是一种存在于各种物体中的硫化合物，在水果、蔬菜、谷物、动物和人体内都有广泛的分布。硫是结缔组织形成所必需的，而在受关节炎影响的软骨中，硫的水平普遍较低。 是人体的第三大主要物质,对于身体组织,酵素,荷尔蒙,抗体和抗氧化的合成和运作起到重要作用.MSM是一种人体需要的重要养分,容易进行细胞氧化,碳水化合物代谢和维持体内酸碱度平衡等,因此我们需要补充足够的硫磺而维持身体正常运作.MSM有巩固结缔组织,维持关节健康,强力止痛,减少发炎,增强血液循环以及消除自由基的功效,MSM亦被视为具有帮助肌肉复原,减少肌肉抽筋和腰背疼痛,加速伤口痊愈,以及疏缓关节炎等作用。 和透明质酸，修复已被磨损的关节软骨，并能够生成新的关节软骨和滑膜。 润滑关节。氨糖能促进关节液的分泌，从而不断润滑关节软骨面，减少磨损。使关节部位灵活自如。 消除关节炎症，减缓关节疼痛。氨糖是关节腔内的“清道夫”不仅能抑制非特异性因子的炎性反应，阻断关节炎症的发展。解除疼痛，而且能消除关节腔内有害酶类，挺高关节和机体的免疫力，通过补充氨糖带来关节免疫力的提高，是消除关节炎症的重要前提。

have_got的详细用法教学内容

h a v e_g o t的详细用 法

复习：have的用法及否定句、一般疑问句的变法。 ①have的意思是：________，它的单数形式是：_______。have是_______词。 例如：我有许多好朋友。_____ _____ many good friends.他有许多好朋友。_____ _____ many good friends. ②把下列两道题改为否定句： 1、I have many good friends：_____________________________ 总结：have 的句子改为否定句要 _______________________________________________________________ 2、He has a dog：_________________________ 总结：has的句子改为否定句要 _____________________________________ 同样的道理：请将下列两道题改为一般疑问句： I have many good friends：_____________________________ 总结： ______________________________________ He has a dog：_________________________ 总结： ________________________________________________________ 练习： 一、用have的正确形式填空： 1、He_____two brothers. 2、I_____a beautiful picture. 3、Betty_____ a lovely dog. 4、They_____some friends here. 二、请将下列的句子改为否定句和一般疑问句。 1-3题改为否定句：1、He has two brothers. ___________________________________________________ 2、I have a beautiful picture. ___________________________________________________ 3、Betty has some friends here. ___________________________________________________ 4-6题改为一般疑问句： 4、They have a good teacher. _________________________________肯定回答：_________________ 5、I have some cards. __________________________________________否定回答： ____________________ 6、Tony has a sister. __________________________________________否定回答： ____________________ 三、请用所给词的适当形式填空。 1、I _________ （have）a brother，but I_________ （not have）a sister. 2、He _________ （have）a beautiful pen. _________ you_________（have）a pen？ 3、Lingling _________ （have）an English dictionary. 4、_________ Tony_________（have）a car？ have got 的用法及否定句、一般疑问句的变法。

morethan的用法(北京四中网校-〉名师答疑-〉高三-〉英语)

more than 的用法（北京四中网校－〉名师答疑－〉高三－〉 英语）... 答： more than 的用法 1. more than后面跟名词，意为“不只是，不仅仅是”。例如：（1）Hibernation is more than sleep. 冬眠不只是睡觉。（2）Kate was more than a teacher. She also did what she could to take care of her pupils. 凯特不仅仅是位教师，她还尽其所能照顾她的学生。 2. more than与数词连用，意思是“多于，大于，超过”。例如：（1）I have known him for more than twenty years. 我认识他已超过二十年了。（2）More than ten policemen turned up at the spot where the accident happened. 十多位警察出现在出事地点。 3. more than与形容词和分词连用，表示“非常、十分”。例如：（1）They were more than happy to see us come back from the expedition. 看到我们远征而归，他们异常高兴。（2）I was more than surprised to see the lion standing at the body. 看到那头狮子站在尸体旁边，我非常惊讶。4. more than 与动词连用，对动词起着加强语气的作用。例如：（1）Repeated advertising will more than increase product sales. 多次做广告意味着增加产品的销售。（2）His

webbernaturals韦博系列保健品1

webbernaturals韦博系列保健品1 【品牌介绍】 Webber Naturals Pharmaceuticals为加拿大10大保健产品厂之一，凭藉50年的经验，生产多款安全有效的健康食品。其健康食品所选取的草药，均由公司自己在加拿大拥有的农场种植，保证新鲜及无农药污染，为了确保产品安全可靠，所有原料及制成品均经由品质控制中心以及根据美国药典（USP）的指示进行测试。食品制造中心的设施皆符合GMP （Good Manufacturing Practices）守则中的Health and Welfare Canada。 一、WebberNaturals伟博氨基葡萄糖软骨素维骨力300粒280元/瓶 【产品规格】300粒 【产品产地】加拿大 【使用方法】每日3次，每次服1粒，随餐服用 【产品介绍】 Webber Naturals伟博天然高浓度维骨力胶囊由天然、高纯度的硫酸氨基葡萄糖和软骨素配制而成，适用于身体关节的部分：手、手腕、膝、背面、脖子、臀部、脚踝和脚。它不象

