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. > > 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. jeff
