[PATCH 6.6 066/143] btrfs: fix race between ordered extent completion and fiemap

Greg Kroah-Hartman
2024-03-04T21:38:50Z

From: Filipe Manana <fdmanana@suse.com>

[ Upstream commit a1a4a9ca77f143c00fce69c1239887ff8b813bec ]

For fiemap we recently stopped locking the target extent range for the
whole duration of the fiemap call, in order to avoid a deadlock in a
scenario where the fiemap buffer happens to be a memory mapped range of
the same file. This use case is very unlikely to be useful in practice but
it may be triggered by fuzz testing (syzbot, etc).

However by not locking the target extent range for the whole duration of
the fiemap call we can race with an ordered extent. This happens like
this:

1) The fiemap task finishes processing a file extent item that covers
the file range [512K, 1M[, and that file extent item is the last item
in the leaf currently being processed;

2) And ordered extent for the file range [768K, 2M[, in COW mode,
completes (btrfs_finish_one_ordered()) and the file extent item
covering the range [512K, 1M[ is trimmed to cover the range
[512K, 768K[ and then a new file extent item for the range [768K, 2M[
is inserted in the inode's subvolume tree;

3) The fiemap task calls fiemap_next_leaf_item(), which then calls
btrfs_next_leaf() to find the next leaf / item. This finds that the
the next key following the one we previously processed (its type is
BTRFS_EXTENT_DATA_KEY and its offset is 512K), is the key corresponding
to the new file extent item inserted by the ordered extent, which has
a type of BTRFS_EXTENT_DATA_KEY and an offset of 768K;

4) Later the fiemap code ends up at emit_fiemap_extent() and triggers
the warning:

if (cache->offset + cache->len > offset) {
WARN_ON(1);
return -EINVAL;
}

Since we get 1M > 768K, because the previously emitted entry for the
old extent covering the file range [512K, 1M[ ends at an offset that
is greater than the new extent's start offset (768K). This makes fiemap
fail with -EINVAL besides triggering the warning that produces a stack
trace like the following:

[1621.677651] ------------[ cut here ]------------
[1621.677656] WARNING: CPU: 1 PID: 204366 at fs/btrfs/extent_io.c:2492 emit_fiemap_extent+0x84/0x90 [btrfs]
[1621.677899] Modules linked in: btrfs blake2b_generic (...)
[1621.677951] CPU: 1 PID: 204366 Comm: pool Not tainted 6.8.0-rc5-btrfs-next-151+ #1
[1621.677954] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
[1621.677956] RIP: 0010:emit_fiemap_extent+0x84/0x90 [btrfs]
[1621.678033] Code: 2b 4c 89 63 (...)
[1621.678035] RSP: 0018:ffffab16089ffd20 EFLAGS: 00010206
[1621.678037] RAX: 00000000004fa000 RBX: ffffab16089ffe08 RCX: 0000000000009000
[1621.678039] RDX: 00000000004f9000 RSI: 00000000004f1000 RDI: ffffab16089ffe90
[1621.678040] RBP: 00000000004f9000 R08: 0000000000001000 R09: 0000000000000000
[1621.678041] R10: 0000000000000000 R11: 0000000000001000 R12: 0000000041d78000
[1621.678043] R13: 0000000000001000 R14: 0000000000000000 R15: ffff9434f0b17850
[1621.678044] FS: 00007fa6e20006c0(0000) GS:ffff943bdfa40000(0000) knlGS:0000000000000000
[1621.678046] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[1621.678048] CR2: 00007fa6b0801000 CR3: 000000012d404002 CR4: 0000000000370ef0
[1621.678053] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[1621.678055] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[1621.678056] Call Trace:
[1621.678074] <TASK>
[1621.678076] ? __warn+0x80/0x130
[1621.678082] ? emit_fiemap_extent+0x84/0x90 [btrfs]
[1621.678159] ? report_bug+0x1f4/0x200
[1621.678164] ? handle_bug+0x42/0x70
[1621.678167] ? exc_invalid_op+0x14/0x70
[1621.678170] ? asm_exc_invalid_op+0x16/0x20
[1621.678178] ? emit_fiemap_extent+0x84/0x90 [btrfs]
[1621.678253] extent_fiemap+0x766/0xa30 [btrfs]
[1621.678339] btrfs_fiemap+0x45/0x80 [btrfs]
[1621.678420] do_vfs_ioctl+0x1e4/0x870
[1621.678431] __x64_sys_ioctl+0x6a/0xc0
[1621.678434] do_syscall_64+0x52/0x120
[1621.678445] entry_SYSCALL_64_after_hwframe+0x6e/0x76

