Comment # 20 on bug 888259 from Jeff Mahoney

Created attachment 609400 [details]
[PATCH] block: copy bi_vcnt in __bio_clone_fast

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 end up allowing more bios (and segments) to
attach the a request until the we finally map it.  That also means we'd
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.