[PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks

[Mark Brown]

On Thu, Sep 28, 2023 at 05:59:25PM +0100, Szabolcs Nagy wrote:
> The 08/23/2023 14:11, Catalin Marinas wrote:

> > > and there is user code doing raw clone threads (such threads are
> > > technically not allowed to call into libc) it's not immediately
> > > clear to me if having gcs in those threads is better or worse.

> i think raw clone / clone3 users may be relevant so we need a
> solution such that they don't fail when gcs args are missing.

Are we sure about that?  Old binaries shouldn't be affected since they
won't turn GCS so we're just talking about new binaries here - are there
really so many of them that we won't be able to get them all converted
over to clone3() and GCS in the timescales we're talking about for GCS
deployment?  I obviously don't particularly mind having the default size
logic but if we allow clone() then that's keeping the existing behaviour
and layering allocation via clone3() on top of it which Catalin didn't
want.  Catalin?

> userspace allocated gcs works for me, but maybe the alternative
> with size only is more consistent (thread gcs is kernel mapped
> with fallback size logic if gcs size is missing):

If we have size only then the handling of GCS and normal stack in struct
clone_args would be inconsistent.  Given that it seems better to have
the field present, we can allow it to be NULL and do the allocation with
the specified size but it should be there.

> > An alternative would be for the clone3() to provide an address _hint_
> > and size for GCS and it would still be the kernel doing the mmap (and
> > munmap on clearing). But at least the user has some control over the
> > placement of the GCS and its size (and maybe providing the address has
> > MAP_FIXED semantics).

> the main thread gcs is still special: the size is provided
> via prctl (if at all).

Either that or we have it do a map_shadow_stack() but that's an extra
syscall during startup.
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