Re: [PATCH v4 1/2] ptrace: add PTRACE_SET_SYSCALL_INFO syscall skipping support

[Oleg Nesterov <oleg@redhat.com>]

On 07/08, Dmitry V. Levin wrote:
>
> On Wed, Jul 08, 2026 at 11:06:54AM +0200, Renzo Davoli wrote:
>
> > System call suppression for PTRACE_SYSCALL_INFO_ENTRY is currently omitted
> > because its implementation is architecture-dependent. On some architectures,
> > the system call number and return value share the same register, making it
> > difficult to suppress a system call without altering the return value. A
> > portable implementation would require an audit of all supported architectures.
>
> I suggest the following wording for the explanation why
> PTRACE_SYSCALL_INFO_ENTRY is not supported yet:
>
> System call suppression via PTRACE_SYSCALL_INFO_ENTRY is currently not
> implemented.  On some architectures (e.g. MIPS), when a system call
> is skipped by setting the syscall number to -1 at the entry stop, the
> architecture entry path unconditionally overwrites the return value
> register with -ENOSYS before the tracer can set a custom return value
> at the exit stop.

Thanks! but looks a bit misleading or (quite possibly) I am confused...

At least for MIPS, I think it should something like

	when a system call is skipped by setting the syscall number to -1

	...

	architecture entry path unconditionally overwrites the return value
	register with -ENOSYS

	...

	_after_ (or even if ) the tracer has already set a custom return value

No?

Oleg.