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How to find lots of bugs with system-specific static analysis

How to find lots of bugs with system-specific static analysis

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How to find lots of bugs with system-specific static analysis

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  1. This talk is about how you can find lots of bugs in real code by making compilers aggressively system specific How to find lots of bugs with system-specific static analysis Dawson Engler Andy Chou, Ben Chelf, Seth Hallem, Ken Ashcraft Stanford University

  2. Never do X (do not use floating point, allocate large vars on 6K kernel stack) Always do X before/after Y (acquire lock before use, release after use) Checking systems software • Systems software has many ad-hoc restrictions: • “acquire lock L before accessing shared variable X” • “do not allocate large variables on 6K kernel stack” • Error = crashed system. How to find errors? • Formal verification • + rigorous • - costly + expensive. *Very* rare to do for software • Testing: • + simple, few false positives • - requires running code: doesn’t scale & can be impractical • Manual inspection • + flexible • - erratic & doesn’t scale well. • What to do?? Some brave pioneers in the audience, but Practically intractable: far too strenous, and even if you do, spec isn’t code. Heuristic: if you see someone trying to verify sw can count on them being an academic trying to get tenure. Method of choice if you build systems for money: test. Problem: O(paths) = exponential in length of code. So if you build systems, what you wind up with is a system that only crashes after a week. Further, mapping crash back to cause can be *really* hard. Inspection is dead: modify code, have to do it again.

  3. Another approach • Observation: rules can be checked with a compiler • scan source for “relevant” acts check if they make sense E.g., to check “disabled interrupts must be re-enabled:” scan for calls to disable()/enable(), check that they match, not done twice • Main problem: • compiler has machinery to automatically check, but not knowledge • implementor has knowledge but not machinery • Metacompilation (MC): • give implementors a framework to add easily-written, system-specific compiler extensions Want to make compilers aggressively system specific: if you design a system or interface and see a use of it, you invariably see ways of doing it better: give you way to articulate this knowledge and have compiler do it for you automatically

  4. ent->data = kmalloc(..) if(!ent->data) free(ent); goto out; … out: return ent; GNU C compiler Linux fs/proc/ generic.c “using ent after free!” free checker Metacompilation (MC) • Implementation: • Extensions dynamically linked into GNU gcc compiler • Applied down all paths in input program source • Scalable: handles millions of lines of code • Precise: says exactly what error was • Immediate: finds bugs without having to execute path • Effective: 1500+ errors in Linux source code The main results: it works really well, and its easy

  5. Simple. Have had freshman write these and post bugs to linux groups. Three parts: start state. Pattern, match does a transition, callouts. Scales with sophistication of analysis. System will kill variables, track when they are assigned to others. No X after Y: do not use freed memory sm free_checker { state decl any_pointer v; decl any_pointer x; start: { kfree(v); } ==> v.freed ; v.freed: { v == x } | { v != x } ==> { /* suppress fp */ } | { v } ==> { err(“Use after free!”); ; } start kfree(v) v.freed use(v) /* 2.4.4:drivers/isdn/isdn_ppp.c */if (!(ippp_table[i] = kmalloc(…)) for (j = 0; j < i; j++)kfree(ippp_table[i]); error

  6. Talk Overview • Overview: metacompilation [OSDI ’00, ASPLOS ‘00] • Next: three examples • Temporal rule: sanitize user data before use • Contextual rule: don’t block with interrupts off • Moving dynamic to static: assert checking • Broader checking: Inferring rules [SOSP ’01] • Find inconsistencies in program belief systems • Great lever: find errors without knowing truth • Deeper checking [ISCA ’01] • Extract formal model from raw C code • Run through model checker

  7. Network packet copyin(&v, p, len) Syscall param v.clean ERROR any<= v <= any v.tainted memcpy(p, q, v) copyin(p,q,v) copyout(p,q,v) array[v] while(i < v) … Use(v) User supplies base functions, we check the rest (9/2 sources, 15/12 sinks). Interesting: written by an undergrad, no compiler course, probably has close to the world record of security holes found. “X before Y”: sanitize integers before use • Security: OS must check user integers before use • MC checker: Warn when unchecked integers from untrusted sources reach trusting sinks • Global; simple to retarget (text file with 2 srcs&12 sinks) • Linux: 125 errors, 24 false; BSD: 12 errors, 4 false

