PING [Patch][Middle-end]Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]

[H.J. Lu]

On Wed, Aug 5, 2020 at 5:34 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Wed, Aug 5, 2020 at 5:31 AM Richard Biener <rguenther@suse.de> wrote:
> >
> > On Wed, 5 Aug 2020, H.J. Lu wrote:
> >
> > > On Wed, Aug 5, 2020 at 12:06 AM Richard Biener <rguenther@suse.de> wrote:
> > > >
> > > > On Tue, 4 Aug 2020, H.J. Lu wrote:
> > > >
> > > > > On Tue, Aug 4, 2020 at 12:35 AM Richard Biener <rguenther@suse.de> wrote:
> > > > > >
> > > > > > On Mon, 3 Aug 2020, Qing Zhao wrote:
> > > > > >
> > > > > > > Hi, Uros,
> > > > > > >
> > > > > > > Thanks a lot for your review on X86 parts.
> > > > > > >
> > > > > > > Hi, Richard,
> > > > > > >
> > > > > > > Could you please take a look at the middle-end part to see whether the
> > > > > > > rewritten addressed your previous concern?
> > > > > >
> > > > > > I have a few comments below - I'm not sure I'm qualified to fully
> > > > > > review the rest though.
> > > > > >
> > > > > > > Thanks a lot.
> > > > > > >
> > > > > > > Qing
> > > > > > >
> > > > > > >
> > > > > > > > On Jul 31, 2020, at 12:57 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
> > > > > > > >
> > > > > > > >
> > > > > > > > 22:05, tor., 28. jul. 2020 je oseba Qing Zhao <QING.ZHAO@oracle.com <mailto:QING.ZHAO@oracle.com>> napisala:
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > Richard and Uros,
> > > > > > > > >
> > > > > > > > > Could you please review the change that H.J and I rewrote based on your comments in the previous round of discussion?
> > > > > > > > >
> > > > > > > > > This patch is a nice security enhancement for GCC that has been requested by security people for quite some time.
> > > > > > > > >
> > > > > > > > > Thanks a lot for your time.
> > > > > > > >
> > > > > > > > I'll be away from the keyboard for the next week, but the patch needs a middle end approval first.
> > > > > > > >
> > > > > > > > That said, x86 parts looks OK.
> > > > > > > >
> > > > > > > >
> > > > > > >
> > > > > > > > Uros.
> > > > > > > > > Qing
> > > > > > > > >
> > > > > > > > > > On Jul 14, 2020, at 9:45 AM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org <mailto:gcc-patches@gcc.gnu.org>> wrote:
> > > > > > > > > >
> > > > > > > > > > Hi, Gcc team,
> > > > > > > > > >
> > > > > > > > > > This patch is a follow-up on the previous patch and corresponding discussion:
> > > > > > > > > > https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545101.html <https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545101.html> <https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545101.html <https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545101.html>>
> > > > > > > > > >
> > > > > > > > > > From the previous round of discussion, the major issues raised were:
> > > > > > > > > >
> > > > > > > > > > A. should be rewritten by using regsets infrastructure.
> > > > > > > > > > B. Put the patch into middle-end instead of x86 backend.
> > > > > > > > > >
> > > > > > > > > > This new patch is rewritten based on the above 2 comments.  The major changes compared to the previous patch are:
> > > > > > > > > >
> > > > > > > > > > 1. Change the names of the option and attribute from
> > > > > > > > > > -mzero-caller-saved-regs=[skip|used-gpr|all-gpr|used|all]  and zero_caller_saved_regs("skip|used-gpr|all-gpr||used|all”)
> > > > > > > > > > to:
> > > > > > > > > > -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]   and  zero_call_used_regs("skip|used-gpr|all-gpr||used|all”)
> > > > > > > > > > Add the new option and  new attribute in general.
> > > > > > > > > > 2. The main code generation part is moved from i386 backend to middle-end;
> > > > > > > > > > 3. Add 4 target-hooks;
> > > > > > > > > > 4. Implement these 4 target-hooks on i386 backend.
> > > > > > > > > > 5. On a target that does not implement the target hook, issue error for the new option, issue warning for the new attribute.
> > > > > > > > > >
> > > > > > > > > > The patch is as following:
> > > > > > > > > >
> > > > > > > > > > [PATCH] Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]
> > > > > > > > > > command-line option and
> > > > > > > > > > zero_call_used_regs("skip|used-gpr|all-gpr||used|all") function attribue:
> > > > > > > > > >
> > > > > > > > > >  1. -fzero-call-used-regs=skip and zero_call_used_regs("skip")
> > > > > > > > > >
> > > > > > > > > >  Don't zero call-used registers upon function return.
> > > > > >
> > > > > > Does a return via EH unwinding also constitute a function return?  I
> > > > > > think you may want to have a finally handler or support in the unwinder
> > > > > > for this?  Then there's abnormal return via longjmp & friends, I guess
> > > > > > there's nothing that can be done there besides patching glibc?
> > > > >
> > > > > Abnormal returns, like EH unwinding and longjmp, aren't covered by this
> > > > > patch. Only normal returns are covered.
> > > >
> > > > What's the point then?  Also specifically thinking about spill slots.
> > > >
> > >
> > > The goal of this patch is to zero caller-saved registers upon normal
> > > function return.  Abnormal returns and spill slots are outside of the
> > > scope of this patch.
> >
> > Sure, I can write a patch that spills some regs, writes zeros to them
> > and then restores them.  And the patch will fulfil what it was designed
> > to do.
> >
> > Still I need to come up with a reason that this is a useful feature
> > by its own for it to be accepted.
> >
> > I am asking for that reason.  What's the reason for the "goal of this
> > patch"?  Why's that a useful goal on its own?
> >
>
> Hi Victor,
>
> Can you provide some background information about how/why this feature
> is used?
>

>From The SECURE project and GCC in GCC Cauldron 2018:

Speaker: Graham Markall

The SECURE project is a 15 month program funded by Innovate UK, to
take well known security techniques from academia and make them
generally available in standard compilers, specfically GCC and LLVM.
An explicit objective is for those techniques to be incorporated in
the upstream versions of compilers. The Cauldron takes place in the
final month of the project and this talk will present the technical
details of some of the techniques implemented, and review those that
are yet to be implemented. A particular focus of this talk will be on
verifying that the implemetnation is correct, which can be a bigger
challenge than the implementation.

Techniques to be covered in the project include the following:

Stack and register erasure. Ensuring that on return from a function,
no data is left lying on the stack or in registers. Particular
challenges are in dealing with inlining, shrink wrapping and caching.

This patch implemens register erasure.


-- 
H.J.