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:

quoted

+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:

quoted

+	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;

quoted

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

Right?

quoted

+	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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