Re: [PATCH] fat: Prevent the race of read/write the FAT16 and FAT32 entry

[Edward Adam Davis]

On Tue, 29 Jul 2025 00:10:31 +0900, OGAWA Hirofumi wrote:
>> The writer and reader access FAT32 entry without any lock, so the data
>> obtained by the reader is incomplete.
>>
>> Add spin lock to solve the race condition that occurs when accessing
>> FAT32 entry.
>>
>> FAT16 entry has the same issue and is handled together.
>
>What is the real issue? Counting free entries doesn't care whether EOF
>(0xffffff) or allocate (0x000068), so it should be same result on both
>case.
>
>We may want to use READ_ONCE/WRITE_ONCE though, I can't see the reason
>to add spin_lock.
Because ent32_p and ent12_p are in the same union [1], their addresses
are the same, and they both have the "read/write race condition" problem,
so I used the same method as [2] to add a spinlock to solve it.

[1]
345 struct fat_entry {
  1         int entry;
  2         union {
  3                 u8 *ent12_p[2];
  4                 __le16 *ent16_p;
  5                 __le32 *ent32_p;
  6         } u;
  7         int nr_bhs;
  8         struct buffer_head *bhs[2];
  9         struct inode *fat_inode;
 10 };

[2] 98283bb49c6c ("fat: Fix the race of read/write the FAT12 entry")

BR,
Edward