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authorNicolai Stange2016-05-05 19:46:19 -0400
committerTheodore Ts'o2016-05-05 19:46:19 -0400
commit935244cd54b86ca46e69bc6604d2adfb1aec2d42 (patch)
tree870253dbfdb7e77a71a85ee1357817a7699855b2 /fs/ext4
parentb5cb316cdf3a3f5f6125412b0f6065185240cfdc (diff)
ext4: silence UBSAN in ext4_mb_init()
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 Cc: stable@vger.kernel.org Signed-off-by: Nicolai Stange <nicstange@gmail.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'fs/ext4')
-rw-r--r--fs/ext4/mballoc.c6
1 files changed, 4 insertions, 2 deletions
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 49e444b51a0c..c1ab3ec30423 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2585,7 +2585,7 @@ 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;
@@ -2614,11 +2614,13 @@ 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);