linux-kernel - Re: [PATCH 2/2] ptrace: add ability to attach a file descriptor to another task

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Date:	Sun, 18 Dec 2011 19:17:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Vagin <avagin@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Roland McGrath <roland@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] ptrace: add ability to attach a file descriptor to
	another task

On 12/16, Andrew Vagin wrote:
>
> Signed-off-by: Andrew Vagin <avagin@...nvz.org>

Nice changelog ;)

I agree with Tejun, this doesn't look like the good idea to me...



As for the patch itself,

> @@ -826,6 +827,32 @@ int ptrace_request(struct task_struct *child, long request,
>  		break;
>  	}
>  #endif
> +	case PTRACE_DUPFD:
> +	{
> +		struct file *file = fget_raw(data);
> +		ret = -EBADF;
> +
> +		if (!file)
> +			break;
> +
> +		task_lock(child);
> +		if (!child->files)
> +			goto out_getfd;
> +
> +		ret = alloc_task_fd(child, 0, 0);

alloc_fdtable() does kmalloc(GFP_KERNEL)/vmalloc(), not good under
spin_lock().

> +		if (ret < 0) {
> +			fput(file);

We can't do this under task_lock() too.

A tracer's sub-thread (or any CLONE_FILES task) can close this file,
it is possible that we have the last reference.


Probably, instead of the new *_task_* helpers, you can add
alloc_file_fd/fd_files_install which take the result of
get_files_struct() as the additional argument. This way you can
avoid task_lock().

Oleg.

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