Date: Sat, 5 Dec 2015 19:48:40 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Will Deacon <will.deacon@arm.com>
Cc: John Blackwood <john.blackwood@ccur.com>, linux-kernel@vger.kernel.org,
        fweisbec@gmail.com
Subject: Re: [PATCH] ARM64: Clear out any singlestep state on a ptrace
	detach operation
Message-ID: <20151205184840.GA6847@redhat.com>
References: <5660A08B.6030204@ccur.com> <20151204100357.GA26172@arm.com>
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On 12/04, Will Deacon wrote:
>
> I hacked up a quick patch below (not even compile-tested), but I'm not
> sure what to do about hardware {break,watch}points. Some architectures
> explicitly clear those on detach, whereas others appear to leave them
> alone. Thoughts?

Heh ;)

Please see fab840fc2d542fabcab "ptrace: PTRACE_DETACH should do
flush_ptrace_hw_breakpoint(child)".

And the next "revert" commit, 35114fcbe0b9b0fa3f6653a2.

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -454,13 +454,20 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
>  	return dead;
>  }
>
> +#ifndef arch_ptrace_detach
> +#define arch_ptrace_detach(child)	do { } while (0)
> +#endif
> +
>  static int ptrace_detach(struct task_struct *child, unsigned int data)
>  {
>  	if (!valid_signal(data))
>  		return -EIO;
>
> -	/* Architecture-specific hardware disable .. */
> -	ptrace_disable(child);
> +	arch_ptrace_detach(child);
> +	user_disable_single_step(child);
> +#ifdef TIF_SYSCALL_EMU
> +	clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
> +#endif
>  	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);

Well, personally I'd prefer to keep the arch-dependent ptrace_disable(), this
just looks safer to me. Although I agree that its name is bad and
arch_ptrace_detach() looks much better.

Oleg.

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