btrfs: initialize device::fs_info always

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+ torvalds#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 initialized
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 dereference device->fs_info in rescue
cases.

Reported-by: Neal Gompa <[email protected]>
CC: [email protected] # 5.11+
Signed-off-by: Josef Bacik <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

fs/btrfs/disk-io.c

@@ -2387,8 +2387,9 @@ 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);
 	}
+	/* Initialize fs_info for all devices in any case */
+	btrfs_init_devices_late(fs_info);
 
 	/* If IGNOREDATACSUMS is set don't bother reading the csum root. */
 	if (!btrfs_test_opt(fs_info, IGNOREDATACSUMS)) {