From: ZhengYuan Huang Date: Wed, 25 Mar 2026 00:43:36 +0000 (+0800) Subject: btrfs: balance: fix potential bg lookup failure in chunk_usage_filter() X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6dde5221f608e0b548fcf43c68034496f1e58542;p=thirdparty%2Flinux.git btrfs: balance: fix potential bg lookup failure in chunk_usage_filter() [BUG] Running btrfs balance with a usage filter (-dusage=N) can trigger a null-ptr-deref when metadata corruption causes a chunk to have no corresponding block group in the in-memory cache: KASAN: null-ptr-deref in range [0x0000000000000070-0x0000000000000077] RIP: 0010:chunk_usage_filter fs/btrfs/volumes.c:3874 [inline] RIP: 0010:should_balance_chunk fs/btrfs/volumes.c:4018 [inline] RIP: 0010:__btrfs_balance fs/btrfs/volumes.c:4172 [inline] RIP: 0010:btrfs_balance+0x2024/0x42b0 fs/btrfs/volumes.c:4604 ... Call Trace: btrfs_ioctl_balance fs/btrfs/ioctl.c:3577 [inline] btrfs_ioctl+0x25cf/0x5b90 fs/btrfs/ioctl.c:5313 vfs_ioctl fs/ioctl.c:51 [inline] ... The bug is reproducible on current development branch. [CAUSE] Two separate data structures are involved: 1. The on-disk chunk tree, which records every chunk (logical address space region) and is iterated by __btrfs_balance(). 2. The in-memory block group cache (fs_info->block_group_cache_tree), which is built at mount time by btrfs_read_block_groups() and holds a struct btrfs_block_group for each chunk. This cache is what the usage filter queries. On a well-formed filesystem, these two are kept in 1:1 correspondence. However, btrfs_read_block_groups() builds the cache from block group items in the extent tree, not directly from the chunk tree. A corrupted image can therefore contain a chunk item in the chunk tree whose corresponding block group item is absent from the extent tree; that chunk's block group is then never inserted into the in-memory cache. When balance iterates the chunk tree and reaches such an orphaned chunk, should_balance_chunk() calls chunk_usage_filter(), which queries the block group cache: cache = btrfs_lookup_block_group(fs_info, chunk_offset); chunk_used = cache->used; /* cache may be NULL */ btrfs_lookup_block_group() returns NULL silently when no cached entry covers chunk_offset. chunk_usage_filter() does not check the return value, so the immediately following dereference of cache->used triggers the crash. [FIX] Add a NULL check after btrfs_lookup_block_group() in chunk_usage_filter(). When the lookup fails, emit a btrfs_err() message identifying the affected bytenr and return -EUCLEAN to indicate filesystem corruption. Since chunk_usage_filter() now has an error path, change its return type from bool to error pointer and 0 if the chunk passes the usage filter, and 1 if it should be skipped. Update should_balance_chunk() accordingly to propagate negative errors from the usage filter. Signed-off-by: ZhengYuan Huang Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index a88e68f905646..cb29bf616e183 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -3988,14 +3988,19 @@ static bool chunk_usage_range_filter(struct btrfs_fs_info *fs_info, u64 chunk_of return ret; } -static bool chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset, - struct btrfs_balance_args *bargs) +static int chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset, + struct btrfs_balance_args *bargs) { struct btrfs_block_group *cache; u64 chunk_used, user_thresh; - bool ret = true; + int ret = 1; cache = btrfs_lookup_block_group(fs_info, chunk_offset); + if (unlikely(!cache)) { + btrfs_err(fs_info, "balance: chunk at bytenr %llu has no corresponding block group", + chunk_offset); + return -EUCLEAN; + } chunk_used = cache->used; if (bargs->usage_min == 0) @@ -4006,7 +4011,7 @@ static bool chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset, user_thresh = mult_perc(cache->length, bargs->usage); if (chunk_used < user_thresh) - ret = false; + ret = 0; btrfs_put_block_group(cache); return ret; @@ -4111,8 +4116,8 @@ static bool chunk_soft_convert_filter(u64 chunk_type, struct btrfs_balance_args return false; } -static bool should_balance_chunk(struct extent_buffer *leaf, struct btrfs_chunk *chunk, - u64 chunk_offset) +static int should_balance_chunk(struct extent_buffer *leaf, struct btrfs_chunk *chunk, + u64 chunk_offset) { struct btrfs_fs_info *fs_info = leaf->fs_info; struct btrfs_balance_control *bctl = fs_info->balance_ctl; @@ -4145,9 +4150,14 @@ static bool should_balance_chunk(struct extent_buffer *leaf, struct btrfs_chunk } /* usage filter */ - if ((bargs->flags & BTRFS_BALANCE_ARGS_USAGE) && - chunk_usage_filter(fs_info, chunk_offset, bargs)) { - return false; + if (bargs->flags & BTRFS_BALANCE_ARGS_USAGE) { + int ret2; + + ret2 = chunk_usage_filter(fs_info, chunk_offset, bargs); + if (ret2 < 0) + return ret2; + if (ret2) + return false; } else if ((bargs->flags & BTRFS_BALANCE_ARGS_USAGE_RANGE) && chunk_usage_range_filter(fs_info, chunk_offset, bargs)) { return false; @@ -4430,6 +4440,10 @@ again: ret = should_balance_chunk(leaf, chunk, found_key.offset); btrfs_release_path(path); + if (ret < 0) { + mutex_unlock(&fs_info->reclaim_bgs_lock); + goto error; + } if (!ret) { mutex_unlock(&fs_info->reclaim_bgs_lock); goto loop;