[cfe-dev] [analyzer] Pointer cast representation problems (original) (raw)

Artem Dergachev via cfe-dev cfe-dev at lists.llvm.org
Sun Dec 13 21:55:05 PST 2020

Ok, first of all, the example false positive you're debugging is not an example of the bug you're trying to look into, but an example of a completely different long-standing bug: https://bugs.llvm.org/show_bug.cgi?id=44114. Both bugzilla entries contain a large number of examples to choose from. None of those are beginner bugs though, the reason why they're not fixed is because they require a relatively large amount of work and a relatively good understanding of the subject.

For your specific example, no, you didn't identify the buggy transition correctly, so you're debugging the part that has nothing to do with the false positive. I strongly recommend finding the buggy transition in your ExplodedGraph dump before jumping into the debugger. This usually goes from bottom to top, exactly like you're expected to read a static analyzer report. Let me explain this procedure on your example.

I attached a screenshot of the tail of the ExplodedGraph dump for your code (i hope this works fine with the mailing list). That's just three last nodes of the graph but they're enough to identify the problem.

Node 53 is the bug node, as stated in red; this is the node that was produced by generateErrorNode() and fed into the BugReport object. The bug report says "Branch condition evaluates to a garbage value". The AST expression p[1] (which acts as the branch condition which we can confirm by looking at the source code) has symbolic value "Undefined". Therefore we can conclude that the bug report was emitted correctly and the our problem is not in the checker callback that emitted the bug report; it was simply fed an incorrect State 1337. Since the only reason why the bug report was emitted was that the value was Undefined, we have to find a node in which "Undefined" was written into the state.

Node 52 doesn't update the state, so we can skip it. In fact it's nicely grouped with node 51 so we barely even notice it.

Node 51 is the nearest node on which the AST expression p[1] was first evaluated into Undefined. Whatever code was responsible for evaluating node 51 is directly responsible for assigning Undefined value to the expression - which ultimately led to the false positive in node 53. It is easy to see what was supposed to happen at this node: this is a memory load (which is also known in C and C++ formalism as "implicit lvalue-to-rvalue conversion" - the operation that converts object-as-in-"location" into object-as-in-"data"). The operation does not change contents of memory, it only reads the existing memory.

Static analyzer represents the knowledge of existing memory in a data structure known as the Store, aka "Region Store". Straightforwardly, it maps "memory regions" (symbolic representations of locations) into arbitrary SVals (symbolic representations of data). No, Balazs, it doesn't map "values to expressions" =/ The Store is written down in the dump and labeled accordingly. In our case there are three entries in the Store:

(1) Variable "myvar" has 16-bit unsigned integer 4386 written into it at byte offset 0; (2) Variable "p" has a pointer to the first char (hence index 0) of variable "myvar" written into it at byte offset 0. (3) Variable "x" has 32-bit signed integer 1 written into it at byte offset 0.

The location we're loading from is already evaluated: it's the value of AST expression p[1] before the load. The Expressions section (aka the Environment) contains two entries for p[1] because implicit lvalue-to-rvalue casts do not have any visual representation in C, however by matching statement identifiers (S780 and S784) with the respective program point dumps (S780: ArraySubscriptExpr, S784: ImplicitCastExpr (LValueToRValue)) you can easily see that S780 is the sub-expression of the load and S784 is the load itself. Therefore the location which we're loading from is the value of AST expression p[1], namely the second char of variable "myvar".

Knowing these facts about the memory of our program, we need to decide whether the load was performed correctly. If the load was performed incorrectly, our job is done: we've found the root cause. If the load was performed correctly that'd have meant that the state is incorrect and we have to go further up in order to find it.

It's fairly obvious that the load was not performed correctly. Looking at Store entry (1), it is easy to see that the second character of variable "myvar" has value 17 rather than "Undefined".

Therefore our problem consists entirely in incorrect construction of node 51. Now's the good time to jump into the debugger, set conditional breakpoint on evalLoad with condition "predecessor node has identifier 50" and debug from there.

The pointer cast that you were trying to debug was not at fault. We can still discuss it separately but it's not what really causes this specific false positive to show up.