  8. Good example: understood once by someone, writes checker and then imposed on everyone. People know in the abstract that they have fixed sized integers; be hard pressed to find anyone that admitted otherwise. However, they prompty program as if they are arbitrarily sized. Some big, gaping security holes. • No checks • Unexpected overflow • Weird security implications 2.4.5-ac8/drivers/usb/se401.c: copy_from_user(&frame, arg, sizeof(int));ret=se401_newframe(se401, frame);se401->frame[frame].grabstate = FRAME_UNUSED; /* 2.4.9: drivers/net/wan/farsync.c */copy_from_user(&wrthdr, addr, sizeof wrthdr);if ( wrthdr.size + wrthdr.offset > FST_MEMSIZE ) return -ENXIO;copy_from_user(card->mem+wrthdr.offset,data,wrthdr.size) /* 2.4.1/kernel/sysctl.c:455:do_sysctl_strategy */ get_user(len, oldlenp); if (len > table->maxlen) len = table->maxlen; copy_to_user(oldval, table->data, len);

  9. Good example: understood once by someone, writes checker and then imposed on everyone. Some more big, gaping security holes. • Remote exploit, no checks • A more subtle overflow /* 2.4.9/drivers/isdn/act2000/capi.c:actcapi_dispatch */isdn_ctrl cmd;...while ((skb = skb_dequeue(&card->rcvq))) {msg = skb->data; ... memcpy(,msg->msg.connect_ind.addr.num,msg->msg.connect_ind.addr.len - 1); /* 2.4.9-ac7/fs/intermezzo/psdev.c:presto_psdev_ioctl */error = copy_from_user(&input, (char *)arg, sizeof(input));...input.path = kmalloc(input.path_len + 1, GFP_KERNEL);if ( !input.path ) return -ENOMEM;error =copy_from_user(input.path,user_path, input.path_len);

  10. NoBlock 400 lines later have this violation. This is a common pattern: implementor just doesn’t know about the rule, so keeps violating it. Happens: since rules manually enforced and poorly documented. Heavy clustering: 14 in one, 19 in another, 34 “In context Y, don’t do X”: blocking • Linux: if interrupts are disabled, or spin lock held, do not call an operation that could block: • Compute transitive closure of all potentially blocking fn’s • Hit disable/lock: warn of any calls • 123 errors, 8 false pos • Heavy clustering: • net/atm: 152 checks, 22 bugs (exp 1.9) P =3.1x10^-15 • drivers/i2o: 692 checks, 35 bugs (exp 8.8) P= 2.6x10^-10 clean lock(l) unlock(l) enable() disable() /* drivers/net/pcmcia/wavelan_cs.c */ spin_lock_irqsave (&lp->lock, flags);/* 1889 */ switch(cmd) ... case SIOCGIWPRIV: /* 2304 */ if(copy_to_user(wrq->, …)) Block call error

  11. Another way to use MC is to push dynamic checks to static. Usually have some amount of dynamic type checking going on where you have a series of if statements at the beginning of your routine to check for error conditions. So just pull into the compiler and check. Example: statically checking assert • Assert(x) used to check “x” at runtime. Abort if false • compiler oblivious, so cannot analyze statically • Use MC to build an assert-aware extension • Result: found 5 errors in FLASH. • Common: code cut&paste from other context • Manual detection questionable: 300-line path explosion between violation and check General method to push dynamic checks to static msg.len = 0; ... assert(msg.len !=0); line 211:assert failure! assert checker

  12. Given a set of uses of some interface you’ve built, you invariably see better ways of doing things. This gives you a way to articulate this knowlege and have the compiler do it for you automatically. Let one person do it. Summary • Metacompilation: • Correctness rules map clearly to concrete source actions • Check by making compilers aggressively system-specific • Easy: digest sentence fragment, write checker. • Result: precise, immediate error diagnosis As outsiders found errors in every system looked at 1000s bugs, many capable of crashing system • Next: • Inferring errors by checking program belief systems • Deeper checking Easier to write code to check than it is to write code that obeys

  13. Reduced to playing where’s waldo with grep on millions of line of code, or documentation, hoping you can find all cases Goal: find as many serious bugs as possible • Problem: what are the rules?!?! • 100-1000s of rules in 100-1000s of subsystems. • To check, must answer: Must a() follow b()? Can foo() fail? Does bar(p) free p? Does lock l protect x? • Manually finding rules is hard. So don’t. Instead infer what code believes, cross check for contradiction • Intuition: how to find errors without knowing truth? • Contradiction. To find lies: cross-examine. Any contradiction is an error. • Deviance. To infer correct behavior: if 1 person does X, might be right or a coincidence. If 1000s do X and 1 does Y, probably an error. • Crucial: we know contradiction is an error without knowing the correct belief!

