[cfe-dev] [analyzer] Calling SymbolReaper::markInUse on BinarySymExprs are ignored
Artem Dergachev via cfe-dev
cfe-dev at lists.llvm.org
Wed Aug 19 14:18:12 PDT 2020
Gabor, we've talked about this two days ago >_> +Nithin because we're
facing the same decision with the smart pointer checker to which raw
pointer value would make a lot of sense as SymbolMetadata.
The way SymbolMetadata used in CStringChecker always bothered me. It
*is* used *as if* it's SymbolConjured: it represents an unknown value of
an expression of a specific type. As such it doesn't do much more than
preserve its identity for debugging purposes. We indeed don't need
another SymbolConjured.
I believe we should remove SymbolConjured's identity elements from
SymbolMetadata and instead make it work more like SymbolRegionValue
works in RegionStore. Namely:
1. Remove statement, location context, block count from SymbolMetadata's
identity. Let it solely depend on the region.
2. Get rid of the metadata-in-use set. From now on SymbolMetadata, like
SymbolRegionValue, is live whenever its region is live.
3. When strlen(R) is used for the first time on a region R, produce
$meta<size_t R> as the answer, but *do not store* it in the string
length map. We don't need to change the state because the state didn't
in fact change. On subsequent strlen(R) calls we'll simply return the
same symbol (*because* the map will remain empty), until the string is
changed.
4. If the string is mutated, record its length as usual. But if the
string is invalidated (or the new length is completely unknown), *do not
remove* the length from the state; instead, set it to SymbolConjured.
Only remove it from the map when the region dies.
5. Keep string length symbols alive as long as they're in the map.
This model is obviously correct because it mimics RegionStore which is
obviously correct (i mean, it's obviously broken beyond repair, but
*this* part seems to be correct). In particular, this model doesn't have
any obvious flaws that you point out in this thread. It also makes sure
that the identity of the state correctly reflects mutations in the
state. I think it's very much superior to the existing model. This also
more or less matches my belief of "everything is a Store" that i wanted
to push on a few years ago (though that one was more radical; back then
i wanted to make a "shadow" region for a string and use the actual
SymbolRegionValue of that shadow region instead of the
SymbolMetadata-as-suggested-in-this-mail).
Balasz, are you willing to implement something like that?
On 8/19/20 7:59 AM, Gábor Horváth wrote:
> Hi!
>
> Since the metadata symbol describes a property of the region I think
> in this example it might make sense to duplicate the metadata symbol
> for dst. But I can also imagine code using the length of a dead string
> which would be nice to support.
>
> I also wonder what is the advantage of not using a conjured symbol.
>
> Regards,
> Gabor
>
>
> On Wed, Aug 19, 2020, 3:08 PM Balázs Benics <benicsbalazs at gmail.com
> <mailto:benicsbalazs at gmail.com>> wrote:
>
> 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 <mailto: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
>> <mailto: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 <mailto: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 <mailto: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
>>> <mailto: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.
>>>
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>>>
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>>
>
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