fat: Fix the race of read/write the FAT12 entry

FAT12 entry is 12bits, so it needs 2 phase to update the value.  And
writer and reader access it without any lock, so reader can get the
half updated value.

This fixes the long standing race condition by adding a global
spinlock to only FAT12 for avoiding any impact against FAT16/32.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index ab171ea8e869b2cc9da6369abad3a83c970d2a96..2c1b73fb82ae29f5a135342c75c2d73fe0d5d931 100644 (file)
--- a/fs/fat/fatent.c
+++ b/fs/fat/fatent.c
@@ -17,6 +17,8 @@ struct fatent_operations {
        int (*ent_next)(struct fat_entry *);
 };
 
+static DEFINE_SPINLOCK(fat12_entry_lock);
+
 static void fat12_ent_blocknr(struct super_block *sb, int entry,
                              int *offset, sector_t *blocknr)
 {
@@ -116,10 +118,13 @@ static int fat12_ent_get(struct fat_entry *fatent)
        u8 **ent12_p = fatent->u.ent12_p;
        int next;
 
+       spin_lock(&fat12_entry_lock);
        if (fatent->entry & 1)
                next = (*ent12_p[0] >> 4) | (*ent12_p[1] << 4);
        else
                next = (*ent12_p[1] << 8) | *ent12_p[0];
+       spin_unlock(&fat12_entry_lock);
+
        next &= 0x0fff;
        if (next >= BAD_FAT12)
                next = FAT_ENT_EOF;
@@ -151,6 +156,7 @@ static void fat12_ent_put(struct fat_entry *fatent, int new)
        if (new == FAT_ENT_EOF)
                new = EOF_FAT12;
 
+       spin_lock(&fat12_entry_lock);
        if (fatent->entry & 1) {
                *ent12_p[0] = (new << 4) | (*ent12_p[0] & 0x0f);
                *ent12_p[1] = new >> 4;
@@ -158,6 +164,7 @@ static void fat12_ent_put(struct fat_entry *fatent, int new)
                *ent12_p[0] = new & 0xff;
                *ent12_p[1] = (*ent12_p[1] & 0xf0) | (new >> 8);
        }
+       spin_unlock(&fat12_entry_lock);
 
        mark_buffer_dirty(fatent->bhs[0]);
        if (fatent->nr_bhs == 2)