[PATCH] avoid false positives due to compute_objsize (PR 95353)

[Richard Biener]

On Mon, Jun 15, 2020 at 7:11 PM Martin Sebor via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On 6/14/20 12:37 PM, Jeff Law wrote:
> > On Sat, 2020-06-13 at 17:49 -0600, Martin Sebor wrote:
> >> On 6/13/20 3:50 PM, Sandra Loosemore wrote:
> >>> On 6/2/20 6:12 PM, Martin Sebor via Gcc-patches wrote:
> >>>> The compute_objsize() function started out as a thin wrapper around
> >>>> compute_builtin_object_size(), but over time developed its own
> >>>> features to compensate for the other function's limitations (such
> >>>> as its inability to work with ranges).  The interaction of these
> >>>> features and the limitations has started to become increasingly
> >>>> problematic as the former function is used in more contexts.
> >>>>
> >>>> A complete "fix" for all the problems (as well as some other
> >>>> limitations) that I'm working on will be more extensive and won't
> >>>> be appropriate for backports.  Until then, the attached patch
> >>>> cleans up the extensions compute_objsize() has accumulated over
> >>>> the years to avoid a class of false positives.
> >>>>
> >>>> To make the warnings issued based on the results of the function
> >>>> easier to understand and fix, the patch also adds an informative
> >>>> message to many instances of -Wstringop-overflow to point to
> >>>> the object to which the warning refers.  This is especially
> >>>> helpful when the object is referenced by a series of pointer
> >>>> operations.
> >>>>
> >>>> Tested by boostrapping on x86_64-linux and building Binutils/GDB,
> >>>> Glibc, and the Linux kernel with no new warnings.
> >>>>
> >>>> Besides applying it to trunk I'm looking to backport the fix to
> >>>> GCC 10.
> >>>
> >>> This patch (commit a2c2cee92e5defff9bf23d3b1184ee96e57e5fdd) has broken
> >>> glibc builds on nios2-linux-gnu, when building sysdeps/posix/getaddrinfo.c:
> >>>
> >>> ../sysdeps/posix/getaddrinfo.c: In function 'gaih_inet.constprop':
> >>> ../sysdeps/posix/getaddrinfo.c:1081:3: error: 'memcpy' writing 16 bytes
> >>> into a region of size 8 overflows the destination
> >>> [-Werror=stringop-overflow=]
> >>>    1081 |   memcpy (&sin6p->sin6_addr,
> >>>         |   ^~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>    1082 |    at2->addr, sizeof (struct in6_addr));
> >>>         |    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>> In file included from ../include/netinet/in.h:3,
> >>>                    from ../resolv/bits/types/res_state.h:5,
> >>>                    from ../include/bits/types/res_state.h:1,
> >>>                    from ../nptl/descr.h:35,
> >>>                    from ../sysdeps/nios2/nptl/tls.h:45,
> >>>                    from ../sysdeps/generic/libc-tsd.h:44,
> >>>                    from ../include/../locale/localeinfo.h:224,
> >>>                    from ../include/ctype.h:26,
> >>>                    from ../sysdeps/posix/getaddrinfo.c:57:
> >>> ../inet/netinet/in.h:249:19: note: destination object 'sin_zero'
> >>>     249 |     unsigned char sin_zero[sizeof (struct sockaddr)
> >>>         |                   ^~~~~~~~
> >>>
> >>>
> >>> I have to say that I don't understand the "note" diagnostic here at all.
> >>>    :-(  Why does it think the destination object is a field of struct
> >>> sockaddr_in, while this memcpy is filling in a field of struct
> >>> sockaddr_in6?  (And, the sin6_addr field is indeed of type struct
> >>> in6_addr, matching the sizeof expression.)
> >>
> >> Most likely because some earlier pass (from my exchange with Jeff
> >> about this instance of the warning I suspect it's PRE) substitutes
> >> one member for the other in the IL when offsets into them happen
> >> to evaluate to the same offset from the start of the enclosing
> >> object.  The Glibc code does this:
> > Yes, this is the same issue we were discussing privately.
> >
> >>
> >>              struct sockaddr_in6 *sin6p =
> >>                (struct sockaddr_in6 *) ai->ai_addr;
> >>
> >>              sin6p->sin6_port = st2->port;
> >>              sin6p->sin6_flowinfo = 0;
> >>              memcpy (&sin6p->sin6_addr,
> >>                      at2->addr, sizeof (struct in6_addr));
> >>
> >> and the warning doesn't see sin6p->sin6_addr as the destination
> >> but something like
> >>
> >>     &MEM <struct sockaddr_in> [(void *)ai_10 + 4B].sin_zero;
> >>
> >> The details in this and all other middle end warnings are only as
> >> reliable as the IL they work with.  If the IL that doesn't correspond
> >> to the original source code they're going to be confusing (and may
