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* [GSoC][analyzer-c++] Enable enough C++ support for self-analysis
@ 2023-03-31 23:33 Benjamin Priour
  2023-04-02 22:38 ` David Malcolm
  0 siblings, 1 reply; 5+ messages in thread
From: Benjamin Priour @ 2023-03-31 23:33 UTC (permalink / raw)
  To: David Malcolm; +Cc: gcc

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Hi David,


On Thu, Mar 30, 2023 at 2:04 AM David Malcolm <dmalcolm@redhat.com> wrote:

> Note that the analyzer doesn't properly work yet on C++; see:
>   https://gcc.gnu.org/bugzilla/showdependencytree.cgi?id=97110
> I'm hoping to address much of this in GCC 14.
>
> Other notes inline below...
>
> I'm guessing that the CFG and thus the supergraph contains edges for
> handling exceptions, and the analyzer currently doesn't know anything
> about these, and mishandled them:
>   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97111
>
> You might want to have a look at the supergraph (via -fdump-analyzer-
> supergraph); I recently added more debugging notes here:
>  https://gcc.gnu.org/onlinedocs/gccint/Debugging-the-Analyzer.html
>
> If that's the case, then -fno-exceptions might affect the behavior.
>
> Thanks for the additional debugging tips!

> >
> >
> > Another observation was in the distinction between delete and free in
> > the
> > case of mixing them.
> > With 'a' being allocated with malloc:
> > > A* a = (A*) malloc(sizeof(A));
> > > free(a);
> > > delete a; // -Wanalyzer-use-after-free, OK, might expect warning
> > > for
> > double free though ?
> >
> > but with allocation through new and inversion of free/delete
> > > A* a = new A();
> > > delete a;
> > > free(a); // No diagnostics at all from the analyzer, got
> > -Wmismatched-new-delete from the front-end though.
> >
> > I believe this might come from a similar issue as above, but I still
> > have
> > to investigate on that front.
> >
> > I just noticed another unexpected behavior. Let's consider
> > > struct A {int x; int y;};
> > > void* foo() { A* a = (A*) __builtin_malloc(sizeof(A)); return a; }
> > > int main() {
> > >     A* a2 = (A*) __builtin_malloc(sizeof(A));
> > >     foo();
> > >     return 0;
> > > }
> >
> > Then a -Wanalyzer-malloc-leak is correctly ensued for allocation in
> > foo(),
> > but none is emitted for the leak in main(), although I checked the
> > exploded
> > graph it is correctly marked as unchecked(free).
> >
> > Should I file those on bugzilla ?
>
>


> We already know that "C++ doesn't work yet".  Minimal examples of
> problems with C++ support might be worth filing, to isolate specific
> issues.  If you do, please add them to the tracker bug (by adding
> "analyzer-c++" to the "Blocks" field of the new bug(s))
>
> Sure will be doing that.


> >
> >
> > Also I have taken interest in PR106388 - Support for use-after-move
> > in
> > -fanalyzer -.
> > The prerequisite PR106386 - Reuse libstdc++ assertions - would also
> > be of
> > great help in extending the support of -Wanalyzer-out-of-bounds,
> > as we could detect out-of-bounds on vectors through
> > __glibcxx_requires_subscript used in the definition of operator[].
>
> There's a bunch of other C++-enablement work (as per the tracker bug
> above) that I think would need fixing before PR106388 is reasonable to
> tackle.
>

Indeed a bunch of them, the ramification of solving that PR got me too
excited.


> >
> > I already thought about a few ideas to implement that, but I'll
> > develop
> > them more and try to come up with some proof of concept before
> > sending them
> > to you.
> > Hopefully by tomorrow I'll update on this, I'll preferably hop to bed
> > now
> > haha.
> > Do you think this could make a suitable GSoC subject ?
>
> I think working on the C++ enablement prerequisites in the analyzer
> would make more sense.  I'd planned to do this myself for GCC 14, but
> there are plenty of other tasks I could work on if you want to tackle
> C++ support as a GSoC project for GCC 14.
>

Yes, I gladly would.


> A good C++ project might be: enable enough C++ support in the analyzer
> for it to be able to analyze itself.  This could be quite a large,
> difficult project, though it sidesteps having to support exception-
> handling, since we build ourselves with exception-handling disabled.
>
> Hope this is helpful
> Dave
>
>
To that purpose,  the following order of resolutions would make sense into
achieving that:
0. An emphasis on reducing the amount of exploded nodes will have to be put
from the beginning. All of my C++ samples produce graphs quite dense.
1. First thing first, extending the current C tests to cover C++ PR96395
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96395>
(1.bis )
2. I would then go with supporting the options relative to sm-malloc:
  - -Wanalyser-double-free should behave properly (cf the fresh PR109365
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109365> )
  - Add proper support of the non-throwing new operators. At the moment,
any new operators are supposed non-throwing (on_allocator_call, param
returns_nonnull is implicitly set to false). Actually handling this would
result in less false positive of -Wanalyzer-possible-null-dereference I
observed by tweaking it to true. Indeed currently every throwing new
operators get this warning. This would fit the bill of PR94355
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94355> and maybe the new
placement sizes could be handled along the way.
   - -Wanalyzer-malloc-leak doesn't seem to go through the constructor
chain. The reproducer
struct D { int x; int y;};
class A {
    public:
        D* d = nullptr;
        A() : d{new D()} {}
};
class B : public A {
    public:
        int b;
        B(int x) : b{x} {}
};

B foo() {B b(4); return b;}
int main() {
    B r = foo();
    return 0;
}
Emits nothing. The state is 'nonnull' is properly tracked though along the
constructors back to foo, so I will have to dive deeper into this tomorrow.
3. Improve the scope of -Wanalyzer-null-dereference
   - For the analyzer, -Wanalyzer[-possible]-null-dereference should fully
support smart pointers. That is not the case currently (see PR109366
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109366> ), even though shared
pointers are promising.
  - For smart pointers, it might be necessary to review the diagnostic
path, as for shared_ptr they are quite long already.
4. Extension of out-of-bounds
 ( - Extending -Wout-of-bounds to the many vec<...> might be a requirement.
However I have to look into more details for that one, I don't see yet how
it could be done without a similar reuse of the assertions as for the
libstdc++.)

From what I saw, despite the bugs not being FIXED, vfuncs seem to be
working nicely enough after the fix from GSoC 2021.

Unfortunately I couldn't devote as much time as I wanted to gcc yesterday,
I plan to send a proposal draft tomorrow evening. Sincerely sorry for the
short time frame before the deadline.

Thanks again for your review.

Best,
Benjamin

^ permalink raw reply	[flat|nested] 5+ messages in thread
end of thread, other threads:[~2023-04-04  0:39 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-03-31 23:33 [GSoC][analyzer-c++] Enable enough C++ support for self-analysis Benjamin Priour
2023-04-02 22:38 ` David Malcolm
2023-04-03 16:44   ` [GSoC][analyzer-c++] Submission of a draft proposal Benjamin Priour
2023-04-03 16:46     ` Benjamin Priour
2023-04-04  0:39       ` David Malcolm

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