On Thu, Feb 15, 2018 at 4:25 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Thu, Feb 15, 2018 at 04:01:57PM +0100, Arnd Bergmann wrote: >> On Wed, Feb 14, 2018 at 11:45 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: >> > On Wed, Feb 14, 2018 at 04:24:12PM -0600, Josh Poimboeuf wrote: >> >> On Wed, Feb 14, 2018 at 04:11:15PM +0100, Arnd Bergmann wrote: > Ok, I expected something like that. GCC "undefined behavior" strikes > again. > > Kees, I suppose you'll need to obfuscate the code to stay one step ahead > of GCC. > > While this may be an objtool bug, I might not fix it because it served a > useful purpose here in finding GCC crap. > >> I would have expected an actual NULL pointer dereference to remain >> in the function though, or at least another trapping instruction. >> >> > Can you share the config for this one? >> >> https://pastebin.com/qFV6SPWP > > Would be interesting to analyze that config to understand what options > are causing GCC to do that. I don't see this "optimization" with my > config. This seems like a very rare combination, the flags I need to reproduce are "gcc -O2 -mno-red-zone -mpreferred-stack-boundary=3 -march=nocona", however I do see the same behavior with every gcc version since 4.8! Aside from -march=nocona, also bonnell, atom, silvermont, slm, and knl show this, but none of the modern microarchitectures do. Arnd