Re: fuzzing bcachefs with dm-flakey

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, May 29, 2023 at 04:59:40PM -0400, Mikulas Patocka wrote:
> Hi
> 
> I improved the dm-flakey device mapper target, so that it can do random 
> corruption of read and write bios - I uploaded it here: 
> https://people.redhat.com/~mpatocka/testcases/bcachefs/dm-flakey.c
> 
> I set up dm-flakey, so that it corrupts 10% of read bios and 10% of write 
> bios with this command:
> dmsetup create flakey --table "0 `blockdev --getsize /dev/ram0` flakey /dev/ram0 0 0 1 4 random_write_corrupt 100000000 random_read_corrupt 100000000"
> 
> 
> I created a bcachefs volume on a single disk (metadata and data checksums 
> were turned off) and mounted it on dm-flakey. I got:
> 
> crash: https://people.redhat.com/~mpatocka/testcases/bcachefs/crash1.txt
> deadlock: https://people.redhat.com/~mpatocka/testcases/bcachefs/crash2.txt
> infinite loop: https://people.redhat.com/~mpatocka/testcases/bcachefs/crash3.txt
> 
> Here I uploaded an image that causes infnite loop when we run bcachefs 
> fsck on it or when we attempt mount it:
> https://people.redhat.com/~mpatocka/testcases/bcachefs/inf-loop.gz
> 
> 
> I tried to run bcachefs on two block devices and fuzzing just one of them 
> (checksums and replication were turned on - so bcachefs shold correct the 
> corrupted data) - in this scenario, bcachefs doesn't return invalid data, 
> but it sometimes returns errors and sometimes crashes.
> 
> This script will trigger an oops on unmount:
> 	https://people.redhat.com/~mpatocka/testcases/bcachefs/crash4.txt
> or nonsensical errors returned to userspace:
> 	rm: cannot remove '/mnt/test/test/cmd_migrate.c': Unknown error 2206
> or I/O errors returned to userspace:
> 	diff: /mnt/test/test/rust-src/target/release/.fingerprint/bch_bindgen-f0bad16858ff0019/lib-bch_bindgen.json: Input/output error
> 
> #!/bin/sh -ex
> umount /mnt/test || true
> dmsetup remove_all || true
> rmmod brd || true
> SRC=/usr/src/git/bcachefs-tools
> while true; do
>         modprobe brd rd_size=1048576
>         bcachefs format --replicas=2 /dev/ram0 /dev/ram1
>         dmsetup create flakey --table "0 `blockdev --getsize /dev/ram0` linear /dev/ram0 0"
>         mount -t bcachefs /dev/mapper/flakey:/dev/ram1 /mnt/test
>         dmsetup load flakey --table "0 `blockdev --getsize /dev/ram0` flakey /dev/ram0 0 0 1 4 random_write_corrupt 100000000 random_read_corrupt 100000000"

Hey, that's really neat!

Any chance you'd be willing to get the dm-flakey changes merged into
upstream so that someone can write a recoveryloop fstest to test all the
filesystems systematically?

:D

--D

>         dmsetup suspend flakey
>         dmsetup resume flakey
>         cp -a "$SRC" /mnt/test/test
>         diff -r "$SRC" /mnt/test/test
>         echo 3 >/proc/sys/vm/drop_caches
>         diff -r "$SRC" /mnt/test/test
>         echo 3 >/proc/sys/vm/drop_caches
>         diff -r "$SRC" /mnt/test/test
>         echo 3 >/proc/sys/vm/drop_caches
>         rm -rf /mnt/test/test
>         echo 3 >/proc/sys/vm/drop_caches
>         cp -a "$SRC" /mnt/test/test
>         echo 3 >/proc/sys/vm/drop_caches
>         diff -r "$SRC" /mnt/test/test
>         umount /mnt/test
>         dmsetup remove flakey
>         rmmod brd
> done
> 
> The oops happens in set_btree_iter_dontneed and it is caused by the fact 
> that iter->path is NULL. The code in try_alloc_bucket is buggy because it 
> sets "struct btree_iter iter = { NULL };" and then jumps to the "err" 
> label that tries to dereference values in "iter".
> 
> 
> Bcachefs gives not much usefull error messages, like "Fatal error: Unknown 
> error 2184" or "Error in recovery: cannot allocate memory" or "mount(2) 
> system call failed: Unknown error 2186." or "rm: cannot remove 
> '/mnt/test/xfstests-dev/tools/fs-walk': Unknown error 2206".
> 
> Mikulas
>