From: Omar Sandoval osandov@fb.com

[ Upstream commit 7081929ab2572920e94d70be3d332e5c9f97095a ]

If the source file descriptor to the snapshot ioctl refers to a deleted subvolume, we get the following abort:

BTRFS: Transaction aborted (error -2) WARNING: CPU: 0 PID: 833 at fs/btrfs/transaction.c:1875 create_pending_snapshot+0x1040/0x1190 [btrfs] Modules linked in: pata_acpi btrfs ata_piix libata scsi_mod virtio_net blake2b_generic xor net_failover virtio_rng failover scsi_common rng_core raid6_pq libcrc32c CPU: 0 PID: 833 Comm: t_snapshot_dele Not tainted 6.7.0-rc6 #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-1.fc39 04/01/2014 RIP: 0010:create_pending_snapshot+0x1040/0x1190 [btrfs] RSP: 0018:ffffa09c01337af8 EFLAGS: 00010282 RAX: 0000000000000000 RBX: ffff9982053e7c78 RCX: 0000000000000027 RDX: ffff99827dc20848 RSI: 0000000000000001 RDI: ffff99827dc20840 RBP: ffffa09c01337c00 R08: 0000000000000000 R09: ffffa09c01337998 R10: 0000000000000003 R11: ffffffffb96da248 R12: fffffffffffffffe R13: ffff99820535bb28 R14: ffff99820b7bd000 R15: ffff99820381ea80 FS: 00007fe20aadabc0(0000) GS:ffff99827dc00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000559a120b502f CR3: 00000000055b6000 CR4: 00000000000006f0 Call Trace: <TASK> ? create_pending_snapshot+0x1040/0x1190 [btrfs] ? __warn+0x81/0x130 ? create_pending_snapshot+0x1040/0x1190 [btrfs] ? report_bug+0x171/0x1a0 ? handle_bug+0x3a/0x70 ? exc_invalid_op+0x17/0x70 ? asm_exc_invalid_op+0x1a/0x20 ? create_pending_snapshot+0x1040/0x1190 [btrfs] ? create_pending_snapshot+0x1040/0x1190 [btrfs] create_pending_snapshots+0x92/0xc0 [btrfs] btrfs_commit_transaction+0x66b/0xf40 [btrfs] btrfs_mksubvol+0x301/0x4d0 [btrfs] btrfs_mksnapshot+0x80/0xb0 [btrfs] __btrfs_ioctl_snap_create+0x1c2/0x1d0 [btrfs] btrfs_ioctl_snap_create_v2+0xc4/0x150 [btrfs] btrfs_ioctl+0x8a6/0x2650 [btrfs] ? kmem_cache_free+0x22/0x340 ? do_sys_openat2+0x97/0xe0 __x64_sys_ioctl+0x97/0xd0 do_syscall_64+0x46/0xf0 entry_SYSCALL_64_after_hwframe+0x6e/0x76 RIP: 0033:0x7fe20abe83af RSP: 002b:00007ffe6eff1360 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00007fe20abe83af RDX: 00007ffe6eff23c0 RSI: 0000000050009417 RDI: 0000000000000003 RBP: 0000000000000003 R08: 0000000000000000 R09: 00007fe20ad16cd0 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 00007ffe6eff13c0 R14: 00007fe20ad45000 R15: 0000559a120b6d58 </TASK> ---[ end trace 0000000000000000 ]--- BTRFS: error (device vdc: state A) in create_pending_snapshot:1875: errno=-2 No such entry BTRFS info (device vdc: state EA): forced readonly BTRFS warning (device vdc: state EA): Skipping commit of aborted transaction. BTRFS: error (device vdc: state EA) in cleanup_transaction:2055: errno=-2 No such entry

This happens because create_pending_snapshot() initializes the new root item as a copy of the source root item. This includes the refs field, which is 0 for a deleted subvolume. The call to btrfs_insert_root() therefore inserts a root with refs == 0. btrfs_get_new_fs_root() then finds the root and returns -ENOENT if refs == 0, which causes create_pending_snapshot() to abort.

Fix it by checking the source root's refs before attempting the snapshot, but after locking subvol_sem to avoid racing with deletion.

CC: stable@vger.kernel.org # 4.14+ Reviewed-by: Sweet Tea Dorminy sweettea-kernel@dorminy.me Reviewed-by: Anand Jain anand.jain@oracle.com Signed-off-by: Omar Sandoval osandov@fb.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com (cherry picked from commit 7081929ab2572920e94d70be3d332e5c9f97095a) [Larry: backport to 5.4.y. Minor conflict resolved due to missing commit 92a7cc425223 btrfs: rename BTRFS_ROOT_REF_COWS to BTRFS_ROOT_SHAREABLE] Signed-off-by: Larry Bassel larry.bassel@oracle.com --- fs/btrfs/ioctl.c | 3 +++ 1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index 674d774eb662..a29182be29fa 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -793,6 +793,9 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir, if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state)) return -EINVAL;

+ if (btrfs_root_refs(&root->root_item) == 0) + return -ENOENT; + if (atomic_read(&root->nr_swapfiles)) { btrfs_warn(fs_info, "cannot snapshot subvolume with active swapfile");