2013-04-23 13:00:55

by Theodore Y. Ts'o

[permalink] [raw]
Subject: Re: Unsigned widening casts of binary "not" operations..

On Mon, Apr 22, 2013 at 05:15:19PM -0700, Linus Torvalds wrote:
> Here's a ext4 code snippet that looks like an actual bug (but seems to only
> hit read-ahead):
>
> ext4_fsblk_t b, block;
>
> b = block & ~(EXT4_SB(sb)->s_inode_readahead_blks-1);
>
> where "b" actually ends up having the upper bits cleared, because the
> s_inode_readahead_blks thing is an unsigned int, so you're masking off not
> just the low bits, but the high bits too. Ted? Of course, it's just
> read-ahead, so it probably doesn't matter, but.

Yep, it's a bug alright. Thanks for catching it!

- Ted


>From 0d606e2c9fccdd4e67febf1e2da500e1bfe9e045 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <[email protected]>
Date: Tue, 23 Apr 2013 08:59:35 -0400
Subject: [PATCH] ext4: fix type-widening bug in inode table readahead code

Due to a missing cast, the high 32-bits of a 64-bit block number used
when calculating the readahead block for inode tables can get lost.
This means we can end up fetching the wrong blocks for readahead for
file systems > 16TB.

Linus found this when experimenting with an enhacement to the sparse
static code checker which checks for missing widening casts before
binary "not" operators.

Reported-by: Linus Torvalds <[email protected]>
Signed-off-by: "Theodore Ts'o" <[email protected]>
---
fs/ext4/inode.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d7518e2..793d44b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4011,13 +4011,14 @@ make_io:
if (EXT4_SB(sb)->s_inode_readahead_blks) {
ext4_fsblk_t b, end, table;
unsigned num;
+ __u32 ra_blks = EXT4_SB(sb)->s_inode_readahead_blks;

table = ext4_inode_table(sb, gdp);
/* s_inode_readahead_blks is always a power of 2 */
- b = block & ~(EXT4_SB(sb)->s_inode_readahead_blks-1);
+ b = block & ~((ext4_fsblk_t) ra_blks - 1);
if (table > b)
b = table;
- end = b + EXT4_SB(sb)->s_inode_readahead_blks;
+ end = b + ra_blks;
num = EXT4_INODES_PER_GROUP(sb);
if (ext4_has_group_desc_csum(sb))
num -= ext4_itable_unused_count(sb, gdp);
--
1.7.12.rc0.22.gcdd159b