There's also another case where before calling btrfs_next_leaf() we are
processing a hole or a prealloc extent and we had several delalloc ranges
within that hole or prealloc extent. In that case if the ordered extents
complete before we find the next key, we may end up finding an extent item
with an offset smaller than (or equals to) the offset in cache->offset.

So fix this by changing emit_fiemap_extent() to address these three
scenarios like this:

1) For the first case, steps listed above, adjust the length of the
previously cached extent so that it does not overlap with the current
extent, emit the previous one and cache the current file extent item;

2) For the second case where he had a hole or prealloc extent with
multiple delalloc ranges inside the hole or prealloc extent's range,
and the current file extent item has an offset that matches the offset
in the fiemap cache, just discard what we have in the fiemap cache and
assign the current file extent item to the cache, since it's more up
to date;

3) For the third case where he had a hole or prealloc extent with
multiple delalloc ranges inside the hole or prealloc extent's range
and the offset of the file extent item we just found is smaller than
what we have in the cache, just skip the current file extent item
if its range end at or behind the cached extent's end, because we may
have emitted (to the fiemap user space buffer) delalloc ranges that
overlap with the current file extent item's range. If the file extent
item's range goes beyond the end offset of the cached extent, just
emit the cached extent and cache a subrange of the file extent item,
that goes from the end offset of the cached extent to the end offset
of the file extent item.

Dealing with those cases in those ways makes everything consistent by
reflecting the current state of file extent items in the btree and
without emitting extents that have overlapping ranges (which would be
confusing and violating expectations).

This issue could be triggered often with test case generic/561, and was
also hit and reported by Wang Yugui.

Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Link: https://lore.kernel.org/linux-btrfs/20240223104619.701F.409509F4@e16-tech.com/
Fixes: b0ad381fa769 ("btrfs: fix deadlock with fiemap and extent locking")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/extent_io.c | 103 ++++++++++++++++++++++++++++++++++++++++---
1 file changed, 96 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 70c6c7a0d4014..f7a10e421f128 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2437,6 +2437,7 @@ static int emit_fiemap_extent(struct fiemap_extent_info *fieinfo,
struct fiemap_cache *cache,
u64 offset, u64 phys, u64 len, u32 flags)
{
+ u64 cache_end;
int ret = 0;

/* Set at the end of extent_fiemap(). */
@@ -2446,15 +2447,102 @@ static int emit_fiemap_extent(struct fiemap_extent_info *fieinfo,
goto assign;

