Re: Process killed by seccomp looks live by tracer

[Oleg Nesterov <oleg@redhat.com>]

That said...

__seccomp_filter() does

	case SECCOMP_RET_KILL_PROCESS:
	...
		/* Show the original registers in the dump. */
		syscall_rollback(current, current_pt_regs());

		/* Trigger a coredump with SIGSYS */
		force_sig_seccomp(this_syscall, data, true);

This means that after syscall_rollback() regs->ax == orig_ax, so
ptrace_get_syscall_info_exit() will always report .is_error == 0.

And since force_sig_seccomp() uses force_coredump == true, SIGSYS
won't be reported (see the SA_IMMUTABLE check in get_signal()).

Again, it is not that I think this wrong. But perhaps Kees and Andy
can take a look and confirm that this is what we actually want.

Oleg.

On 03/05, Oleg Nesterov wrote:
>
> Hi Max,
>
> On 03/05, Max Ver wrote:
> >
> > >This is expected; PTRACE_GET_SYSCALL_INFO is at syscall entry before seccomp filtering has run.
> >
> > It also happens at the syscall exit. Take a look at the result, it
> > shows 'exit ok' twice.
>
> Why do you think this is wrong? (and I don't think this has something to
> do with seccomp, btw).
>
> > If we can agree on this is a bug, I suggest the kernel give a hint
> > about tracee exit in waitpid return value, what do you think?
>
> But the kernel already gives you a hint, no?
>
> Perhaps I missed your point, but see the change of your test-case below.
>
> Oleg.
>
>
> --- /tmp/PT.c~	2026-03-05 15:18:18.397319905 +0100
> +++ /tmp/PT.c	2026-03-05 15:40:11.044415647 +0100
> @@ -15,6 +15,8 @@
>  #include <sys/wait.h>
>  #include <threads.h>
>  #include <unistd.h>
> +#include <assert.h>
> +#include <errno.h>
>
>  void
>  child ()
> @@ -57,6 +59,14 @@
>        puts ("child exit");
>        exit (1);
>      }
> +
> +	if (WIFSIGNALED(status)) {
> +		printf("signalled pid=%d sig=%d\n", pid, WTERMSIG(status));
> +		assert(kill(pid, 0) == -1 && errno == ESRCH);
> +		exit(0);
> +	}
> +
> +
>    if (ptrace (PTRACE_GET_SYSCALL_INFO, pid,
>                sizeof (struct ptrace_syscall_info), &info)
>        == -1)