1 ext4: address UBSAN warning in mb_find_order_for_block()
3 From: Nicolai Stange <nicstange@gmail.com>
5 Currently, in mb_find_order_for_block(), there's a loop like the following:
7 while (order <= e4b->bd_blkbits + 1) {
9 bb += 1 << (e4b->bd_blkbits - order);
12 Note that the updated bb is used in the loop's next iteration only.
14 However, at the last iteration, that is at order == e4b->bd_blkbits + 1,
15 the shift count becomes negative (c.f. C99 6.5.7(3)) and UBSAN reports
17 UBSAN: Undefined behaviour in fs/ext4/mballoc.c:1281:11
18 shift exponent -1 is negative
21 [<ffffffff818c4d35>] dump_stack+0xbc/0x117
22 [<ffffffff818c4c79>] ? _atomic_dec_and_lock+0x169/0x169
23 [<ffffffff819411bb>] ubsan_epilogue+0xd/0x4e
24 [<ffffffff81941cbc>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
25 [<ffffffff81941ac1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
26 [<ffffffff816e93a0>] ? ext4_mb_generate_from_pa+0x590/0x590
27 [<ffffffff816502c8>] ? ext4_read_block_bitmap_nowait+0x598/0xe80
28 [<ffffffff816e7b7e>] mb_find_order_for_block+0x1ce/0x240
31 Unless compilers start to do some fancy transformations (which at least
32 GCC 6.0.0 doesn't currently do), the issue is of cosmetic nature only: the
33 such calculated value of bb is never used again.
35 Silence UBSAN by introducing another variable, bb_incr, holding the next
36 increment to apply to bb and adjust that one by right shifting it by one
37 position per loop iteration.
39 Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=114701
40 Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=112161
42 Cc: stable@vger.kernel.org
43 Signed-off-by: Nicolai Stange <nicstange@gmail.com>
44 Signed-off-by: Theodore Ts'o <tytso@mit.edu>
46 fs/ext4/mballoc.c | 4 +++-
47 1 file changed, 3 insertions(+), 1 deletion(-)
49 diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
50 index 50e05df..4bc89fe 100644
51 --- a/fs/ext4/mballoc.c
52 +++ b/fs/ext4/mballoc.c
53 @@ -1266,6 +1266,7 @@ static void ext4_mb_unload_buddy(struct ext4_buddy *e4b)
54 static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
57 + int bb_incr = 1 << (e4b->bd_blkbits - 1);
60 BUG_ON(e4b->bd_bitmap == e4b->bd_buddy);
61 @@ -1278,7 +1279,8 @@ static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
62 /* this block is part of buddy of order 'order' */
65 - bb += 1 << (e4b->bd_blkbits - order);