Re: [PATCH v5 15/30] arm64/sve: Signal handling support

[Dave Martin <Dave.Martin@arm.com>]

On Tue, Dec 12, 2017 at 10:40:30AM +0000, Will Deacon wrote:
> On Mon, Dec 11, 2017 at 11:23:09AM -0800, Kees Cook wrote:
> > On Mon, Dec 11, 2017 at 6:07 AM, Will Deacon <will.deacon@arm.com> wrote:
> > > On Thu, Dec 07, 2017 at 10:50:38AM -0800, Kees Cook wrote:
> > >> My question is mainly: why not just use copy_*() everywhere instead?
> > >> Having these things so spread out makes it fragile, and there's very
> > >> little performance benefit from using __copy_*() over copy_*().
> > >
> > > I think that's more of a general question. Why not just remove the __
> > > versions from the kernel entirely if they're not worth the perf?
> > 
> > That has been something Linus has strongly suggested in the past, so
> > I've kind of been looking for easy places to drop the __copy_*
> > versions. :)
> 
> Tell you what then: I'll Ack the arm64 patch if it's part of a series
> removing the thing entirely :p
> 
> I guess we'd still want to the validation of the whole sigframe though,
> so we don't end up pushing half a signal stack before running into an
> access_ok failure?

That's an interesting question.  In many cases access_ok() might become
redundant, but for syscalls that you don't want to have side-effects
on user memory on failure it's still relevant.

In the signal case we'd still an encompassing access_ok() to prevent
stack guard overruns, because the signal frame can be large and isn't
written or read contiguously or in a well-defined order.

Cheers
---Dave