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

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

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On Thu, Sep 27, 2018 at 7:58 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?
>

Here is the updated patch to zero caller-saved vector registers.   I don't
mind a different option name if it is preferred.  I may be able to create
some generic utility functions which can be used by other backends.  But
actual implementation must be target specific.

Any comments?

Thanks.


-- 
H.J.

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