On 07/13/2017 09:47 PM, Matthias Kaehlcke wrote: > Thanks for your analysis! > >> What happens if you try the below patch instead of the revert? Any >> chance the offending instruction goes away? >> >> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h >> index 11433f9..beac907 100644 >> --- a/arch/x86/include/asm/uaccess.h >> +++ b/arch/x86/include/asm/uaccess.h >> @@ -171,7 +171,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) > >> sizeof(0UL), 0ULL, 0UL)) >> might_fault(); \ >> asm volatile("call __get_user_%P4" \ >> : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \ >> - : "0" (ptr), "i" (sizeof(*(ptr)))); \ >> + : "0" (ptr), "i" (sizeof(*(ptr))), "r" (__sp)); \ >> (x) = (__force __typeof__(*(ptr))) __val_gu; \ >> __builtin_expect(__ret_gu, 0); \ >> }) > > The generated code is basically the same, only that now the value from > the stack is stored in a register and written twice to RSP: >
AFAIR clang works much better with global named registers. Could you check if the patch bellow helps? --- arch/x86/include/asm/uaccess.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h index a059aac9e937..121204387978 100644 --- a/arch/x86/include/asm/uaccess.h +++ b/arch/x86/include/asm/uaccess.h @@ -157,15 +157,18 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL)) * Clang/LLVM cares about the size of the register, but still wants * the base register for something that ends up being a pair. */ + +register unsigned long __current_sp asm(_ASM_SP); + #define get_user(x, ptr) \ ({ \ 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) \ + : "=a" (__ret_gu), "=r" (__val_gu), \ + "+r" (__current_sp) \ : "0" (ptr), "i" (sizeof(*(ptr)))); \ (x) = (__force __typeof__(*(ptr))) __val_gu; \ __builtin_expect(__ret_gu, 0); \ -- 2.13.0