Re: [patch 13/18] entry: Make trace_syscall_enter() return type bool

On 07/10, Michal Suchánek wrote:
>
> On Fri, Jul 10, 2026 at 01:40:43PM +0200, Oleg Nesterov wrote:
> >
> > 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/[email protected]/
> >
> > 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.