  14. B(); // MUST: B() need not // be preceded by A() A(); … B(); A(); … B(); A(); … B(); A(); … B(); Specification = checkable redundancy. Can cross check code against itself for same effect. Others: that x was not already equal to value. Cross-checking program belief systems • MUST beliefs: • Inferred from acts that imply beliefs code *must* have. • Check using internal consistency: infer beliefs at different locations, then cross-check for contradiction • MAY beliefs: could be coincidental • Inferred from acts that imply beliefs code *may* have • Check as MUST beliefs; rank errors by belief confidence. x = *p / z; // MUST belief: p not null // MUST: z != 0 unlock(l); // MUST: l acquired x++; // MUST: x not protected by l // MAY: A() and B() // must be paired

  15. Hello world of internal consistency checkers. Show because it is one of the simplest possible checkers, and because it finds hundreds of errors. This is a case where the rule is obvious, but we commonly don’t know the state of the system --- instead infer what state the code believes the system is in. Trivial consistency: NULL pointers • *p implies MUST belief: • p is not null • A check (p == NULL) implies two MUST beliefs: • POST: p is null on true path, not null on false path • PRE: p was unknown before check • Cross-check these for three different error types. • Check-then-use (79 errors, 26 false pos) /* 2.4.1: drivers/isdn/svmb1/capidrv.c */ if(!card) printk(KERN_ERR, “capidrv-%d: …”, card->contrnr…) Any NULL pointer analysis is going to get this one. It’s good we do too, but not wildly impressive. The next errors are where the action is.

  16. Had been doing this for a year, but had missed these errors --- as soon as you put in terms of beliefs, becomes obvious. Null pointer fun • Use-then-check: 102 bugs, 4 false • Contradiction/redundant checks (24 bugs, 10 false) /* 2.4.7: drivers/char/mxser.c */ struct mxser_struct *info = tty->driver_data; unsigned flags; if(!tty || !info->xmit_buf) return 0; /* 2.4.7/drivers/video/tdfxfb.c */ fb_info.regbase_virt = ioremap_nocache(...); if(!fb_info.regbase_virt) return -ENXIO; fb_info.bufbase_virt = ioremap_nocache(...); /* [META: meant fb_info.bufbase_virt!] */ if(!fb_info.regbase_virt) { iounmap(fb_info.regbase_virt);

  17. Can look for redundancy in general: deadcode elim is an error finder. Can look for: writes never read, lock acquired that protects nothing. Redundant transition means we’re missing something with analysis. Aside: redundancy checking • Assume: code supposed to be useful • Like types: high-level bugs map to low-level redundancies • Identity operations: “x = x”, “1 * y”, “x & x”, “x | x” • Assignments never read (126 bugs, 26 fp, 1.8K uninsp): /* 2.4.5-ac8/net/appletalk/aarp.c */ da.s_node = sa.s_node; da.s_net = da.s_net; /* 2.4.5-ac8/net/decnet/af_decnet.c:dn_wait_run */do { if (signal_pending(current)) { err = -ERESTARTSYS; break; } ...} while(scp->state != DN_RUN);return 0;

  18. Can look for redundancy in general: deadcode elim is an error finder. Can look for: writes never read, lock acquired that protects nothing. Redundant transition means we’re missing something with analysis. Redundancy checking • Dead code (66 bugs, 26 false): • Detect incomplete specifications: • Detect missed sinks in range checker: flag when data read from untrusted source, sanitized, but then not used for any dangerous operation. • Lock checker: critical section with no shared state, lock with no bound variables for(entry=priv->lec_arp_tables[i];entry != NULL; entry=next){ next = entry->next; if (…) lec_arp_remove(priv->lec_arp_tables, entry); lec_arp_unlock(priv); return 0;}

