linux-kernel - Re: [PATCH] ptrace: clear TIF_SYSCALL

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Message-ID: <20160901164613.GA13138@redhat.com>
Date:   Thu, 1 Sep 2016 18:46:13 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Ales Novak <alnovak@...e.cz>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH] ptrace: clear TIF_SYSCALL_TRACE on ptrace detach

On 09/01, Ales Novak wrote:
>
> On ptrace_detach(), the TIF_SYSCALL_TRACE in thread->flags of the tracee
> is not cleared up. This results in the tracehook_report_syscall_* being
> called (though there's no longer a tracer listening to that) upon its
> further syscalls.
>
> Example scenario - attach "strace" to a running process and kill it (the
> strace) with SIGKILL. You'll see that the syscall trace hooks are still
> being called.

We do not really care, if the tracer dies the tracee can be in the
inconsistent state anyway.

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -85,6 +85,7 @@ void __ptrace_unlink(struct task_struct *child)
>  	task_clear_jobctl_pending(child, JOBCTL_TRAP_MASK);
>  	task_clear_jobctl_trapping(child);
>
> +	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);

OK, probably this change makes sense anyway, but then you should
remove another clear_tsk_thread_flag(TIF_SYSCALL_TRACE) in ptrace_detach?

And I'd suggest to move this clear_tsk_thread_flag() up, before
spin_lock(siglock). Otherwise it looks as if we should clear this flag
with ->siglock held.

Oleg.