/*
- * Sanity check, extent_fiemap() should have ensured that new
- * fiemap extent won't overlap with cached one.
- * Not recoverable.
+ * When iterating the extents of the inode, at extent_fiemap(), we may
+ * find an extent that starts at an offset behind the end offset of the
+ * previous extent we processed. This happens if fiemap is called
+ * without FIEMAP_FLAG_SYNC and there are ordered extents completing
+ * while we call btrfs_next_leaf() (through fiemap_next_leaf_item()).
*
- * NOTE: Physical address can overlap, due to compression
+ * For example we are in leaf X processing its last item, which is the
+ * file extent item for file range [512K, 1M[, and after
+ * btrfs_next_leaf() releases the path, there's an ordered extent that
+ * completes for the file range [768K, 2M[, and that results in trimming
+ * the file extent item so that it now corresponds to the file range
+ * [512K, 768K[ and a new file extent item is inserted for the file
+ * range [768K, 2M[, which may end up as the last item of leaf X or as
+ * the first item of the next leaf - in either case btrfs_next_leaf()
+ * will leave us with a path pointing to the new extent item, for the
+ * file range [768K, 2M[, since that's the first key that follows the
+ * last one we processed. So in order not to report overlapping extents
+ * to user space, we trim the length of the previously cached extent and
+ * emit it.
+ *
+ * Upon calling btrfs_next_leaf() we may also find an extent with an
+ * offset smaller than or equals to cache->offset, and this happens
+ * when we had a hole or prealloc extent with several delalloc ranges in
+ * it, but after btrfs_next_leaf() released the path, delalloc was
+ * flushed and the resulting ordered extents were completed, so we can
+ * now have found a file extent item for an offset that is smaller than
+ * or equals to what we have in cache->offset. We deal with this as
+ * described below.
*/
- if (cache->offset + cache->len > offset) {
- WARN_ON(1);
- return -EINVAL;
+ cache_end = cache->offset + cache->len;
+ if (cache_end > offset) {
+ if (offset == cache->offset) {
+ /*
+ * We cached a dealloc range (found in the io tree) for
+ * a hole or prealloc extent and we have now found a
+ * file extent item for the same offset. What we have
+ * now is more recent and up to date, so discard what
+ * we had in the cache and use what we have just found.
+ */
+ goto assign;
+ } else if (offset > cache->offset) {
+ /*
+ * The extent range we previously found ends after the
+ * offset of the file extent item we found and that
+ * offset falls somewhere in the middle of that previous
+ * extent range. So adjust the range we previously found
+ * to end at the offset of the file extent item we have
+ * just found, since this extent is more up to date.
+ * Emit that adjusted range and cache the file extent
+ * item we have just found. This corresponds to the case
+ * where a previously found file extent item was split
+ * due to an ordered extent completing.
+ */
+ cache->len = offset - cache->offset;
+ goto emit;
+ } else {
+ const u64 range_end = offset + len;
+
+ /*
+ * The offset of the file extent item we have just found
+ * is behind the cached offset. This means we were
+ * processing a hole or prealloc extent for which we
+ * have found delalloc ranges (in the io tree), so what
+ * we have in the cache is the last delalloc range we
+ * found while the file extent item we found can be
+ * either for a whole delalloc range we previously
+ * emmitted or only a part of that range.
+ *
+ * We have two cases here:
+ *
+ * 1) The file extent item's range ends at or behind the
+ * cached extent's end. In this case just ignore the
+ * current file extent item because we don't want to
+ * overlap with previous ranges that may have been
+ * emmitted already;
+ *
+ * 2) The file extent item starts behind the currently
+ * cached extent but its end offset goes beyond the
+ * end offset of the cached extent. We don't want to
+ * overlap with a previous range that may have been
+ * emmitted already, so we emit the currently cached
+ * extent and then partially store the current file
+ * extent item's range in the cache, for the subrange
+ * going the cached extent's end to the end of the
+ * file extent item.
+ */
+ if (range_end <= cache_end)
+ return 0;
+
+ if (!(flags & (FIEMAP_EXTENT_ENCODED | FIEMAP_EXTENT_DELALLOC)))
+ phys += cache_end - offset;
+
+ offset = cache_end;
+ len = range_end - cache_end;
+ goto emit;
+ }
}

/*
@@ -2474,6 +2562,7 @@ static int emit_fiemap_extent(struct fiemap_extent_info *fieinfo,
return 0;
}

+emit:
/* Not mergeable, need to submit cached one */
ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,
cache->len, cache->flags);
--
2.43.0