[cfe-dev] Analyser - variadic arguments

Michael Tandy via cfe-dev cfe-dev at lists.llvm.org
Mon Aug 1 05:09:18 PDT 2016


Thanks for your reply! I agree that va_start/copy/arg/end call ordering
should be reasonably easy to check.

Unfortunately I'm still having trouble getting the number of arguments - I
suspect this is because IPA is disabled for variadic functions at
https://github.com/llvm-mirror/clang/blob/4ab9d6e02b29c24ca44638cc61b52cde2df4a888/lib/StaticAnalyzer/Core/ExprEngineCallAndReturn.cpp#L735

Unfortunately, the details are tracked at rdar://problem/12147064 which
seems to be private so I don't know the full details. Can anyone give me
any pointers as to what the issue is, and whether it's a task it would be
feasible for a new developer to attempt?

On 28 July 2016 at 20:53, Artem Dergachev via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>  If i was to write such checker, i'd try the following:
>
> Map va_list-type variables (as memory regions) upon va_start(), map them
> to the number of read arguments (0 so far), and to the current
> StackFrameContext pointer, which represents a particular function call
> within which we are now (CheckerContext::getCurrentStackFrame()) (second
> program state map - or map to structure with two items).
>
> You can retrieve [path-sensitive] function declaration and
> [path-insensitive] call expression from the stack frame context, and
> compute the number of variadic arguments passed based on that.
>
> If va_start() is called upon a variable that is already tracked, warn.
>
> If va_start() is called twice in the same StackFrameContext, warn.
>
> Upon va_arg(), increment the number of reads for the respective variable.
> If the variable is not tracked, then the user has forgotten to call
> va_start(), hence warn. If the number exceeds the remembered total number
> of arguments, warn.
>
> Upon va_copy(), copy the whole map item to the new variable. If the source
> variable is not va_start()'ed, warn.
>
> Upon va_end(), erase the map item, which means that you no longer track
> the variable. If the variable is not va_start()'ed, warn.
>
> Upon checkDeadSymbols, see if any of the tracked va_list variables is dead
> (in the sense of SymbolReaper::isLiveRegion()). If it is dead, then
> va_end() will never be called on it (and it wasn't so far, because the
> variable is still in the map). Hence warn on forgotten va_end().
>
> You don't need to track end of function to warn on forgotten va_end() -
> checkDeadSymbols is already more frequent.
>
> If you expect seeing a lot of passing va_list variables by pointers (which
> is insane but i guess possible), then consider the following.
>
> Upon checkPointerEscape, see if any of the tracked va_list variables
> escapes. If so, mark it in the map as escaped: it might have va_end()'ed
> elsewhere, so we should not warn if it dies. Hence you cannot erase it from
> the program state - instead, you'd essentially need a separate trait flag
> (though you may put some magic constant into the existing trait).
>
> If the memory region of the va_list variable has symbolic base (came to us
> by pointer, rather than variable declaration), then it might have already
> been initialized with va_start. Hence you shouldn't warn if
> va_arg()/va_end()/va_copy() is called upon it without prior va_start().
>
> Hence the checker looks pretty similar to SimpleStreamChecker described in
> the video, and it's a very typical path-sensitive checker. I don't expect
> much problems on this path - it may sound like a lot of info, but basically
> it should be easy.
>
> If you are only interested in argument count overflows, but not in other
> va_* API misuse, then probably you may simplify this "typestate machine".
> But you'd still need to catch va_start() and va_args() and most likely
> va_copy(), and once you do, you get all other checks for almost-free.
>
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