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
