[cfe-dev] [analyzer] Calling SymbolReaper::markInUse on BinarySymExprs are ignored

Wed Aug 19 06:08:42 PDT 2020

It turns out that if the region of `src` is alive, then the required
metadata symbol will be kept alive as well.
Here is the modified example:
void strcat_symbolic_src_length(char *src) {
  char dst[8] = "1234";
  strcat(dst, src);
  clang_analyzer_eval(strlen(dst) >= 4); // expected-warning{{TRUE}}
  (void)*dst;
  *(void)*src;* // Now we keep the `src` alive, thus any metadata symbols
to that region will be alive as well at the eval call.
}
```

It seems slightly confusing to me that depending on that `src` is used
later or not, the `clang_analyzer_eval(strlen(dst) >= 4)` will show either
`TRUE` or `UNKNOWN`.
I think it should always give `TRUE` as an answer.

Note that a metadata symbol is alive only if marked in use AND its region
is also alive.
Without the `(void)*src;` the region of `src` is dead, thus the symbol
($meta{src} + 4) representing the cstring length of the region `dst` will
be dead too.

Since this problem was caused by the handling of metadata symbols,
shouldn't we use conjured ones instead?
In that way, we would decouple the liveness of the cstring length of a
region and the region itself.

What is the use-case for using a metadata symbol instead of a conjured one?

Artem Dergachev <noqnoqneo at gmail.com> ezt írta (időpont: 2020. aug. 7., P,
0:05):

>
>
> On 8/6/20 1:38 PM, Balázs Benics wrote:
>
> Gábor
> > How much work would it be to prototype keeping these expressions alive
> and measuring performance and memory implications?
> I'm not sure, since I'm not really experienced in this liveness stuff. I
> will try it.
>
> Artem
> > It sounds to me as if putting metadata symbols into the live set
> directly would have worked just fine.
> Ehm, I' not sure about this.
> If you look my example closely, you can note that the CStringChecker maps
> directly the SymRegion{reg_$0<char * src>} to the meta_$2{SymRegion{reg_$0<char
> * src>},unsigned long} symbol.
> So the SymbolReaper::markInUse will in fact place that meta_$2 symbol in
> the Live set.
> However later, when you query the SymbolReaper if the mentioned meta_$2
> symbol is dead or not, it will answer you that it is *dead*.
>
>
> That's not how that works.
>
> markInUse() doesn't put anything into the live set, it puts things into an
> auxiliary "metadata-in-use" set:
> https://github.com/llvm/llvm-project/blob/llvmorg-11.0.0-rc1/clang/lib/StaticAnalyzer/Core/SymbolManager.cpp#L396
>
> On the other hand, isDead()/isLive() has to return true if the symbol is
> present in the live set. In fact, that's the first thing it checks for:
> https://github.com/llvm/llvm-project/blob/llvmorg-11.0.0-rc1/clang/lib/StaticAnalyzer/Core/SymbolManager.cpp#L437
>
>
> I'm quoting the related trace log:
>
>> [...]
>> *CStringChecker::**checkLiveSymbols marks 'meta_$2{SymRegion{reg_$0<char
>> * src>},unsigned long}' in use*
>>
>> # Even we marked the given symbols in use, we still removes them for some
>> reason...
>> CStringChecker::checkDeadSymbols finds the
>> '(meta_$2{SymRegion{reg_$0<char * src>},unsigned long}) + 4U' as dead; so
>> removes the mapping from 'dst'
>> *CStringChecker::**checkDeadSymbols finds the
>> 'meta_$2{SymRegion{reg_$0<char * src>},unsigned long}' as dead; so removes
>> the mapping from 'SymRegion{reg_$0<char * src>}'*
>>
>
> Why does the ExprEngine conjure a return symbol, if an evalCall already
> evaluated the call?
>
> Artem
> > See how RegionStore does that within `ScanReachableSymbols::scan(const
> SymExpr *)`
> I managed to implement a metadata collector visitor using the new
> SymExprVisitor infrastructure, in just a couple of lines of code. I was
> amazed by that :)
>
> Artem Dergachev <noqnoqneo at gmail.com> ezt írta (időpont: 2020. aug. 6.,
> Cs, 19:04):
>
>> I- i- i was about to reply to that!
>>
>> I don't know why metadata-in-use is a thing at all. It sounds to me as if
>> putting metadata symbols into the live set directly would have worked just
>> fine. If you find any interesting counterexamples please let me know.
>>
>> Apart from that, indeed, the correct way to implement checkLiveSymbols
>> when you're tracking arbitrary symbols is to iterate over these arbitrary
>> symbols and mark all sub-symbols as live. See how RegionStore does that
>> within `ScanReachableSymbols::scan(const SymExpr *)`. I.e., the following
>> example works correctly and i expect CStringChecker to work similarly:
>>
>> ```
>> int conjure();
>>
>> int foo() {
>>   int x = conjure();
>>   clang_analyzer_warnOnDeadSymbol(x);
>>   return x + 1;
>> }
>>
>> void bar() {
>>   int y = foo(); // At this point `conj_$2` is no longer directly present
>> in the state; only `conj_$2 + 1` is.
>>   (void)y;
>> } // But despite that, `conj_$2` only dies here.
>> ```
>>
>>
>> On 8/6/20 11:35 AM, Gábor Horváth via cfe-dev wrote:
>>
>> +Artem
>>
>> It would be great if the analyzer could reason about code like that. I
>> think Artem is the most competent in these liveness related problems.
>> Aside from performance, I do not see any downside for keeping the whole
>> symbolic expression alive after markInUse was called on it (hopefully Artem
>> corrects me if I'm wrong).
