[BUG]
There are several different KASAN reports for balance + snapshot
workloads.
Involved call paths include:
should_ignore_root+0x54/0xb0 [btrfs]
build_backref_tree+0x11af/0x2280 [btrfs]
relocate_tree_blocks+0x391/0xb80 [btrfs]
relocate_block_group+0x3e5/0xa00 [btrfs]
btrfs_relocate_block_group+0x240/0x4d0 [btrfs]
btrfs_relocate_chunk+0x53/0xf0 [btrfs]
btrfs_balance+0xc91/0x1840 [btrfs]
btrfs_ioctl_balance+0x416/0x4e0 [btrfs]
btrfs_ioctl+0x8af/0x3e60 [btrfs]
do_vfs_ioctl+0x831/0xb10
ksys_ioctl+0x67/0x90
__x64_sys_ioctl+0x43/0x50
do_syscall_64+0x79/0xe0
entry_SYSCALL_64_after_hwframe+0x49/0xbe
create_reloc_root+0x9f/0x460 [btrfs]
btrfs_reloc_post_snapshot+0xff/0x6c0 [btrfs]
create_pending_snapshot+0xa9b/0x15f0 [btrfs]
create_pending_snapshots+0x111/0x140 [btrfs]
btrfs_commit_transaction+0x7a6/0x1360 [btrfs]
btrfs_mksubvol+0x915/0x960 [btrfs]
btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs]
btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs]
btrfs_ioctl+0x241b/0x3e60 [btrfs]
do_vfs_ioctl+0x831/0xb10
ksys_ioctl+0x67/0x90
__x64_sys_ioctl+0x43/0x50
do_syscall_64+0x79/0xe0
entry_SYSCALL_64_after_hwframe+0x49/0xbe
btrfs_reloc_pre_snapshot+0x85/0xc0 [btrfs]
create_pending_snapshot+0x209/0x15f0 [btrfs]
create_pending_snapshots+0x111/0x140 [btrfs]
btrfs_commit_transaction+0x7a6/0x1360 [btrfs]
btrfs_mksubvol+0x915/0x960 [btrfs]
btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs]
btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs]
btrfs_ioctl+0x241b/0x3e60 [btrfs]
do_vfs_ioctl+0x831/0xb10
ksys_ioctl+0x67/0x90
__x64_sys_ioctl+0x43/0x50
do_syscall_64+0x79/0xe0
entry_SYSCALL_64_after_hwframe+0x49/0xbe
[CAUSE]
All these call sites are only relying on root->reloc_root, which can
undergo btrfs_drop_snapshot(), and since we don't have real refcount
based protection to reloc roots, we can reach already dropped reloc
root, triggering KASAN.
[FIX]
To avoid such access to unstable root->reloc_root, we should check
BTRFS_ROOT_DEAD_RELOC_TREE bit first.
This patch introduces a new wrapper, have_reloc_root(), to do the proper
check for most callers who don't distinguish merged reloc tree and no
reloc tree.
The only exception is should_ignore_root(), as merged reloc tree can be
ignored, while no reloc tree shouldn't.
[CRITICAL SECTION ANALYSE]
Although test_bit()/set_bit()/clear_bit() doesn't imply a barrier, the
DEAD_RELOC_TREE bit has extra help from transaction as a higher level
barrier, the lifespan of root::reloc_root and DEAD_RELOC_TREE bit are:
NULL: reloc_root is NULL PTR: reloc_root is not NULL
0: DEAD_RELOC_ROOT bit not set DEAD: DEAD_RELOC_ROOT bit set
(NULL, 0) Initial state __
| /\ Section A
btrfs_init_reloc_root() \/
| __
(PTR, 0) reloc_root initialized /\
| |
btrfs_update_reloc_root() | Section B
| |
(PTR, DEAD) reloc_root has been merged \/
| __
=== btrfs_commit_transaction() ====================
| /\
clean_dirty_subvols() |
| | Section C
(NULL, DEAD) reloc_root cleanup starts \/
| __
btrfs_drop_snapshot() /\
| | Section D
(NULL, 0) Back to initial state \/
Very have_reloc_root() or test_bit(DEAD_RELOC_ROOT) caller has hold a
transaction handler, so none of such caller can cross transaction
boundary.
In Section A, every caller just found no DEAD bit, and grab reloc_root.
In the cross section A-B, caller may get no DEAD bit, but since
reloc_root is still completely valid thus accessing reloc_root is
completely safe.
No test_bit() caller can cross the boundary of Section B and Section C.
In Section C, every caller found the DEAD bit, so no one will access
reloc_root.
In the cross section C-D, either caller gets the DEAD bit set, avoiding
access reloc_root no matter if it's safe or not.
Or caller get the DEAD bit cleared, then access reloc_root, which is
already NULL, nothing will be wrong.
Here we need extra memory barrier in cross section C-D, to ensure
proper memory order between reloc_root and clear_bit().
In Section D, since no DEAD bit and no reloc_root, it's back to initial
state.
With this lifespan, it should be clear only one memory barrier is
needed, between setting reloc_root to NULL and clearing DEAD_RELOC_ROOT
bit.
Reported-by: Zygo Blaxell <ce3g8jdj@xxxxxxxxxxxxxxxxxxxxx>
Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion
after merge_reloc_roots")
Suggested-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
---
Changelog:
v2:
- Add the [CRITICAL SECTION ANALYSE] part
This gets me into the rabbit hole of memory ordering, but thanks for
the help from David (initially mentioning the mb hell) and Nikolay
(for the proper doc), finally I could explain clearly why only
one mb is needed.
- Add comment for the only needed memory barrier.
---
fs/btrfs/relocation.c | 32 ++++++++++++++++++++++++++++----
1 file changed, 28 insertions(+), 4 deletions(-)
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index d897a8e5e430..17a2484f76a5 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -517,6 +517,22 @@ static int update_backref_cache(struct
btrfs_trans_handle *trans,
return 1;
}
+/*
+ * Check if this subvolume tree has valid reloc(*) tree.
+ *
+ * *: Reloc tree after swap is considered dead, thus not considered
as valid.
+ * This is enough for most callers, as they don't distinguish dead
reloc
+ * root from no reloc root.
+ * But should_ignore_root() below is a special case.
+ */
+static bool have_reloc_root(struct btrfs_root *root)
+{
+ if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
+ return false;