This entire debugging procedure is very straightforward and requires almost no thinking. You simply match static analyzer's simulation of the program's behavior to the actual runtime behavior of the program and check whether the former is a correct abstraction over the latter. Static analyzer is just an interpreter (cf. "abstract interpretation") so it's very easy to reason about the correctness of every transition as long as you know the language it's trying to interpret (in this case, C). Which is why i made this 2018 conference talk in order to explain it. The only non-trivial part is knowing the structure of the program state (store vs environment, what are regions and symbols and how to read them) which is why i did this whole workbook thing on that subject in 2016. I wish i did it properly so that all the documentation was in one place but for now that's where we are i guess.

On 12/7/20 7:40 AM, Balázs Benics via cfe-dev wrote:

Ok, you are saying that we should know that p[1] points to an object which was initialized. And according to the target's endianness, resolve the DeclRefExpr to the appropriate constant value.

The dumps are probably from the line char *p = &myvar; if I'm right. That is the reason why the right-hand side is evaluated to the location of myvar, but wrapped into an ElementRegion of type char representing the reinterpret cast. IMO, it's the correct behavior to this point. You should probably dig around the evaluation of the DeclRefExpr of the expression p[1]. That should return your concrete value. AFAIK, the store maps the values to expressions and the problem probably resides there. But it's just a wide guess :D Unfortunately, that is all I can say about this right now :( Balazs. Vince Bridgers via cfe-dev <cfe-dev at lists.llvm.org_ _<mailto:cfe-dev at lists.llvm.org>> ezt írta (időpont: 2020. dec. 7., H, 11:54): Hi all, I thought I'd look at this problem https://bugs.llvm.org/showbug.cgi?id=43364 <https://bugs.llvm.org/showbug.cgi?id=43364> (Pointer cast representation problems). maybe use it as an opportunity to dig into the inner workings of the analyzer and maybe even solve a concrete problem :)   But it seems I need guidance about possible paths to pursue. Starting with a concrete case, a simple reproducer is below. I debugged this down to SimpleSValBuilder.cpp:evalCastFromLoc(). val is an SVal and castTy is a Loc, and this code path attempts to extract a concrete integer from the SVal (did I get this right?). So I think a solution to this specific case would be to dig into the SVal to see if casted data is concrete, and extract that data. Seems to me this would be the location for that (evalCastFromLoc seems appropropriate enough). If that's true, can someone point me to an example that's similar? I'll keep digging, but thought I'd ask in case this is easy for someone to drop a few helpful hints. I've included some select dumps, state and a bt below in case this is helpful. Best (gdb) p val.dump() &Element{myvar,0 S64b,char}$2 = void (gdb) p castTy.dump() PointerType 0xf931740 'char *' `-BuiltinType 0xf930b50 'char' $3 = void

