On Wed, Jul 19, 2017 at 12:52 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Wed, Jul 19, 2017 at 12:45:19PM -0700, Kees Cook wrote:
>> > diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
>> > index 13b91e850a02..e7587db3487c 100644
>> > --- a/arch/x86/include/asm/refcount.h
>> > +++ b/arch/x86/include/asm/refcount.h
>> > @@ -15,6 +15,7 @@
>> > ".pushsection .text.unlikely\n" \
>> > "111:\tlea %[counter], %%" _ASM_CX "\n" \
>> > "112:\t" ASM_UD0 "\n" \
>> > + ASM_UNREACHABLE \
>> > ".popsection\n" \
>> > "113:\n" \
>> > _ASM_EXTABLE_REFCOUNT(112b, 113b)
>> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
>> > index cd4bbe8242bd..85e0b8f42ca0 100644
>> > --- a/include/linux/compiler-gcc.h
>> > +++ b/include/linux/compiler-gcc.h
>> > @@ -202,15 +202,25 @@
>> > #endif
>> >
>> > #ifdef CONFIG_STACK_VALIDATION
>> > +
>> > #define annotate_unreachable() ({ \
>> > asm("%c0:\t\n" \
>> > - ".pushsection .discard.unreachable\t\n" \
>> > - ".long %c0b - .\t\n" \
>> > - ".popsection\t\n" : : "i" (__LINE__)); \
>> > + ".pushsection .discard.unreachable\n\t" \
>> > + ".long %c0b - .\n\t" \
>> > + ".popsection\n\t" : : "i" (__LINE__)); \
>>
>> Is this just an indentation change?
>
> This was sneaking in a fix to put the tab after the newline instead of
> before it. I figured it's not worth its own commit.
Ah! Now I see it. Gotcha.
>> > })
>> > +
>> > +#define ASM_UNREACHABLE \
>> > + "999: .pushsection .discard.unreachable\n\t" \
>> > + ".long 999b - .\n\t" \
>> > + ".popsection\n\t"
>>
>> Just so I understand, we'll get a single byte added for each exception
>> case, but it'll get discarded during final link?
>
> I think it's four bytes actually, but yeah, the section gets stripped at
> vmlinux link time.
Right, yes.
BTW, I think this needs compiler.h coverage instead of the #else in
compiler-gcc.h (since it's different from how annotate_unreachable is
used only in compiler-gcc.h. I'll adjust.
Also, in looking at CONFIG_STACK_VALIDATION, do you want it to just
warn and skip, or do you want to error out the build if validation
isn't available but it's in the .config?
-Kees
--
Kees Cook
Pixel Security
