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

On 07/10, Michal Suchánek wrote:

quoted

On Wed, Jul 08, 2026 at 10:34:38PM +0200, Thomas Gleixner wrote:

quoted

  1) The set in stone rule is that if the entry code returns -1L as the
     syscall number then the architecture code has to skip the syscall
     invocation _and_ is not supposed to change the return value.

Which stone?

Pics or it did not happen.

quoted

  2) There is no guarantee and never has been that any of the involved
     mechanisms (ptrace, seccomp, tracing) will change the return value
     when it sets the syscall number to -1L.

For ptrace to correctly emulate a syscall it needs to set the syscall nr
to an invalid value on entry, and the desired result if the syscall on
exit AFAICT.

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.

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.

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.

Thanks

Michal