linux-kernel - Re: [RFC PATCH RESEND v3 3/3] ptrace: add PTRACE_EVENT_SECCOMP support to PTRACE_GET_SYSCALL

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Date:   Tue, 27 Nov 2018 13:31:17 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Elvira Khabirova <lineprinter@...linux.org>
Cc:     rostedt@...dmis.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, ldv@...linux.org, esyr@...hat.com,
        luto@...nel.org, strace-devel@...ts.strace.io,
        linux-api@...r.kernel.org
Subject: Re: [RFC PATCH RESEND v3 3/3] ptrace: add PTRACE_EVENT_SECCOMP
 support to PTRACE_GET_SYSCALL_INFO

On 11/27, Elvira Khabirova wrote:
>
> On Mon, 26 Nov 2018 15:35:24 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > On 11/25, Elvira Khabirova wrote:
> > >
> > > Extend PTRACE_GET_SYSCALL_INFO to support PTRACE_EVENT_SECCOMP stops.
> > > The information returned is the same as for syscall-enter-stops.
> >
> > Oh, this is not nice ;) there must be a better option, I hope... Plus
> >
> >
> > Can't ptrace_get_syscall() check
> >
> > 	child->exit_code == (PTRACE_EVENT_SECCOMP << 8) | SIGTRAP;
> >
> > to detect the PTRACE_EVENT_SECCOMP case?
>
> Nope; looks like exit_code is zeroed after wait().

Yes, thanks for correcting me,

but we can use child->last_siginfo->si_code. Just like ptrace_request(PTRACE_LISTEN)
does but you can do this lockless (no need to lock_task_sighand()).

And if we require that the user of ptrace_get_syscall() should also use TRACESYSGOOD
then ptrace_get_syscall() can probably do something like

	int entry;

	if (!child->last_siginfo)
		return -EINVAL;
	else if (child->last_siginfo->si_code == (PTRACE_EVENT_SECCOMP << 8) | SIGTRAP)
		entry = 1;
	else if (child->last_siginfo->si_code == SIGTRAP | 0x80)
		entry = child->ptrace_message == PTRACE_EVENTMSG_SYSCALL_ENTRY;
	else
		return -EINVAL;

and this way PTRACE_EVENTMSG_SYSCALL_ENTRY/EXIT can't confict with seccomp or
anything else.

No?

Of course, debugger can do PTRACE_SETSIGINFO and confuse itself but probably we
do not care?

Oleg.