> >> trigger false positives).
> > True, but the transformation done by PRE is valid.  PRE is concerned only with
> > value equivalences and the two addresses are the same and PRE can and will
> > replace one with the other.
>
> That's fine.  Since they are treated as equivalent it shouldn't
> matter which of the equivalent alternatives is chosen (there
> may be many).  It's the particular choice of the smaller member
> that makes it a problem: both in the terms of triggering a false
> positive and in terms of the note referencing a member the source
> code doesn't use.
>
> If PRE instead picked the bigger member it wouldn't necessarily
> trigger the warning.  But if that member was also too small,
> the note might still reference the wrong member.
>
> But if PRE picked another equivalent representation not involving
> any member at all but rather an offset from the base object (i.e.,
> just a MEM_REF) there would be no problem either way: no false
> positive, and if it overflowed, the warning wouldn't reference
> any member but just the base object.
>
> >
> >>
> >> Instead of substituting one member for another in the COMPONENT_REF
> >> when both happen to be accessed at the same offset, using a MEM_REF
> >> alone into the enclosing struct or union plus the offset of
> >> the members would avoid the problem.  Something like this:
> > Ultimately that's just a bandaid over a flawed implementation.  Fundamentally the
> > problem is the diagnostics should not be depending on the type of those MEM
> > expressions.  As long as we continue to do that we're going to run into problems.
> > Hence my suggestion we look at attaching suitable type information to the calls
> > early in the pipeline, possibly at AST generation time.
>
> It's not the MEM_REF type that's a problem here.  The general
> issue with the reliability of late warnings like this one also
> isn't isolated to just function calls.  Any checked statement
> is susceptible to them.
>
> In this case, the problem is that the COMPONENT_REF member is
> not the one referenced in the source code.  So more reliable
> type information for MEM_REFs wouldn't solve it: we would also
> need more reliable COMPONENT_REFs.
>
> I can think of a few ways to deal with it.
>
> 1) Introduce some alternate, more reliable, on-the-side
>     representation for warnings (your suggestion).
>
> 2) Accept that warnings that depend on these transforms are
>     inherently prone to false positives as a result (the status
>     quo).
>
> 3) Tighten up the requirements on the transforms/representations
>     to more faithfully correspond to the source code (my suggestion).
>
> Taken to its natural conclusion, (1) means we would need a whole
> separate IL just for warnings.  That doesn't seem sustainable.
>
> (2) is clearly suboptimal for users.  Issuing some false positives
> may be defensible but referencing code that doesn't even exist in
> the source is wrong.
>
> Meeting the needs of both optimizations and warnings within the same
> IL (i.e., (3)), seems like the right and only viable approach to me.
>
> For the case of this warning, when both of these
>
>    &MEM[(struct sockaddr_in6 *)ai_10 + 4B].sin6_addr;
>    &MEM <struct sockaddr_in> [(void *)ai_10 + 4B].sin_zero;
>
> are treated as equivalent (because they evaluate to the same
> address), is there some disadvantage or difficulty in encoding
> them as
>
>    &MEM <TYPE> [(void *)ai_10 + 4B + offsetof sin_zero];
>
> instead?

Well, we are _explicitely_ not doing this for the sake of diagnostics...

It also falls apart if you have

&MEM[].a[i_1]  vs. &MEM[].b[i_1]

do you want us to look for which array, a or b, is the bigger?

But sure - specifically talking about PRE - we can simplify
expressions inserted on the fly easily - we even pre-compute
that offset and it is readily available.  What we cannot
do is do this _only_ when there are "conflicting" expressions
(because we only know one of them at that point), we'd do
it always.  Look into create_component_ref_by_pieces_1
and the vn_reference_op_t's off member (see vn_reference_eq
how it's used to only consider offsets).  It will be a bit iffy
to fold trailing recursive elements into the MEM_REF base
but I guess by modifying the MEM_REF type and offset
in-place instead of attaching a component with non -1 offset
would be easiest.

You are suggesting to drop information (the access path that
carries no semantics on GIMPLE).  So you're fine with
removing the crippling of FRE before inlining (which is
there to preserve access paths to make __builtin_object_size
"happy"?).

Mind you can't have both ;)

Richard.

>
> Martin