Re: RFC: [PATCH] x86: Add -mzero-caller-saved-regs=[skip|used|all]

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Sep 27, 2018 at 7:57 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Thu, Sep 27, 2018 at 3:16 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Thu, Sep 27, 2018 at 6:08 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>> > On 26/09/18 19:10, H.J. Lu wrote:
>> >>
>> >> Add -mzero-caller-saved-regs=[skip|used|all] command-line option and
>> >> zero_caller_saved_regs("skip|used|all") function attribue:
>> >>
>> >> 1. -mzero-caller-saved-regs=skip and zero_caller_saved_regs("skip")
>> >>
>> >> Don't zero caller-saved integer registers upon function return.
>> >>
>> >> 2. -mzero-caller-saved-regs=used and zero_caller_saved_regs("used")
>> >>
>> >> Zero used caller-saved integer registers upon function return.
>> >>
>> >> 3. -mzero-caller-saved-regs=all and zero_caller_saved_regs("all")
>> >>
>> >> Zero all caller-saved integer registers upon function return.
>> >>
>> >> Tested on i686 and x86-64 with bootstrapping GCC trunk and
>> >> -mzero-caller-saved-regs=used as well as -mzero-caller-saved-regs=all
>> >> enabled by default.
>> >>
>> >
>> > from this description and the documentation it's
>> > not clear to me what this tries to achieve.
>> >
>> > is it trying to prevent information leak?
>> > or some pcs hack the caller may rely on?
>> >
>> > if it's for information leak then i'd expect such
>> > attribute to be used on crypto code.. however i'd
>> > expect crypto code to use simd registers as well,
>> > so integer only cleaning needs explanation.
>>
>> The target usage is in Linux kernel.
>
> Maybe still somehow encode that in the option since it otherwise raises
> expectations that are not met?
> -mzero-call-clobbered-regs=used-int|all-int|skip|used-simd|used-fp,etc.?
> and sorry() on unimplemented ones?  Or simply zero also non-integer
> regs the same way?  I suppose
> there isn't sth like vzeroupper that zeros all SIMD regs and completely?
>

The problem with SIMD regs is how to cover different ISAs, widths and
number of SIMD regs.  We need to generate at least 3 different code paths
to clear SSE, AVX and AVX512 registers.

-- 
H.J.