Re: [PATCH] block: copy bi_vcnt in __bio_clone_fast

From: Jeff Moyer
Date: Thu Oct 09 2014 - 09:54:18 EST

Jeff Mahoney <jeffm@xxxxxxxx> writes:

> Commit 05f1dd53152173 (block: add queue flag for disabling SG merging) uses
> bi_vcnt to assign bio->bi_phys_segments if sg merging is disabled. When
> using device mapper on top of a blk-mq device (virtio_blk in my test),
> we'd end up overflowing the scatterlist in __blk_bios_map_sg.
> __bio_clone_fast copies bi_iter and bi_io_vec but not bi_vcnt, so
> blk_recount_segments would report bi_phys_segments as 0. Since
> rq->nr_phys_segments is 0 as well, the checks to ensure that we don't
> exceed the queue's segment limit end up allowing more bios (and segments) to
> attach the a request until we finally map it. That also means we
> pass the BUG_ON at the beginning of virtio_queue_rq, ultimately causing
> memory corruption and a crash.
> If we copy bi_vcnt in __bio_clone_fast, the bios and requests properly
> report the number of segments and everything works as expected.
> Originally reported at

Hi, Jeff,

Did you manage to reproduce this problem with commit 0738854 (blk-merge:
fix blk_recount_segments) applied? Or perhaps with commit 200612e (dm
table: propagate QUEUE_FLAG_NO_SG_MERGE)?

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