Invocation: clang -cc1 -analyze -analyzer-checker=core case.c struct mystruct { unsigned short u16; }; int main(void) { struct mystruct myvar = { 0x1122 }; char *p = &myvar; int x = 0; if (p[0]) x+=1; if (p[1]) // Branch condition evaluates to a garbage value [core.uninitialized.Branch] x+=1; return x; } (gdb) p state $4 = {Obj = 0xf9b81b8} (gdb) p state->dump() "programstate": { "store": { "pointer": "0xf9b6fa2", "items": [ { "cluster": "myvar", "pointer": "0xf9b2250", "items": [ { "kind": "Direct", "offset": 0, "value": "4386 U16b" } ]}, { "cluster": "p", "pointer": "0xf9b2aa8", "items": [ { "kind": "Direct", "offset": 0, "value": "&Element{myvar,0 S64b,char}" } ]}, { "cluster": "x", "pointer": "0xf9b6ed0", "items": [ { "kind": "Direct", "offset": 0, "value": "1 S32b" } ]} ]}, "environment": { "pointer": "0xf9b16e0", "items": [ { "lctxid": 1, "locationcontext": "#0 Call", "calling": "main", "location": null, "items": [ { "stmtid": 898, "pretty": "p", "value": "&p" } ]} ]}, "constraints": null, "dynamictypes": null, "dynamiccasts": null, "constructingobjects": null, "checkermessages": null }$5 = void (gdb) bt #0  (anonymous namespace)::SimpleSValBuilder::evalCastFromLoc (this=0xf9aedf0, val=..., castTy=...) at ../../clang/lib/StaticAnalyzer/Core/SimpleSValBuilder.cpp:136 #1  0x0000000007ec6ecb in (anonymous namespace)::SimpleSValBuilder::dispatchCast (this=0xf9aedf0, Val=..., CastTy=...) at ../../clang/lib/StaticAnalyzer/Core/SimpleSValBuilder.cpp:70 #2  0x0000000007edd75e in clang::ento::StoreManager::CastRetrievedVal (this=0xf9b02a0, V=..., R=0xf9b2aa8, castTy=...) at ../../clang/lib/StaticAnalyzer/Core/Store.cpp:438 #3  0x0000000007ea0480 in (anonymous namespace)::RegionStoreManager::getBinding (this=0xf9b02a0, B=..., L=..., T=...) at ../../clang/lib/StaticAnalyzer/Core/RegionStore.cpp:1511 #4  0x0000000007e9d059 in (anonymous namespace)::RegionStoreManager::getBinding (this=0xf9b02a0, S=0xf9b6fa2, L=..., T=...) at ../../clang/lib/StaticAnalyzer/Core/RegionStore.cpp:551 #5  0x0000000007753856 in clang::ento::ProgramState::getRawSVal (this=0xf9b81b8, LV=..., T=...) at ../../clang/include/clang/StaticAnalyzer/Core/PathSensitive/ProgramState.h:785 #6  0x0000000007e64d61 in clang::ento::ProgramState::getSVal (this=0xf9b81b8, location=..., T=...) at ../../clang/lib/StaticAnalyzer/Core/ProgramState.cpp:267 #7  0x0000000007df0d77 in clang::ento::ExprEngine::evalLoad (this=0x7fffffffa828, Dst=..., NodeEx=0xf985e00, BoundEx=0xf985e00, Pred=0xf9b8230, state=..., location=..., tag=0x0, LoadTy=...) at ../../clang/lib/StaticAnalyzer/Core/ExprEngine.cpp:2982 #8  0x0000000007e10402 in clang::ento::ExprEngine::VisitCast (this=0x7fffffffa828, CastE=0xf985e00, Ex=0xf985dc0, Pred=0xf9b8230, Dst=...) at ../../clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp:313 #9  0x0000000007de88a8 in clang::ento::ExprEngine::Visit (this=0x7fffffffa828, S=0xf985e00, Pred=0xf9b8230, DstTop=...) at ../../clang/lib/StaticAnalyzer/Core/ExprEngine.cpp:1729 #10 0x0000000007de4d9c in clang::ento::ExprEngine::ProcessStmt (this=0x7fffffffa828, currStmt=0xf985e00, Pred=0xf9b8230) at ../../clang/lib/StaticAnalyzer/Core/ExprEngine.cpp:791 #11 0x0000000007de4a89 in clang::ento::ExprEngine::processCFGElement (this=0x7fffffffa828, E=..., Pred=0xf9b8230, StmtIdx=1, Ctx=0x7fffffffa338) at ../../clang/lib/StaticAnalyzer/Core/ExprEngine.cpp:637 #12 0x0000000007db6fa9 in clang::ento::CoreEngine::HandlePostStmt (this=0x7fffffffa848, B=0xf9a8860, StmtIdx=1, Pred=0xf9b8230) at ../../clang/lib/StaticAnalyzer/Core/CoreEngine.cpp:466 #13 0x0000000007db6717 in clang::ento::CoreEngine::dispatchWorkItem (this=0x7fffffffa848, Pred=0xf9b8230, Loc=..., WU=...) at ../../clang/lib/StaticAnalyzer/Core/CoreEngine.cpp:191 #14 0x0000000007db6323 in clang::ento::CoreEngine::ExecuteWorkList (this=0x7fffffffa848, L=0xf9b16e0, Steps=224975, InitState=...) at ../../clang/lib/StaticAnalyzer/Core/CoreEngine.cpp:147 #15 0x000000000734dc74 in clang::ento::ExprEngine::ExecuteWorkList (this=0x7fffffffa828, L=0xf9b16e0, Steps=225000) at ../../clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h:192

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