>> But mainly due to constraint solver limitations it might not make sense
>> to keep arbitrarily complex expressions alive.
>>
>> @Balázs Benics <benicsbalazs at gmail.com>
>> How much work would it be to prototype keeping these expressions alive
>> and measuring performance and memory implications?
>>
>>
>> On Wed, 5 Aug 2020 at 16:17, Balázs Benics via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> If you are really curious, here is some context.
>>>
>>> Imagine the following code (test.c):
>>> ```
>>> typedef typeof(sizeof(int)) size_t;
>>>
>>> void clang_analyzer_eval(int);
>>> char  *strcat(char *restrict s1, const char *restrict s2);
>>> size_t strlen(const char *s);
>>>
>>> void strcat_symbolic_src_length(char *src) {
>>>   char dst[8] = "1234";
>>>   strcat(dst, src);
>>>   clang_analyzer_eval(strlen(dst) >= 4); // expected-warning{{TRUE}}
>>>   (void)*dst;
>>> }
>>> ```
>>>
>>> One would expect that the 'strlen(dst) >= 4' is TRUE, but it returns
>>> UNKOWN.
>>>
>>> After doing a little bit of investigation - and debug prints - I came up
>>> with the following trace.
>>>
>>> ---
>>>
>>> # In the CStringChecker::evalStrcat creates a metadata symbol
>>> representing the cstring length of the region pointed by 'src'.
>>> created metadata symbol 'meta_$2{SymRegion{reg_$0<char * src>},unsigned
>>> long}'
>>>
>>> # After the evalStrcat evaluated the call, the state contains the
>>> necessary mapping that the 'dst' points to a cstring which is 4 + meta$4
>>> long.
>>> # Note that meta$4 represents the cstring length of the region pointed
>>> by 'src'.
>>> # So far so good. We know that resulting cstring length precisely.
>>> strcpy common END State.dump: "program_state": {
>>>   [...]
>>>   "checker_messages": [
>>>     { "checker": "unix.cstring.CStringModeling", "messages": [
>>>       "CString lengths:",
>>>       "dst: (meta_$2{SymRegion{reg_$0<char * src>},unsigned long}) +
>>> 4U",
>>>       "SymRegion{reg_$0<char * src>}: meta_$2{SymRegion{reg_$0<char *
>>> src>},unsigned long}",
>>>       ""
>>>     ]}
>>>   ]
>>> }
>>>
>>> # We mark all symbols as live in the cstring length map.
>>> # At least we think so...
>>> CStringChecker::checkLiveSymbols marks '(meta_$2{SymRegion{reg_$0<char *
>>> src>},unsigned long}) + 4U' in use
>>> CStringChecker::checkLiveSymbols marks 'meta_$2{SymRegion{reg_$0<char *
>>> src>},unsigned long}' in use
>>> CStringChecker::checkLiveSymbols marks 'meta_$2{SymRegion{reg_$0<char *
>>> src>},unsigned long}' in use
>>>
>>> # Even we marked the given symbols in use, we still removes them for
>>> some reason...
>>> CStringChecker::checkDeadSymbols finds the
>>> '(meta_$2{SymRegion{reg_$0<char * src>},unsigned long}) + 4U' as dead; so
>>> removes the mapping from 'dst'
>>> CStringChecker::checkDeadSymbols finds the
>>> 'meta_$2{SymRegion{reg_$0<char * src>},unsigned long}' as dead; so removes
>>> the mapping from 'SymRegion{reg_$0<char * src>}'
>>>
>>> # Now that state does not contain the cstring length of the region
>>> pointed by 'dst'.
>>>
>>> ---
>>>
>>> Further investigation showed that even if I would visit all the sub
>>> SymExprs looking for SymbolMetadata would not help.
>>> SymbolReaper::isDead would still show that the
>>> '(meta_$2{SymRegion{reg_$0<char * src>},unsigned long}) + 4U' SymExpr is
>>> dead.
>>>
>>> ---
>>>
>>> How can we preserve such metadata information?
>>>
>>> ---
>>>
>>> You can also reproduce this following these steps:
>>>
>>> Apply the add-debug-prints.patch on top of 7f1556f292ccfd80c4ffa986d5b849f915e5cd82
>>> "Fix typo: s/epomymous/eponymous/ NFC".
>>> Analyze the 'test.c' file using this command:
>>> ./bin/clang -cc1 -internal-isystem lib/clang/12.0.0/include
>>> -nostdsysteminc -analyze -analyzer-constraints=range -setup-static-analyzer
>>> -analyzer-checker=unix.cstring,alpha.unix.cstring,alpha.security.taint,debug.ExprInspection
>>> -analyzer-config eagerly-assume=false test.c
>>>
>>> Balázs Benics <benicsbalazs at gmail.com> ezt írta (időpont: 2020. aug.
>>> 5., Sze, 10:29):
>>>
>>>> Why does the SymbolReaper::markInUse only work on SymbolMetadata
>>>> symbols?
>>>>
>>>> void SymbolReaper::markInUse(SymbolRef sym) {
>>>>   if (isa<SymbolMetadata>(sym))
>>>>     MetadataInUse.insert(sym);}
>>>>
>>>> I think it is flawed if the Symbol is a SymIntExpr holding an
>>>> expression tree referring to SymbolMetadata symbols. In such case, those
>>>> symbols would not be kept alive - causing some confusion on the checker
>>>> developers' side and potentially losing some information about the analysis.
>>>>
>>>> Should we walk the expression tree instead of the current
>>>> implementation?
>>>> What performance impact should we expect by doing so?
>>>>
>>>> Any ideas?
>>>>
>>>> Balazs.
>>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>
>> _______________________________________________
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>>
>>
>>
>
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