Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 13 Jul 2017 16:34:06 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm" |
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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
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