Currently, in mb_find_order_for_block(), there's a loop like the following:
while (order <= e4b->bd_blkbits + 1) {
...
bb += 1 << (e4b->bd_blkbits - order);
}
Note that the updated bb is used in the loop's next iteration only.
However, at the last iteration, that is at order == e4b->bd_blkbits + 1,
the shift count becomes negative (c.f. C99 6.5.7(3)) and UBSAN reports
UBSAN: Undefined behaviour in fs/ext4/mballoc.c:1281:11
shift exponent -1 is negative
[...]
Call Trace:
[<ffffffff818c4d35>] dump_stack+0xbc/0x117
[<ffffffff818c4c79>] ? _atomic_dec_and_lock+0x169/0x169
[<ffffffff819411bb>] ubsan_epilogue+0xd/0x4e
[<ffffffff81941cbc>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
[<ffffffff81941ac1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
[<ffffffff816e93a0>] ? ext4_mb_generate_from_pa+0x590/0x590
[<ffffffff816502c8>] ? ext4_read_block_bitmap_nowait+0x598/0xe80
[<ffffffff816e7b7e>] mb_find_order_for_block+0x1ce/0x240
[...]
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 bb is never used again.
Silence UBSAN by introducing another variable, bb_incr, holding the next
increment to apply to bb and adjust that one by right shifting it by one
position per loop iteration.
Signed-off-by: Nicolai Stange <nicstange@xxxxxxxxx>
---
fs/ext4/mballoc.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 50e05df..4bc89fe 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1266,6 +1266,7 @@ static void ext4_mb_unload_buddy(struct ext4_buddy *e4b)
static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
{
int order = 1;
+ int bb_incr = 1 << (e4b->bd_blkbits - 1);
void *bb;
BUG_ON(e4b->bd_bitmap == e4b->bd_buddy);
@@ -1278,7 +1279,8 @@ static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
/* this block is part of buddy of order 'order' */
return order;
}
- bb += 1 << (e4b->bd_blkbits - order);
+ bb += bb_incr;
+ bb_incr >>= 1;
order++;
}
return 0;
--
2.7.3
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