Re: [PATCH 1/3] btrfs: init devices always

[Anand Jain <anand.jain@oracle.com>]

On 12/3/21 12:23 am, Josef Bacik wrote:
> Neal reported a panic trying to use -o rescue=all
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000030
> PGD 0 P4D 0
> Oops: 0000 [#1] SMP NOPTI
> CPU: 0 PID: 696 Comm: mount Tainted: G        W         5.12.0-rc2+ #296
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
> RIP: 0010:btrfs_device_init_dev_stats+0x1d/0x200
> RSP: 0018:ffffafaec1483bb8 EFLAGS: 00010286
> RAX: 0000000000000000 RBX: ffff9a5715bcb298 RCX: 0000000000000070
> RDX: ffff9a5703248000 RSI: ffff9a57052ea150 RDI: ffff9a5715bca400
> RBP: ffff9a57052ea150 R08: 0000000000000070 R09: ffff9a57052ea150
> R10: 000130faf0741c10 R11: 0000000000000000 R12: ffff9a5703700000
> R13: 0000000000000000 R14: ffff9a5715bcb278 R15: ffff9a57052ea150
> FS:  00007f600d122c40(0000) GS:ffff9a577bc00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000030 CR3: 0000000112a46005 CR4: 0000000000370ef0
> Call Trace:
>   ? btrfs_init_dev_stats+0x1f/0xf0
>   ? kmem_cache_alloc+0xef/0x1f0
>   btrfs_init_dev_stats+0x5f/0xf0
>   open_ctree+0x10cb/0x1720
>   btrfs_mount_root.cold+0x12/0xea
>   legacy_get_tree+0x27/0x40
>   vfs_get_tree+0x25/0xb0
>   vfs_kern_mount.part.0+0x71/0xb0
>   btrfs_mount+0x10d/0x380
>   legacy_get_tree+0x27/0x40
>   vfs_get_tree+0x25/0xb0
>   path_mount+0x433/0xa00
>   __x64_sys_mount+0xe3/0x120
>   do_syscall_64+0x33/0x40
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> This happens because when we call btrfs_init_dev_stats we do
> device->fs_info->dev_root.  However device->fs_info isn't init'ed
> because we were only calling btrfs_init_devices_late() if we properly
> read the device root.  


> However we don't actually need the device root to
> init the devices, this function simply assigns the devices their
> ->fs_info pointer properly, so this needs to be done unconditionally
> always so that we can properly deref device->fs_info in rescue cases.
  btrfs_device_init_dev_stats() calls btrfs_search_slot() leading
  to btrfs_search_slot_get_root(), and does de-reference root (dev_root)
  to get fs_info.

-------------
  static int btrfs_device_init_dev_stats(struct btrfs_device *device,
                                        struct btrfs_path *path)
::
         ret = btrfs_search_slot(NULL, device->fs_info->dev_root, &key, 
path, 0, 0);


int btrfs_search_slot(struct btrfs_trans_handle *trans, struct 
btrfs_root *root, ...)
::
         b = btrfs_search_slot_get_root(root, p, write_lock_level);


static struct extent_buffer *btrfs_search_slot_get_root(struct 
btrfs_root *root, ...)
{
         struct btrfs_fs_info *fs_info = root->fs_info;
--------------

  Can we allocate a dummy dev_root and set its dev_root::fs_info?

Thanks, Anand


> Reported-by: Neal Gompa <ngompa13@gmail.com>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>   fs/btrfs/disk-io.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 41b718cfea40..63656bf23ff2 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2387,8 +2387,8 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
>   	} else {
>   		set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
>   		fs_info->dev_root = root;
> -		btrfs_init_devices_late(fs_info);
>   	}
> +	btrfs_init_devices_late(fs_info);
>   
>   	/* If IGNOREDATACSUMS is set don't bother reading the csum root. */
>   	if (!btrfs_test_opt(fs_info, IGNOREDATACSUMS)) {
>