Messages in this thread | | | Date | Thu, 13 Jul 2017 14:57:04 -0700 | From | Matthias Kaehlcke <> | Subject | Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm" |
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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
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