  19. First pass: mark all pointers treated as user pointers. Second pass: make sure they are never dereferenced. Internal Consistency: finding security holes • Applications are bad: • Rule: “do not dereference user pointer <p>” • One violation = security hole • Detect with static analysis if we knew which were “bad” • Big Problem: which are the user pointers??? • Sol’n: forall pointers, cross-check two OS beliefs • “*p” implies safe kernel pointer • “copyin(p)/copyout(p)” implies dangerous user pointer • Error: pointer p has both beliefs. • Implemented as a two pass global checker • Result: 24 security bugs in Linux, 18 in OpenBSD • (about 1 bug to 1 false positive)

  20. An example Marked as tainted because passed as the first argument to copy_to_user, which is used to access potentientially bad user pointers. Does global analysis to detect that the pointer will be dereferenced by ippd_… • Still alive in linux 2.4.4: • Tainting marks “rt” as a tainted pointer, checking warns that rt is passed to a routine that dereferences it • 3 other examples in same routine • Can combine with earlier range checker (12 errors): /* drivers/net/appletalk/ipddp.c:ipddp_ioctl */ case SIOFCINDIPDDPRT: if(copy_to_user(rt, ipddp_find_route(rt), sizeof(struct ipddp_route))) return –EFAULT; /* 2.4.9/drivers/telephony/ixj.c:ixj_ioctl */ case IXJCTL_INIT_TONE: copy_from_user(&ti, (char *) arg, sizeof(ti)); ... case IXJCTL_INTERCOM_START: ... ixj[arg]->intercom = board;

  21. Parameter features: Can a param be null? What are legal values of integer parameter Return code: What are allowable error code to return & when? • Execution context: Are interrupts off or on when code runs? When it exits? Does it run concurrently? Cross checking beliefs related abstractly • Common: multiple implementations of same interface. • Beliefs of one implementation can be checked against those of the others! • User pointer (3 errors): If one implementation taints its argument, all others must • How to tell? Routines assigned to same function pointer • More general: infer execution context, arg preconditions… • Interesting q: what spec properties can be inferred? bar_write(void *p, void *arg,…){ *p = *(int *)arg; … do something … disable(); return 0; } foo_write(void *p, void *arg,…){ copy_from_user(p, arg, 4); disable(); … do something … enable(); return 0; } If one does it right, we can cross check all: if one dev gets it right we are in great shape.

  22. Intuition: the more often x is obeyed correctly, the more likely it is to be a valid instance. Handling MAY beliefs • MUST beliefs: only need a single contradiction • MAY beliefs: need many examples to separate fact from coincidence. General approach: • Assume MAY beliefs are MUST beliefs & check them • Count number of times belief passed check • Count number of times belief failed check • Use the test statistic to rank errors based on ratio of checks (n) to errors (err): • Intuition: the most likely errors are those where n is large, and err is small. • BAD idea: pick threshold t, if z(n,c) > t treat as MUST z(n, err) = ((n-err)/n-p0)/sqrt(p0*(1-p0)/n)

  23. Can cross-correlate: free is on error path, has dealloc in name, etc, bump up ranking. Foo has 3 errors, and 3 checks. Bar, 3 checks, one error. Essentially every passed check implies belief held, every error = not held Statistical: Deriving deallocation routines • Use-after free errors are horrible. • Problem: lots of undocumented sub-system free functions • Soln: derive behaviorally: pointer “p” not used after call “foo(p)” implies MAY belief that “foo” is a free function • Conceptually: Assume all functions free all arguments • (in reality: filter functions that have suggestive names) • Emit a “check” message at every call site. • Emit an “error” message at every use • Rank errors using z test statistic: z(checks, errors) • E.g., foo.z(3, 3) < bar.z(3, 1) so rank bar’s error first • Results: 23 free errors, 11 false positives bar(p); *p = x; bar(p); p = 0; foo(p); *p = x; foo(p); *p = x; foo(p); *p = x; bar(p); p = 0;

