Re: [PATCH] bcache: Prevent granting write refs when filesystem is read-only

[Kent Overstreet]

On Sat, Apr 12, 2025 at 02:39:33PM -0400, Gabriel Shahrouzi wrote:
> Fix a shutdown WARNING in bch2_dev_free caused by active write I/O
> references (ca->io_ref[WRITE]) on a device being freed.
> 
> The problem occurs when:
> - The filesystem is marked read-only (BCH_FS_rw clear in c->flags).
> - A subsequent operation (e.g., error handling for device removal)
>   incorrectly tries to grant write references back to a device.
> - During final shutdown, the read-only flag causes the system to skip
>   stopping write I/O references (bch2_dev_io_ref_stop(ca, WRITE)).
> - The leftover active write reference triggers the WARN_ON in
>   bch2_dev_free.
> 
> Prevent this by checking if the filesystem is read-only before
> attempting to grant write references to a device in the problematic
> code path. Ensure consistency between the filesystem state flag
> and the device I/O reference state during shutdown.
> 
> ---
> Not sure what to put for the fixes tag so I omitted it. The bisection
> that Syzkaller found technically is correct but only because additional
> warn_on checks were added recently. The git blame shows code from 8
> years ago for the specific lines being modified.
> 
> Also not sure if devices should have read and write permissions
> (ca->mi.state = BCH_MEMBER_STATE_rw) when filesystem is in read-only
> mode. If that is what intended, then I believe this solution works.There
> could potentially be other places where a similar scenario occurs.

Yes, that is intended. BCH_MEMBER_STATE is persistent state, stored in
the superblock and controlled by the user or when we notice a device is
going bad.

> 
> Reported-by: syzbot+aec9606169fbc3a12ca6@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/67f50e3e.050a0220.396535.0560.GAE@xxxxxxxxxx/T/
> Signed-off-by: Gabriel Shahrouzi <gshahrouzi@xxxxxxxxx>

Nice find - applied.

> ---
>  fs/bcachefs/super.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/bcachefs/super.c b/fs/bcachefs/super.c
> index b79e80a435e09..788e870bfef6a 100644
> --- a/fs/bcachefs/super.c
> +++ b/fs/bcachefs/super.c
> @@ -1757,7 +1757,8 @@ int bch2_dev_remove(struct bch_fs *c, struct bch_dev *ca, int flags)
>  	up_write(&c->state_lock);
>  	return 0;
>  err:
> -	if (ca->mi.state == BCH_MEMBER_STATE_rw &&
> +	if (test_bit(BCH_FS_rw, &c->flags) &&
> +	    ca->mi.state == BCH_MEMBER_STATE_rw &&
>  	    !percpu_ref_is_zero(&ca->io_ref[READ]))
>  		__bch2_dev_read_write(c, ca);
>  	up_write(&c->state_lock);
> -- 
> 2.43.0
>