Messages in this thread | | | Date | Fri, 22 May 2015 09:32:12 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v4 0/3] Compile-time stack frame pointer validation |
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On Fri, May 22, 2015 at 12:01:58AM +0200, Borislav Petkov wrote: > On Thu, May 21, 2015 at 03:54:25PM -0500, Josh Poimboeuf wrote: > > stackvalidate: arch/x86/lib/memmove_64.o: return instruction outside of a function at .altinstr_replacement+0x5 > > That must be something like this: > > 0000000000000000 <.altinstr_replacement>: > 0: 48 89 d1 mov %rdx,%rcx > 3: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi) > 5: c3 retq > > right? > > In any case, anything with alternatives is probably a false positive > because even if instructions appear outside of the containing function, > they get patched in and are actually inside. Jump offsets get fixed up > properly too. Should, at least :-)
Hm, alternatives do complicate things a bit. It *is* a false positive, but not necessarily because its part of an alternative instruction block.
The above code would be patched into memmove(), which is a leaf function because it doesn't call any other functions. Leaf functions don't need frame pointer logic, so we can ignore them.
If instead the above code were patched into a non-leaf function, we'd have to change it to restore the frame pointer before returning.
-- Josh
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