linux-kernel - Re: [PATCH] ptrace: kill BKL in ptrace syscall

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Message-ID: <20100329114046.GB16971@redhat.com>
Date:	Mon, 29 Mar 2010 13:40:46 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Roland McGrath <roland@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ptrace: kill BKL in ptrace syscall

On 03/28, Frederic Weisbecker wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> The comment suggests that this usage is stale. There is no bkl in the
> exec path so if there is a race lurking there, the bkl in ptrace is
> not going to help in this regard.

I never understood this comment too, and I do not see any potentional
races bkl could prevent.

> Overview of the possibility of "accidental" races this bkl might
> protect:
>
> - ptrace_traceme() is protected against task removal and concurrent
> read/write on current->ptrace as it locks write tasklist_lock.
>
> - arch_ptrace_attach() is serialized by ptrace_traceme() against
> concurrent PTRACE_TRACEME or PTRACE_ATTACH
>
> - ptrace_attach() is protected the same way ptrace_traceme() and
> in turn serializes arch_ptrace_attach()
>
> - ptrace_check_attach() does its own well described serializing too.
>
> There is no obvious race here.

Yes, nothing inside sys_ptrace() pathes relies on bkl, and all recent
changes were done assuming that lock_kernel() doesn't exist.

I think the patch is correct.

Acked-by: Oleg Nesterov <oleg@...hat.com>

> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  kernel/ptrace.c |   10 ----------
>  1 files changed, 0 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 42ad8ae..5357502 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -666,10 +666,6 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, long, addr, long, data)
>  	struct task_struct *child;
>  	long ret;
>
> -	/*
> -	 * This lock_kernel fixes a subtle race with suid exec
> -	 */
> -	lock_kernel();
>  	if (request == PTRACE_TRACEME) {
>  		ret = ptrace_traceme();
>  		if (!ret)
> @@ -703,7 +699,6 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, long, addr, long, data)
>   out_put_task_struct:
>  	put_task_struct(child);
>   out:
> -	unlock_kernel();
>  	return ret;
>  }
>
> @@ -813,10 +808,6 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
>  	struct task_struct *child;
>  	long ret;
>
> -	/*
> -	 * This lock_kernel fixes a subtle race with suid exec
> -	 */
> -	lock_kernel();
>  	if (request == PTRACE_TRACEME) {
>  		ret = ptrace_traceme();
>  		goto out;
> @@ -846,7 +837,6 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
>   out_put_task_struct:
>  	put_task_struct(child);
>   out:
> -	unlock_kernel();
>  	return ret;
>  }
>  #endif	/* CONFIG_COMPAT */
> --
> 1.6.2.3
>

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