[PATCH v4 1/2] aarch64: clear ZA state of SME before clone and clone3

[Rich Felker]

On Wed, Sep 03, 2025 at 02:53:55PM +0100, Yury Khrustalev wrote:
> On Tue, Sep 02, 2025 at 11:59:32AM -0400, enh wrote:
> > On Mon, Sep 1, 2025 at 5:51 AM Yury Khrustalev <yury.khrustalev@arm.com> wrote:
> > >
> > > This change adds a call to the __arm_za_disable() function immediately
> > > before the SVC instruction inside clone() and clone3() wrappers. This
> > > sets the ZA state of SME to "off" on return from these functions (for
> > > both the child and the parent).
> > >
> > > This commit also adds relevant tests for the clone() and clone3() use cases.
> > > While the former is trivial, the latter is a bit complex since the clone3()
> > > symbol is not public.
> > 
> > is it time to make clone3() public? i've had a few requests over in Android too.
> > 
> > (avoiding this kind of "oh, but for $ARCH you also have to do $TRICK"
> > in callers being the answer to "why not just use syscall()?".)
> 
> As far as I understand, making clone3() public will require significant
> changes throughout the library to make sure any internal machinery that
> exists to make sure child processes and threads are working correctly is
> also exposed correctly. This is a big undertaking, and without
> significant popular demand I don't think anyone would want to do that.

clone3 is largely just not useful to expose. Pretty much all of the
things you can set through it are things that have to be forbidden
unless you're writing a low level no-libc runtime.

Probably the right thing to do is expose a fancier fork that uses
clone3 only in fork-mode and lets you do the pidfd and cgroup stuff
that might actually be useful.

Rich