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    Subject[PATCH v2 1/2] fs/ext4: mb_find_order_for_block(): silence UBSAN
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    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@gmail.com>
    ---
    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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