El Thu, Jul 13, 2017 at 04:34:06PM -0500 Josh Poimboeuf ha dit:
> 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.
I totally agree, I was mainly concerned about not breaking the
solution that currently works with gcc.
> 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.
Eventually I expect that there will be interest in live patching
clang-compiled kernels, however at this stage it probably isn't an
overly important feature.
> 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).
Sounds great.
Thanks again for looking into this and coming up with a solution!
Matthias
