On Wed, Aug 23, 2023 at 02:11:07PM +0100, Catalin Marinas wrote:

Yes, this should work. Any invocation of clone() or clone3() without a
shadow stack would disable GCS. What about the reverse, should GCS be
enabled for a thread even if the clone3() caller has GCS disabled? I
guess we shouldn't since GCS enabling depends on the prctl() state set
previously.

It has a fairly obvious intended meaning so we could do it easily enough
but OTOH allowing it opens up the idea of people wanting to specify GCS
flags which starts to seem like more trouble than it's worth compared to
just having them do the prctl() in the new thread.