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?
Martin
