re: dm: Forbid requeue of writes to zones

[Colin Ian King <colin.king@canonical.com>]

Hi,

Static analysis with Coverity on Linux next has found and issue in
drivers/md/dm.c with the following commit:

commit 2c243153d1d4be4e23735cd10984ac17c7a54531
Author: Damien Le Moal <damien.lemoal@wdc.com>
Date:   Wed May 26 06:24:58 2021 +0900

    dm: Forbid requeue of writes to zones

The analysis is as follows:

 828 static void dec_pending(struct dm_io *io, blk_status_t error)
 829 {
 830        unsigned long flags;
 831        blk_status_t io_error;

    1. var_decl: Declaring variable bio without initializer.

 832        struct bio *bio;
 833        struct mapped_device *md = io->md;
 834
 835        /* Push-back supersedes any I/O errors */

    2. Condition !!error, taking true branch.

 836        if (unlikely(error)) {
 837                spin_lock_irqsave(&io->endio_lock, flags);

    3. Condition io->status == 11 /* (blk_status_t)11 */, taking false
branch.

 838                if (!(io->status == BLK_STS_DM_REQUEUE &&
__noflush_suspending(md)))
 839                        io->status = error;
 840                spin_unlock_irqrestore(&io->endio_lock, flags);
 841        }
 842

    4. Condition atomic_dec_and_test(&io->io_count), taking true branch.

 843        if (atomic_dec_and_test(&io->io_count)) {

    5. Condition io->status == 11 /* (blk_status_t)11 */, taking true
branch.

 844                if (io->status == BLK_STS_DM_REQUEUE) {
 845                        /*
 846                         * Target requested pushing back the I/O.
 847                         */
 848                        spin_lock_irqsave(&md->deferred_lock, flags);

    6. Condition __noflush_suspending(md), taking true branch.

 849                        if (__noflush_suspending(md) &&

Uninitialized pointer read
    7. uninit_use_in_call: Using uninitialized value bio when calling
dm_is_zone_write.

 850                            !WARN_ON_ONCE(dm_is_zone_write(md, bio)))
 851                                /* NOTE early return due to
BLK_STS_DM_REQUEUE below */
 852                                bio_list_add_head(&md->deferred,
io->orig_bio);

The pointer bio is not initialized and yet is being used in the call to
function dm_is_zone_write where pointer bio is being accessed. I'm not
sure what the original intent was, but this looks incorrect.

Colin