一般的止痛药只能给患者短暂的舒缓，维骨力与软骨素可刺激磨损的软骨结缔组织生长，有益于重建流失的软骨，增加软骨的保水性，以减少关节硬骨的相互摩擦。 中老年人容易有骨骼退化症状，退化性关节炎的发生是由于软骨遭到破坏所致。现代人缺乏运动，易造成关节老化。而骨刺及膝关节退化都与之有关。 【营养成分】 每1粒胶囊含量 硫酸氨基葡萄糖(Glucosamine Sulphate) 500mg 硫酸软骨素(Chondroitin Sulfate Sodium) 400mg 【产品功效】 －干扰并阻断关节炎病理过程、缓解疼痛肿胀、消除增生、校正畸形、改善活动能力、延缓骨骼衰老； －加速软骨细胞的修复和再生，提供关节建造、修补、软骨组织、肌腱、韧带的主要材料； －刺激软骨弹性组织的修复、再造、预防软骨失去弹性；－控制滑膜分泌液的平衡，避免关节枯萎； －减轻关节发炎，舒解神经炎、韧带炎、肌腱炎； －减轻坐骨神经痛 氨基葡萄糖(Glucosamine Sulphate) 氨基葡萄糖是目前市场上最被普遍认可的关节恢复补剂，对减轻骨关节炎病人的疼痛有很好的效果，它能防止健美运

have_got的详细用法

__________________________________________________ 复习：have的用法及否定句、一般疑问句的变法。 ①have的意思是：________，它的单数形式是：_______。have是_______词。 例如：我有许多好朋友。_____ _____ many good friends.他有许多好朋友。_____ _____ many good friends. ②把下列两道题改为否定句： 1、I have many good friends：_____________________________ 总结：have 的句子改为否定句要_____________________________________________________________ __ 2、He has a dog：_________________________ 总结：has的句子改为否定句要_____________________________________ 同样的道理：请将下列两道题改为一般疑问句： I have many good friends：_____________________________ 总结：______________________________________ He has a dog：_________________________ 总结：________________________________________________________ 练习： 一、用have的正确形式填空： 1、He_____two brothers. 2、I_____a beautiful picture. 3、Betty_____ a lovely dog. 4、They_____some friends here. 二、请将下列的句子改为否定句和一般疑问句。 1-3题改为否定句：1、He has two brothers. ___________________________________________________ 2、I have a beautiful picture. ___________________________________________________ 3、Betty has some friends here. ___________________________________________________ 4-6题改为一般疑问句：4、They have a good teacher. _________________________________肯定回答：_________________ 5、I have some cards. __________________________________________否定回答：____________________ 6、Tony has a sister. __________________________________________否定回答：____________________ 三、请用所给词的适当形式填空。 1、I _________ （have）a brother，but I_________ （not have）a

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have-got的详细用法

复习：have的用法及否定句、一般疑问句的变法。 ①have的意思是：________，它的单数形式是：_______。have是_______词。 例如：我有许多好朋友。_____ _____ many good friends.他有许多好朋友。_____ _____ many good friends. ②把下列两道题改为否定句： 1、I have many good friends：_____________________________ 总结：have 的句子改为否定句要_______________________________________________________________ 2、He has a dog：_________________________ 总结：has的句子改为否定句要_____________________________________同样的道理：请将下列两道题改为一般疑问句： I have many good friends：_____________________________ 总结：______________________________________ He has a dog：_________________________ 总结：________________________________________________________ 练习： 一、用have的正确形式填空： 1、He_____two brothers. 2、I_____a beautiful picture. 3、Betty_____ a lovely dog. 4、They_____some friends here. 二、请将下列的句子改为否定句和一般疑问句。 1-3题改为否定句：1、He has two brothers. ___________________________________________________ 2、I have a beautiful picture. ___________________________________________________ 3、Betty has some friends here. ___________________________________________________ 4-6题改为一般疑问句： 4、They have a good teacher. _________________________________肯定回答：_________________ 5、I have some cards. __________________________________________否定回答：____________________ 6、Tony has a sister. __________________________________________否定回答：____________________ 三、请用所给词的适当形式填空。 1、I _________ （have）a brother，but I_________ （not have）a sister. 2、He _________ （have）a beautiful pen. _________ you_________（have）a pen 3、Lingling _________ （have）an English dictionary. 4、_________ Tony_________（have）a car have got 的用法及否定句、一般疑问句的变法。 ①have got 表示_________________________________ 例：我有一只猫。I have got a cat. have got 的第三人称单数形式是：____________________________ ②have got可以缩写为：_______________ 例如：I have got a cat = ________________________ has got 可以缩写为：_______________ 例如：He has got a cat = ________________________ 练习:请用has got或have got填空。 1、I a bike. 2、He a bike. 3、You a bike. 4、Your sister a bike. 5、We some books. 6、Our