On Thu, Jul 13, 2017 at 02:12:45PM -0700, Matthias Kaehlcke wrote:
> El Thu, Jul 13, 2017 at 03:34:16PM -0500 Josh Poimboeuf ha dit:
> > And yet another one to try (clobbering sp) :-)
> >
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 11433f9..21f0c39 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -166,12 +166,12 @@ __typeof__(__builtin_choose_expr(sizeof(x) >
> > sizeof(0UL), 0ULL, 0UL))
> > ({ \
> > int __ret_gu; \
> > register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX); \
> > - register void *__sp asm(_ASM_SP); \
> > __chk_user_ptr(ptr); \
> > might_fault(); \
> > - asm volatile("call __get_user_%P4" \
> > - : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \
> > - : "0" (ptr), "i" (sizeof(*(ptr)))); \
> > + asm volatile("call __get_user_%P3" \
> > + : "=a" (__ret_gu), "=r" (__val_gu) \
> > + : "0" (ptr), "i" (sizeof(*(ptr))) \
> > + : "sp"); \
> > (x) = (__force __typeof__(*(ptr))) __val_gu; \
> > __builtin_expect(__ret_gu, 0); \
> > })
>
> This compiles with both gcc and clang, clang does not corrupt the
> stack pointer. I wouldn't be able to tell though if it forces a stack
> frame if it doesn't already exist, as the original patch intends.
Whether it forces the stack frame on clang is a very minor issue
compared to the double fault. That really only matters when you want to
use CONFIG_STACK_VALIDATION to get 100% reliable stacktraces with frame
pointers. And that feature is currently very GCC-specific. So you
probably don't need to worry about that for now, at least until you want
to do live patching with a clang-compiled kernel.
IIRC, clobbering SP does at least force the stack frame on GCC, though I
need to double check that. I can try to work up an official patch in
the next week or so (need to do some testing first).
--
Josh
