RE: [RFC][PATCH 01/22] x86 user stack frame reads: switch to explicit __get_user()

[David Laight]

From: Linus Torvalds
> Sent: 29 March 2020 19:16
> On Sun, Mar 29, 2020 at 11:03 AM David Laight <David.Laight@xxxxxxxxxx> wrote:
> >
> > > That's how get_user() already works.
> > >
> > > It is a polymorphic function (done using macros, sizeof() and ugly
> > > compiler tricks) that generates a call, yes. But it's not a normal C
> > > call. On x86-64, it returns the error code in %rax, and the value in
> > > %rdx
> >
> > I must be mis-remembering the object code from last time
> > I looked at it.
> 
> On an object code level, the end result actually almost looks like a
> normal call, until you start looking at the exact register passing
> details.
> 
> On a source level, it's anything but.
> 
> This is "get_user()" on x86:
> 
>   #define get_user(x, ptr)                                              \
>   ({                                                                    \
>         int __ret_gu;                                                   \
>         register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX);            \
>         __chk_user_ptr(ptr);                                            \
>         might_fault();                                                  \
>         asm volatile("call __get_user_%P4"                              \
>                      : "=a" (__ret_gu), "=r" (__val_gu),                \
>                         ASM_CALL_CONSTRAINT                             \
>                      : "0" (ptr), "i" (sizeof(*(ptr))));                \
>         (x) = (__force __typeof__(*(ptr))) __val_gu;                    \
>         __builtin_expect(__ret_gu, 0);                                  \
>   })

Can't you simplify that by using the =d constraint rather
than relying on a asm register variable.

	David

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