Messages in this thread | | | From | Kees Cook <> | Date | Thu, 15 Feb 2018 10:06:03 -0800 | Subject | Re: objtool warnings on 4.14-stable/gcc-7.3.0 |
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On Thu, Feb 15, 2018 at 7:59 AM, Arnd Bergmann <arnd@arndb.de> wrote: > On Thu, Feb 15, 2018 at 4:25 PM, Josh Poimboeuf <jpoimboe@redhat.com> 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@redhat.com> 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.
Uuhhh... I don't see the NULL deref, and even if it was eliminating later stuff, I'd still expect a pr_info() ...
void lkdtm_CORRUPT_LIST_ADD(void) { /* * Initially, an empty list via LIST_HEAD: * test_head.next = &test_head * test_head.prev = &test_head */ LIST_HEAD(test_head); struct lkdtm_list good, bad; void *target[2] = { }; void *redirection = ⌖
pr_info("attempting good list addition\n"); ...
>>> > 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.
I'll see if I can reproduce this...
-Kees
-- Kees Cook Pixel Security
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