linux-kernel - Re: [RFC v10][PATCH 08/13] Dump open file descriptors

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Date:	Mon, 01 Dec 2008 09:47:25 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Oren Laadan <orenl@...columbia.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...l.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Serge Hallyn <serue@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC v10][PATCH 08/13] Dump open file descriptors

On Fri, 2008-11-28 at 10:19 +0000, Al Viro wrote:
> On Wed, Nov 26, 2008 at 08:04:39PM -0500, Oren Laadan wrote:
> > +int cr_scan_fds(struct files_struct *files, int **fdtable)
> > +{
> > +	struct fdtable *fdt;
> > +	int *fds;
> > +	int i, n = 0;
> > +	int tot = CR_DEFAULT_FDTABLE;
> > +
> > +	fds = kmalloc(tot * sizeof(*fds), GFP_KERNEL);
> > +	if (!fds)
> > +		return -ENOMEM;
> > +
> > +	/*
> > +	 * We assume that the target task is frozen (or that we checkpoint
> > +	 * ourselves), so we can safely proceed after krealloc() from where
> > +	 * we left off; in the worst cases restart will fail.
> > +	 */
> 
> Task may be frozen, but it may share the table with any number of other
> tasks...

First of all, thanks for looking at this, Al.  

I think Oren's assumption here is that all tasks possibly sharing the
table would be frozen.  I don't think that's a good assumption,
either. :)

This would be a lot safer and bulletproof if we size the allocation
ahead of time, take all the locks, then retry if the size has changed.

I think that will just plain work of we do this:

> > +	spin_lock(&files->file_lock);
> > +	rcu_read_lock();
> > +	fdt = files_fdtable(files);
> > +	for (i = 0; i < fdt->max_fds; i++) {
> > +		if (!fcheck_files(files, i))
> > +			continue;
> > +		if (n == tot) {
> > +			/*
> > +			 * fcheck_files() is safe with drop/re-acquire
> > +			 * of the lock, because it tests:  fd < max_fds
> > +			 */
> > +			spin_unlock(&files->file_lock);
> > +			rcu_read_unlock();
> > +			tot *= 2;	/* won't overflow: kmalloc will fail */

			  free(fds);
			  goto first_kmalloc_in_this_function;

> > +		}
> > +		fds[n++] = i;
> > +	}
> > +	rcu_read_unlock();
> > +	spin_unlock(&files->file_lock);
> > +
> > +	*fdtable = fds;
> > +	return n;
> > +}

Right?

> > +	switch (inode->i_mode & S_IFMT) {
> > +	case S_IFREG:
> > +		fd_type = CR_FD_FILE;
> > +		break;
> > +	case S_IFDIR:
> > +		fd_type = CR_FD_DIR;
> > +		break;
> > +	case S_IFLNK:
> > +		fd_type = CR_FD_LINK;
> 
> Opened symlinks?  May I have whatever you'd been smoking, please?

Ugh, that certainly doesn't have any place here.  I do wonder if Oren
had some use for that in the fully put together code, but it can
certainly go for now.

I'll send patches for these shortly.

-- Dave

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