fuzzing bcachefs with dm-flakey

[Mikulas Patocka <mpatocka@redhat.com>]

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"
        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