[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-Id: <a573a1da9861fa2ffb4d2a4d89e5bcb8f7e4bde4.1465975780.git.jslaby@suse.cz>
Date: Wed, 15 Jun 2016 09:31:11 +0200
From: Jiri Slaby <jslaby@...e.cz>
To: stable@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Nicolai Stange <nicstange@...il.com>,
Theodore Ts'o <tytso@....edu>, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 47/56] ext4: silence UBSAN in ext4_mb_init()
From: Nicolai Stange <nicstange@...il.com>
3.12-stable review patch. If anyone has any objections, please let me know.
===============
commit 935244cd54b86ca46e69bc6604d2adfb1aec2d42 upstream.
Currently, in ext4_mb_init(), there's a loop like the following:
do {
...
offset += 1 << (sb->s_blocksize_bits - i);
i++;
} while (i <= sb->s_blocksize_bits + 1);
Note that the updated offset is used in the loop's next iteration only.
However, at the last iteration, that is at i == sb->s_blocksize_bits + 1,
the shift count becomes equal to (unsigned)-1 > 31 (c.f. C99 6.5.7(3))
and UBSAN reports
UBSAN: Undefined behaviour in fs/ext4/mballoc.c:2621:15
shift exponent 4294967295 is too large for 32-bit type 'int'
[...]
Call Trace:
[<ffffffff818c4d25>] dump_stack+0xbc/0x117
[<ffffffff818c4c69>] ? _atomic_dec_and_lock+0x169/0x169
[<ffffffff819411ab>] ubsan_epilogue+0xd/0x4e
[<ffffffff81941cac>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
[<ffffffff81941ab1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
[<ffffffff814b6dc1>] ? kmem_cache_alloc+0x101/0x390
[<ffffffff816fc13b>] ? ext4_mb_init+0x13b/0xfd0
[<ffffffff814293c7>] ? create_cache+0x57/0x1f0
[<ffffffff8142948a>] ? create_cache+0x11a/0x1f0
[<ffffffff821c2168>] ? mutex_lock+0x38/0x60
[<ffffffff821c23ab>] ? mutex_unlock+0x1b/0x50
[<ffffffff814c26ab>] ? put_online_mems+0x5b/0xc0
[<ffffffff81429677>] ? kmem_cache_create+0x117/0x2c0
[<ffffffff816fcc49>] ext4_mb_init+0xc49/0xfd0
[...]
Observe that the mentioned shift exponent, 4294967295, equals (unsigned)-1.
Unless compilers start to do some fancy transformations (which at least
GCC 6.0.0 doesn't currently do), the issue is of cosmetic nature only: the
such calculated value of offset is never used again.
Silence UBSAN by introducing another variable, offset_incr, holding the
next increment to apply to offset and adjust that one by right shifting it
by one position per loop iteration.
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=114701
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=112161
Signed-off-by: Nicolai Stange <nicstange@...il.com>
Signed-off-by: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
fs/ext4/mballoc.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4d42e50ab0a0..4a79ce1ecaa1 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2537,7 +2537,7 @@ int ext4_mb_init(struct super_block *sb)
{
struct ext4_sb_info *sbi = EXT4_SB(sb);
unsigned i, j;
- unsigned offset;
+ unsigned offset, offset_incr;
unsigned max;
int ret;
@@ -2566,11 +2566,13 @@ int ext4_mb_init(struct super_block *sb)
i = 1;
offset = 0;
+ offset_incr = 1 << (sb->s_blocksize_bits - 1);
max = sb->s_blocksize << 2;
do {
sbi->s_mb_offsets[i] = offset;
sbi->s_mb_maxs[i] = max;
- offset += 1 << (sb->s_blocksize_bits - i);
+ offset += offset_incr;
+ offset_incr = offset_incr >> 1;
max = max >> 1;
i++;
} while (i <= sb->s_blocksize_bits + 1);
--
2.9.0
Powered by blists - more mailing lists