+ fat-fix-fat_ent_write-for-reverting-the-value.patch added to mm-nonmm-unstable branch

+ fat-fix-fat_ent_write-for-reverting-the-value.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: fat: fix fat_ent_write() for reverting the value
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     fat-fix-fat_ent_write-for-reverting-the-value.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fat-fix-fat_ent_write-for-reverting-the-value.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Subject: fat: fix fat_ent_write() for reverting the value
Date: Tue, 25 Aug 2026 21:11:32 +0900

commit 64d9183203ee ("fat: restore original value when fat_ent_write
failed") try to revert the fatent value to old value when got the error on
mirror FAT.

However it didn't work if the error is when writing the fatent bh.  In
that case, the bh is cleared the uptodate flag, so reuse bh is invalid.

Fix this by reverting the fatent only if got the error on mirror FAT.

Link: https://lore.kernel.org/87ik4yz9fv.fsf_-_@mail.parknet.co.jp
Fixes: 64d9183203ee ("fat: restore original value when fat_ent_write failed")
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: syzbot+e64c6472a3d96a75172a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e64c6472a3d96a75172a
Reported-by: syzbot+26461e903494e689c24f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=26461e903494e689c24f
Cc: Yemu Lu <prcups@krgm.moe>
Cc: Ren Wei <n05ec@lzu.edu.cn>
Cc: Yuan Tan <yuantan098@gmail.com>
Cc: Yifan Wu <yifanwucs@gmail.com>
Cc: Juefei Pu <tomapufckgml@gmail.com>
Cc: Xin Liu <bird@lzu.edu.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/fat/fat.h    |    2 +-
 fs/fat/fatent.c |   21 ++++++++++++++++++---
 fs/fat/file.c   |    3 ++-
 fs/fat/misc.c   |    6 ++----
 4 files changed, 23 insertions(+), 9 deletions(-)

--- a/fs/fat/fatent.c~fat-fix-fat_ent_write-for-reverting-the-value
+++ a/fs/fat/fatent.c
@@ -413,7 +413,7 @@ error:
 }
 
 int fat_ent_write(struct inode *inode, struct fat_entry *fatent,
-		  int new, int wait)
+		  int new, int old, int wait)
 {
 	struct super_block *sb = inode->i_sb;
 	const struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;
@@ -422,10 +422,25 @@ int fat_ent_write(struct inode *inode, s
 	ops->ent_put(fatent, new);
 	if (wait) {
 		err = fat_sync_bhs(fatent->bhs, fatent->nr_bhs);
-		if (err)
+		if (err) {
+			/*
+			 * bhs are not uptodate after I/O error. So we
+			 * can't simply re-dirty to revert. And it
+			 * would not have value to write again on I/O
+			 * error.
+			 */
 			return err;
+		}
 	}
-	return fat_mirror_bhs(sb, fatent->bhs, fatent->nr_bhs);
+
+	err = fat_mirror_bhs(sb, fatent->bhs, fatent->nr_bhs);
+	if (err) {
+		/* Try to revert if got the error on mirror FAT */
+		ops->ent_put(fatent, old);
+		if (wait)
+			fat_sync_bhs(fatent->bhs, fatent->nr_bhs);
+	}
+	return err;
 }
 
 static inline int fat_ent_next(struct msdos_sb_info *sbi,
--- a/fs/fat/fat.h~fat-fix-fat_ent_write-for-reverting-the-value
+++ a/fs/fat/fat.h
@@ -392,7 +392,7 @@ extern void fat_ent_access_init(struct s
 extern int fat_ent_read(struct inode *inode, struct fat_entry *fatent,
 			int entry);
 extern int fat_ent_write(struct inode *inode, struct fat_entry *fatent,
-			 int new, int wait);
+			 int new, int old, int wait);
 extern int fat_alloc_clusters(struct inode *inode, int *cluster,
 			      int nr_cluster);
 extern int fat_free_clusters(struct inode *inode, int cluster);
--- a/fs/fat/file.c~fat-fix-fat_ent_write-for-reverting-the-value
+++ a/fs/fat/file.c
@@ -364,7 +364,8 @@ static int fat_free(struct inode *inode,
 				     __func__, MSDOS_I(inode)->i_pos);
 			ret = -EIO;
 		} else if (ret > 0) {
-			err = fat_ent_write(inode, &fatent, FAT_ENT_EOF, wait);
+			err = fat_ent_write(inode, &fatent, FAT_ENT_EOF, ret,
+					    wait);
 			if (err)
 				ret = err;
 		}
--- a/fs/fat/misc.c~fat-fix-fat_ent_write-for-reverting-the-value
+++ a/fs/fat/misc.c
@@ -133,11 +133,9 @@ int fat_chain_add(struct inode *inode, i
 		ret = fat_ent_read(inode, &fatent, last);
 		if (ret >= 0) {
 			int wait = inode_needs_sync(inode);
-			int old = ret;
 
-			ret = fat_ent_write(inode, &fatent, new_dclus, wait);
-			if (ret < 0)
-				fat_ent_write(inode, &fatent, old, wait);
+			ret = fat_ent_write(inode, &fatent, new_dclus, ret,
+					    wait);
 			fatent_brelse(&fatent);
 		}
 		if (ret < 0)
_

Patches currently in -mm which might be from hirofumi@mail.parknet.co.jp are

fat-fix-fat_ent_write-for-reverting-the-value.patch