linux-kernel - __getblk infinite loop

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Date:	Thu, 4 Sep 2008 23:24:11 -0400
From:	Bob Copeland <me@...copeland.com>
To:	linux-kernel@...r.kernel.org
Cc:	snakebyte@....de
Subject: __getblk infinite loop

Hi all,

Eric Sesterhenn and I were puzzling over a lockup found by his fsfuzzer.

sb_bread() calls __getblk, which says:

/*
 * __getblk will locate (and, if necessary, create) the buffer_head
 * which corresponds to the passed block_device, block and size. The
 * returned buffer has its reference count incremented.
 *
 * __getblk() cannot fail - it just keeps trying.  If you pass it an
 * illegal block number, __getblk() will happily return a buffer_head
 * which represents the non-existent block.  Very weird.
 *
 * __getblk() will lock up the machine if grow_dev_page's try_to_free_buffers()
 * attempt is failing.  FIXME, perhaps?
 */

In fact the following will cause an infinite loop when mounting omfs 
loopback (on 32 bit x86 at least):

diff --git a/fs/omfs/inode.c b/fs/omfs/inode.c
index a95fe59..80eacc8 100644
--- a/fs/omfs/inode.c
+++ b/fs/omfs/inode.c
@@ -413,6 +413,15 @@ static int omfs_fill_super(struct super_block *sb, void *data, int silent)
 	sector_t start;
 	int ret = -EINVAL;

+	if (1) {
+		sector_t foo = 0x1d4000004ULL;
+
+		sb_set_blocksize(sb, 2048);
+		bh = sb_bread(sb, foo);
+		brelse(bh);
+		goto end;
+	}
+
 	save_mount_options(sb, (char *) data);

 	sbi = kzalloc(sizeof(struct omfs_sb_info), GFP_KERNEL);

What's supposed to happen here?  I would have thought that sb_bread
would realize foo was outside the block dev and bail out, but instead
it just gets stuck.  Do I need to bounds-check anything passed to
sb_bread?

-- 
Bob Copeland %% www.bobcopeland.com

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