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

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Date:   Thu, 1 Sep 2016 21:50:51 +0200
From:   Ales Novak <alnovak@...e.cz>
To:     Oleg Nesterov <oleg@...hat.com>
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 Thu, Sep 01, 2016 at 06:46:13PM +0200, Oleg Nesterov wrote:
> 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?

You're right, sorry for not noticing that. Also my changelog was
misleading, I've tried to fix that.

> 
> 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.

Good point also.

> 
> Oleg.
> 

-- 
Ales Novak
SUSE L3

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