Re: [PATCH V2] fat: Prevent the race of read/write the FAT32 entry

[Edward Adam Davis <eadavis@qq.com>]

On Tue, 29 Jul 2025 11:06:35 +0100, Al Viro wrote:
syzbot reports data-race in fat32_ent_get/fat32_ent_put.

	CPU0(Task A)			CPU1(Task B)
	====				====
	vfs_write
	new_sync_write
	generic_file_write_iter
	fat_write_begin
	block_write_begin
	fat_get_block			vfs_statfs
	fat_add_cluster			statfs_by_dentry
	fat_chain_add			fat_statfs
	fat_ent_write			fat_count_free_clusters
	fat32_ent_put			fat32_ent_get

In fat_add_cluster(), fat_alloc_clusters() retrieves an free
cluster and marks the entry with a value of FAT_ENT_EOF, protected
by lock_fat(). There is no lock protection in fat_chain_add().
When fat_ent_write() writes the last entry to new_dclus, this has
no effect on fat_count_free_clusters() in the statfs operation.
The last entry is not FAT_ENT_FREE before or after the write.

Therefore, the race condition reported here is invalid.

BR,
Edward