  24. Stratified error reports: rank all errors for different classes. See that there is a few clear ones, then a longer tail. At the top, 2.6K ok checks and 60 violations (2% error?) the third function was bogus . The next few were good, then there was a tail so we stopped. You decide how deeply to go down. Good for both discovery and for validation that you have everything. Ranked free errors Kfree[0]: 2623 checks, 60 errors, z= 48.872.4.1/drivers/sound/sound_core.c:sound_insert_unit: ERROR:171:178: Use-after-free of 's'! set by 'kfree‘ ... kfree_skb[0]: 1070 checks, 13 errors, z = 31.922.4.1/drivers/net/wan/comx-proto-fr.c:fr_xmit: ERROR:508:510: Use-after-free of 'skb'! set by 'kfree_skb‘ ... [FALSE] page_cache_release[0] ex=117, counter=3, z = 10.3 dev_kfree_skb[0]: 109 checks, 4 errors, z=9.67 2.4.1/drivers/atm/iphase.c:rx_dle_intr: ERROR:1321:1323: Use-after-free of 'skb'! set by 'dev_kfree_skb_any‘ ... cmd_free[1]: 18 checks, 1 error, z=3.772.4.1/drivers/block/cciss.c:667:cciss_ioctl: ERROR:663:667: Use-after-free of 'c'! set by 'cmd_free[1]'drm_free_buffer[1] 15 checks, 1 error, z = 3.352.4.1/drivers/char/drm/gamma_dma.c:gamma_dma_send_buffers: ERROR:Use-after-free of 'last_buf'! [FALSE] cmd_free[0] 18 checks, 2 errors, z = 3.2

  25. A bad free error /* drivers/block/cciss.c:cciss_ioctl */ if (iocommand.Direction == XFER_WRITE){ if (copy_to_user(...)) { cmd_free(NULL, c); if (buff != NULL) kfree(buff); return( -EFAULT); } } if (iocommand.Direction == XFER_READ) { if (copy_to_user(...)) {cmd_free(NULL, c);kfree(buff); } }cmd_free(NULL, c); if (buff != NULL) kfree(buff);

  26. Simple. Have had freshman write these and post bugs to linux groups. Three parts: start state. Pattern, match does a transition, callouts. Scales with sophistication of analysis. Example inferring free checker sm free_checker { state decl any_pointer v; decl any_pointer x; decl any_fn_call call; decl any_args args; start: { call(v) }  { char *n = mc_identifier(call); if(strstr(n, “free”) || strstr(n, “dealloc”) || … ) { mc_v_set_state(v, freed); mc_v_set_data(v, n); note(“NOTE: %s”, n); } }; v.freed: { v == x } | { v != x }  { /* suppress fp */ } | { v }  { err(“Use after free %s!”, mc_v_get_data(v)); ;

  27. Can also use consistency: if a routine calls a routine that fails, then it to can fail. Similarly, if a routine checks foo for failure, but calls bar, which does not, is a type error. (In a sense can use witnesses: take good code and see what it does, reapply to unknown code) Statistical: deriving routines that can fail • Traditional: • Use global analysis to track which routines return NULL • Problem: false positives when pre-conditions hold, difficult to tell statically (“return p->next”?) • Instead: see how often programmer checks. • Rank errors based on number of checks to non-checks. • Algorithm: Assume *all* functions can return NULL • If pointer checked before use, emit “check” message • If pointer used before check, emit “error” • Sort errors based on ratio of checks to errors • Result: 152 bugs, 16 false. p = bar(…); if(!p) return; *p = x; p = bar(…); if(!p) return; *p = x; p = bar(…); if(!p)return; *p = x; p = bar(…); *p = x; p = foo(…); *p = x;

  28. The worst bug • Starts with weird way of checking failure: • So why are we looking for “seg_alloc”? /* 2.3.99: ipc/shm.c:1745:map_zero_setup */if (IS_ERR(shp = seg_alloc(...))) return PTR_ERR(shp);static inline long IS_ERR(const void *ptr) { return (unsigned long)ptr > (unsigned long)-1000L; } /* ipc/shm.c:750:newseg: */if (!(shp = seg_alloc(...)) return -ENOMEM;id = shm_addid(shp); int ipc_addid(…* new…) { ... new->cuid = new->uid =…;new->gid = new->cgid = … ids->entries[id].p = new;

  29. foo(p, …) bar(p, …); foo(p, …); … x(); y(); “check foo-bar” “error:foo, no bar!” “check x-y” Deriving “A() must be followed by B()” • “a(); … b();” implies MAY belief that a() follows b() • Programmer may believe a-b paired, or might be a coincidence. • Algorithm: • Assume every a-b is a valid pair (reality: prefilter functions that seem to be plausibly paired) • Emit “check” for each path that has a() then b() • Emit “error” for each path that has a() and no b() • Rank errors for each pair using the test statistic • z(foo.check, foo.error) = z(2, 1) • Results: 23 errors, 11 false positives.

