Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e

Borislav Petkov
2020-09-16T21:01:33Z

On Tue, Sep 15, 2020 at 05:50:54PM -0400, Arvind Sankar wrote:
> On Tue, Sep 15, 2020 at 10:49:12PM +0200, Borislav Petkov wrote:
> >
> > (Btw, clang doesn't need to add that "xor %eax,%eax" - panic() should not be
> > returning, ever. :-))
> >
>
> I think this is because panic() is varargs, and clang doesn't support
> gcc's -mskip-rax-setup. The normal ABI requires the caller to set RAX to
> the number of arguments in vector registers.
>
> https://patchwork.ozlabs.org/project/gcc/patch/20141218131150.GA32638@intel.com/

Ah, good point. Found this in the ABI doc:

"For calls that may call functions that use varargs or stdargs
(prototype-less calls or calls to functions containing ellipsis (...) in
the declaration) %al is used as hidden argument to specify the number of
vector registers used. The contents of %al do not need to match exactly
the number of registers, but must be an upper bound on the number of
vector registers used and is in the range 0–8 inclusive."

Thx.

--
Regards/Gruss,
Boris.