Messages in this thread | | | Date | Thu, 13 Jul 2017 14:25:54 -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 11:47:48AM -0700, Matthias Kaehlcke wrote: > > 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: > > ffffffff813676ba: 31 c0 xor %eax,%eax > ffffffff813676bc: 48 89 45 c8 mov %rax,-0x38(%rbp) > ffffffff813676c0: 45 31 ff xor %r15d,%r15d > ffffffff813676c3: 48 89 45 a8 mov %rax,-0x58(%rbp) > ... > ffffffff81367918: 48 8b 4d a8 mov -0x58(%rbp),%rcx > ffffffff8136791c: 48 89 cc mov %rcx,%rsp > ffffffff8136791f: 48 89 cc mov %rcx,%rsp > ffffffff81367922: e8 69 26 f1 ff callq ffffffff81279f90 <__get_user_4>
LOL. Why corrupt the stack pointer with a single instruction (reading a zero from memory, no less) when you can instead do it with three instructions, including two duplicates?
Anyway this seems like a clang bug to me. If I specify RSP as an input register then the compiler shouldn't overwrite it first. For that matter it has no reason to overwrite it if it's an output register either.
-- Josh
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