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 
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

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 <ngomp...@gmail.com>
Signed-off-by: Josef Bacik <jo...@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)) {

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