On 07/10, Michal Suchánek wrote:
>
> On Wed, Jul 08, 2026 at 10:34:38PM +0200, Thomas Gleixner wrote:
> >
> > 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.
>
> >
> > 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/[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.
Oleg.
