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Date: Tue, 7 Jul 2009 12:19:33 -0400
From: Neil Horman <nhorman@tuxdriver.com>
To: linux-kernel@vger.kernel.org
Cc: akpm@linux-foundation.org, earl_chew@agilent.com, alan@lxorguk.ukuu.org.uk,
       andi@firstfloor.org, oleg@redhat.com
Subject: Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space
	pipe readers to complete (v8)
Message-ID: <20090707161933.GF28326@hmsreliant.think-freely.org>
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Reposting for Oleg to Ack

On Fri, Jul 03, 2009 at 06:52:33AM -0400, Neil Horman wrote:
> core_pattern: Allow core_pattern pipes to wait for user space to complete
> 
> One of the things that user space processes like to do is look at metadata for a
> crashing process in their /proc/<pid> directory.  this is racy however, since
> do_coredump in the kernel doesn't wait for the user space process to complete
> before it reaps the crashing process.  This patch corrects that.  Allowing the
> kernel to wait for the user space process to complete before cleaning up the
> crashing process.  This is a bit tricky to do for a few reasons:
> 
> 1) The user space process isn't our child, so we can't sys_wait4 on it
> 2) We need to close the pipe before waiting for the user process to complete,
> since the user process may rely on an EOF condition
> 
> I've discussed several solutions with Oleg Nesterov off-list about this, and
> this is the one we've come up with. We add ourselves as a pipe reader (to
> prevent premature cleanup of the pipe_inode_info), and remove ourselves as a
> writer (to provide an EOF condition to the writer in user space), then we
> iterate until the user space process exits (which we detect by pipe->readers ==
> 1, hence the > 1 check in the loop).  When we exit the loop, we restore the
> proper reader/writer values, then we return and let filp_close in do_coredump
> clean up the pipe data properly.
> 
> Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
> Reported-by: Earl Chew <earl_chew@agilent.com>
> 
> 
>  exec.c |   26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 93ab6eb..6b3579e 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -55,6 +55,7 @@
>  #include <linux/kmod.h>
>  #include <linux/fsnotify.h>
>  #include <linux/fs_struct.h>
> +#include <linux/pipe_fs_i.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm/mmu_context.h>
> @@ -1711,6 +1712,29 @@ int get_dumpable(struct mm_struct *mm)
>  	return (ret >= 2) ? 2 : ret;
>  }
>  
> +static void wait_for_dump_helpers(struct file *file)
> +{
> +	struct pipe_inode_info *pipe;
> + 
> +	pipe = file->f_path.dentry->d_inode->i_pipe;
> + 
> +	pipe_lock(pipe);
> +	pipe->readers++;
> +	pipe->writers--;
> +
> +	while ((pipe->readers > 1) && (!signal_pending(current))) {
> +		wake_up_interruptible_sync(&pipe->wait);
> +		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> +		pipe_wait(pipe);
> +	}
> + 
> +	pipe->readers--;
> +	pipe->writers++;
> +	pipe_unlock(pipe);
> + 
> +}
> + 
> +
>  void do_coredump(long signr, int exit_code, struct pt_regs *regs)
>  {
>  	struct core_state core_state;
> @@ -1862,6 +1886,8 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
>  		current->signal->group_exit_code |= 0x80;
>  
>  close_fail:
> +	if (ispipe && core_pipe_limit)
> +		wait_for_dump_helpers(file);
>  	filp_close(file, NULL);
>  fail_dropcount:
>  	if (dump_count)
> --
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