Commit adbecb12 authored May 08, 2008 by Al Viro

[PATCH] dup_fd() part 4 - race fix



Parent _can_ be a clone task, contrary to the comment.  Moreover,
more files could be opened while we allocate a copy, in which case
we end up copying only part into new descriptor table.  Since what
we get _is_ affected by all changes in the old range, we can get
rather weird effects - e.g.
	dup2(0, 1024); close(0);
in parallel with fork() resulting in child that sees the effect of
close(), but not that of dup2() done just before that close().

What we need is to recalculate the open_count after having reacquired
->file_lock and if external fdtable we'd just allocated is too small for
it, free the sucker and redo allocation.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

parent afbec7ff

fs/file.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -308,11 +308,16 @@ struct files_struct dup_fd(struct files_struct oldf, int *errorp)

		/*
		* Check whether we need to allocate a larger fd array and fd set.
		* Note: we're not a clone task, so the open count won't change.
		*/
		if (open_files > new_fdt->max_fds) {
		while (unlikely(open_files > new_fdt->max_fds)) {
		spin_unlock(&oldf->file_lock);

		if (new_fdt != &newf->fdtab) {
		free_fdarr(new_fdt);
		free_fdset(new_fdt);
		kfree(new_fdt);
		}

		new_fdt = alloc_fdtable(open_files - 1);
		if (!new_fdt) {
		*errorp = -ENOMEM;
		@@ -335,6 +340,7 @@ struct files_struct dup_fd(struct files_struct oldf, int *errorp)
		*/
		spin_lock(&oldf->file_lock);
		old_fdt = files_fdtable(oldf);
		open_files = count_open_files(old_fdt);
		}

		old_fds = old_fdt->fd;

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