On 07/10, Michal Suchánek wrote:

On Fri, Jul 10, 2026 at 01:40:43PM +0200, Oleg Nesterov wrote:

quoted

I can only say that ptrace users do want to skip the syscall and set the
return value on entry.

See
	[PATCH v5 1/2] ptrace: add PTRACE_SET_SYSCALL_INFO syscall skipping support
	https://lore.kernel.org/all/20260709100949.94345-2-renzo@cs.unibo.it/ (local)

The changelog explains that currently this doesn't work because
among the arches which define HAVE_ARCH_TRACEHOOK (at least) arch/mips is
broken in this regard.

Or it could be documented that setting the return value has to be done
in the exit trace, and that would than work on any architecture AFAICT.

Well, ptrace users know the problem. And this what they have to do
currently.

With ppc and s390 using the same register for the syscall number and
syscall return value it's very much impossible to poke the return value
on entry into a register using the generic register access function. As
of now there is no place to store the value ot of the return value
outside of the registers, either.

I know nothing about ppc and s390. Can't comment right now.

And the current PTRACE_SET_SYSCALL_INFO indeed sets the syscall nr and
arguments on entry and the syscall return value on exit, that
disctincion is implemented.

Not sure how the patchset you point out is relevant, it only adds
changes in the exit case.

No. It allows to skip-and-set-retval on PTRACE_EVENT_SECCOMP.

But ENTRY -> EXIT transition is not yet allowed due to the problems
above.

Oleg.