On Thu, Aug 03, 2023 at 23:30:31 +0900, Rin Okuyama wrote: > On 2023/08/03 23:23, Valery Ushakov wrote: > > On Thu, Aug 03, 2023 at 13:33:27 +0000, Rin Okuyama wrote: > > > > > -Wuse-after-free for GCC 12 is premature. It fires on a common idiom: > > > > > > newbuf = realloc(buf, size); > > > p = newbuf + (p - buf); > > > > > > Let shut this up for GCC 12 (with hoping it gets improved for 13!). > > > > C99 says > > > > J.2 Undefined behavior > > > > [#1] The behavior is undefined in the following > > circumstances: > > [...] > > -- The value of a pointer to an object whose lifetime has > > ended is used (6.2.4). > > > > > > Yes, for the "obvious" implementation of pointers as addresses the > > above idiom happens to work, but it doesn't make that idiom any less > > UB. > > Ah, I only thought about "obvious" impl. Thank you for kind > explanation! I will revert them for now.
We should fix those cases that gcc12 found. While it may seem like a stretch of imagination (e.g. compiling C to JVM or something like that, where the pointer is actually a nontrivial object), "fat" function pointers on itanium were a mundane thing and caused their fair share of problems for code that naively assumed trivial "address-only" pointers. I would imagine UB sanitizers will trip up on that idiom too... Thanks! -uwe
