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: > >> > Hi Josh, > >> > > >> > I recently did some randconfig testing with a plain 4.14-stable kernel > >> > and gcc-7.3.0, and came across three distinct objtool warnings: > >> > > >> > drivers/misc/lkdtm_bugs.o: warning: objtool: > >> > lkdtm_CORRUPT_LIST_ADD()+0x15: return with modified stack frame > > > > While this is probably an objtool bug, the code is very odd: > > > > 00000000000001a8 <lkdtm_CORRUPT_LIST_ADD>: > > 1a8: e8 00 00 00 00 callq 1ad <lkdtm_CORRUPT_LIST_ADD+0x5> > > 1a9: R_X86_64_PC32 __fentry__-0x4 > > 1ad: 55 push %rbp > > 1ae: 48 89 e5 mov %rsp,%rbp > > 1b1: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp > > 1b5: 48 83 ec 20 sub $0x20,%rsp > > 1b9: 48 89 ec mov %rbp,%rsp > > 1bc: 5d pop %rbp > > 1bd: c3 retq > > > > The function just allocates/aligns its stack space and then returns. It > > seems like GCC was too smart for its own good here, as the function > > doesn't test what it's supposed to. > > AFAIU, there is an optimization step in gcc that eliminates basic blocks > that contain an unconditional NULL pointer dereference, based on the > assumption that it's undefined behavior, and if we ever get here, it is > free to drop not only code after but also before it as long as it doesn't > have any side-effects. 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. -- Josh