  30. Checking derived lock functions /* 2.4.1: drivers/sound/trident.c: trident_release:lock_kernel(); card = state->card; dmabuf = &state->dmabuf;VALIDATE_STATE(state); • Evilest: • And the award for best effort: /* 2.4.0:drivers/sound/cmpci.c:cm_midi_release: */lock_kernel(); if (file->f_mode & FMODE_WRITE) { add_wait_queue(&s->midi.owait, &wait); ... if (file->f_flags & O_NONBLOCK) { remove_wait_queue(&s->midi.owait, &wait); set_current_state(TASK_RUNNING);return –EBUSY; … unlock_kernel();

  31. Code has many similar redundancies: can cross check all callers to routine for consistency, can check multiple implementations of routine or interface to make sure they make the same assumptions. Summary: Belief Analysis • Key ideas: • Check code beliefs: find errors without knowing truth. • Beliefs code MUST have: Contradictions = errors • Beliefs code MAY have: check as MUST beliefs and rank errors by belief confidence • Secondary ideas: • High-level errors map to low-level redundancies • Specification is a checkable redundancy: code has many redundant uses that can be leveraged in same way. • Can use statistical ranking to help traditional analysis!

  32. Interesting: checking high level things easier than low level. Will have set of data structures, function calls, naming conventions. Strip these out. In reality, the code is 18K lines and the specification is 2-3K and takes months Deeper checking • We’d like real assurances of correctness • Verification? Coders don’t write docs, much less specs… • Observation: spec clearly mirrors code. Auto-extract! void PILocalGet(void) { /* ... Boilerplate setup code ... */ nh.len = LEN_CACHELINE; if (!hl.Pending) { if (!hl.Dirty) { /* ... 37 lines deleted ... */ ASSERT(!hl.IO); // The commented out ASSERT is // true 99.99% of the time, // but is not always // ASSERT(hl.Local);/*... deleted 15 lines ... */ PI_SEND(F_DATA, F_FREE, F_SWAP, F_NOWAIT, F_DEC, 1); hl.Local = 1; Rule "PI Local Get”Cache.State = Invalid & ! Cache.Wait& ! DH.Pending & ! DH.Dirty ==> Begin Assert !DH.Local; DH.Local := true; CC_Put(Home, Memory);EndRule;

  33. slicer murphi code mark spec print bugs model Interesting: checking high level things easier than low level. Will have set of data structures, function calls, naming conventions. Strip these out. Esseniall wriing an absracion funcion. Overview: Automatic extraction • Key: abstract models are clearly embedded in code • Implementors use extensions to mark these features • System rips them out & translates to formal model • Implementors can guide translation to rewrite + augment • Example: verifying FLASH protocol • Hard core, asm strewn C. • Tested for 6+ years, manually “verified” • We found 8 errors. • Bonus: Automatically found bugs in manual spec (it’s code) So hard that they got a paper; so easy that we got one.

  34. A simple abstraction function sm len slicer { /* wildcard variables for pattern matching */ decl any_expr type, data, keep, swp, wait, nl; /* match all uses of the length field. */ pat length = { HG_header.nh.len } ; /* match sends */ pat sends = { NI_SEND(type, data, keep, swp, wait, nl) } | { PI_SEND(type, data, keep, swp, wait, nl) } ; /* match accesses to directory entries */ pat entries = { HG_h.hl.Local } | { HG_h.hl.Dirty } ; /* mark patterns for MC slicer */ all: length | sends | entries ==> { mgk_tag(mgk_s); } ;}

  35. Related work • Tool-based checking • PREfix/PREfast • Slam • ESP • Higher level languages • TypeState, Vault • Foster et al’s type qualifier work. • Derivation: • Houdini to infer some ESC specs • Ernst’s Daikon for dynamic invariants • Larus et al dynamic temporal inference • Spec extraction • Bandera • Slam

  36. Summary • MC: Effective static analysis of real code • Write small extension, apply to code, find 100s-1000s of bugs in real systems • Result: Static, precise, immediate error diagnosis • Belief analysis: broader checking • Infer system rules and state using code beliefs • Key feature: find errors without knowing truth. • Model extraction: deeper checking • Common: abstract models clearly embedded in C code • Automatically extract these using extensions • Model check result