[PATCH v6 0/4] fat: add support for the renameat2 RENAME_EXCHANGE flag

[PATCH v6 0/4] fat: add support for the renameat2 RENAME_EXCHANGE flag

[Javier Martinez Canillas]

Hello,

The series adds support for the renameat2 system call RENAME_EXCHANGE flag
(which allows to atomically replace two paths) to the vfat filesystem code.

There are many use cases for this, but we are particularly interested in
making possible for vfat filesystems to be part of OSTree [0] deployments.

Currently OSTree relies on symbolic links to make the deployment updates
an atomic transactional operation. But RENAME_EXCHANGE could be used [1]
to achieve a similar level of robustness when using a vfat filesystem.

Patch #1 is just a preparatory patch to introduce the RENAME_EXCHANGE
support, patch #2 moves some code blocks in vfat_rename() to a set of
helper functions, that can be reused by tvfat_rename_exchange() that's
added by patch #3 and finally patch #4 adds some kselftests to test it.

This is a v6 that addresses issues pointed out in v5:

https://lkml.org/lkml/2022/6/9/361

[0]: https://github.com/ostreedev/ostree
[1]: https://github.com/ostreedev/ostree/issues/1649

Changes in v6:
- Simplify logic to determine if nlink have to modified (OGAWA Hirofumi).

Changes in v5:
- Only update nlink for different parent dirs and file types (OGAWA Hirofumi).

Changes in v4:
- Add new patch from OGAWA Hirofumi to use the helpers in vfat_rename().
- Rebase the patch on top of OGAWA Hirofumi proposed changes.
- Drop iversion increment for old and new file inodes (OGAWA Hirofumi).
- Add Muhammad Usama Anjum Acked-by tag.

Changes in v3:
- Add a .gitignore for the rename_exchange binary (Muhammad Usama Anjum).
- Include $(KHDR_INCLUDES) instead of hardcoding a relative path in Makefile
  (Muhammad Usama Anjum).

Changes in v2:
- Only update the new_dir inode version and timestamps if != old_dir
  (Alex Larsson).
- Add some helper functions to avoid duplicating code (OGAWA Hirofumi).
- Use braces for multi-lines blocks even if are one statement (OGAWA Hirofumi).
- Mention in commit message that the operation is as transactional as possible
  but within the vfat limitations of not having a journal (Colin Walters).
- Call sync to flush the page cache before checking the file contents
  (Alex Larsson).
- Drop RFC prefix since the patches already got some review.

Javier Martinez Canillas (3):
  fat: add a vfat_rename2() and make existing .rename callback a helper
  fat: add renameat2 RENAME_EXCHANGE flag support
  selftests/filesystems: add a vfat RENAME_EXCHANGE test

OGAWA Hirofumi (1):
  fat: factor out reusable code in vfat_rename() as helper functions

 MAINTAINERS                                   |   1 +
 fs/fat/namei_vfat.c                           | 231 +++++++++++++++---
 tools/testing/selftests/Makefile              |   1 +
 .../selftests/filesystems/fat/.gitignore      |   2 +
 .../selftests/filesystems/fat/Makefile        |   7 +
 .../testing/selftests/filesystems/fat/config  |   2 +
 .../filesystems/fat/rename_exchange.c         |  37 +++
 .../filesystems/fat/run_fat_tests.sh          |  82 +++++++
 8 files changed, 324 insertions(+), 39 deletions(-)
 create mode 100644 tools/testing/selftests/filesystems/fat/.gitignore
 create mode 100644 tools/testing/selftests/filesystems/fat/Makefile
 create mode 100644 tools/testing/selftests/filesystems/fat/config
 create mode 100644 tools/testing/selftests/filesystems/fat/rename_exchange.c
 create mode 100755 tools/testing/selftests/filesystems/fat/run_fat_tests.sh

-- 
2.36.1

[PATCH v6 1/4] fat: add a vfat_rename2() and make existing .rename callback a helper

[Javier Martinez Canillas]

Currently vfat only supports the RENAME_NOREPLACE flag which is handled by
the virtual file system layer but doesn't support the RENAME_EXCHANGE flag.

Add a vfat_rename2() function to be used as the .rename callback and move
the current vfat_rename() handler to a helper. This is in preparation for
implementing the RENAME_NOREPLACE flag using a different helper function.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

(no changes since v1)

 fs/fat/namei_vfat.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index c573314806cf..88ccb2ee3537 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -889,9 +889,8 @@ static int vfat_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	return err;
 }
 
-static int vfat_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
-		       struct dentry *old_dentry, struct inode *new_dir,
-		       struct dentry *new_dentry, unsigned int flags)
+static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
+		       struct inode *new_dir, struct dentry *new_dentry)
 {
 	struct buffer_head *dotdot_bh;
 	struct msdos_dir_entry *dotdot_de;
@@ -902,9 +901,6 @@ static int vfat_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
 	int err, is_dir, update_dotdot, corrupt = 0;
 	struct super_block *sb = old_dir->i_sb;
 
-	if (flags & ~RENAME_NOREPLACE)
-		return -EINVAL;
-
 	old_sinfo.bh = sinfo.bh = dotdot_bh = NULL;
 	old_inode = d_inode(old_dentry);
 	new_inode = d_inode(new_dentry);
@@ -1021,13 +1017,24 @@ static int vfat_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
 	goto out;
 }
 
+static int vfat_rename2(struct user_namespace *mnt_userns, struct inode *old_dir,
+			struct dentry *old_dentry, struct inode *new_dir,
+			struct dentry *new_dentry, unsigned int flags)
+{
+	if (flags & ~RENAME_NOREPLACE)
+		return -EINVAL;
+
+	/* VFS already handled RENAME_NOREPLACE, handle it as a normal rename */
+	return vfat_rename(old_dir, old_dentry, new_dir, new_dentry);
+}
+
 static const struct inode_operations vfat_dir_inode_operations = {
 	.create		= vfat_create,
 	.lookup		= vfat_lookup,
 	.unlink		= vfat_unlink,
 	.mkdir		= vfat_mkdir,
 	.rmdir		= vfat_rmdir,
-	.rename		= vfat_rename,
+	.rename		= vfat_rename2,
 	.setattr	= fat_setattr,
 	.getattr	= fat_getattr,
 	.update_time	= fat_update_time,
-- 
2.36.1

[PATCH v6 2/4] fat: factor out reusable code in vfat_rename() as helper functions

[Javier Martinez Canillas]

From: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

The vfat_rename() function is quite big and there are code blocks that can
be moved into helper functions. This not only simplify the implementation
of that function but also allows these helpers to be reused.

For example, the helpers can be used by the handler of the RENAME_EXCHANGE
flag once this is implemented in a subsequent patch.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

(no changes since v4)

Changes in v4:
- Add new patch from OGAWA Hirofumi to use the helpers in vfat_rename().

 fs/fat/namei_vfat.c | 89 +++++++++++++++++++++++++++++----------------
 1 file changed, 57 insertions(+), 32 deletions(-)

diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 88ccb2ee3537..9c04053a8f1c 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -889,16 +889,55 @@ static int vfat_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	return err;
 }
 
+static int vfat_get_dotdot_de(struct inode *inode, struct buffer_head **bh,
+			      struct msdos_dir_entry **de)
+{
+	if (S_ISDIR(inode->i_mode)) {
+		if (fat_get_dotdot_entry(inode, bh, de))
+			return -EIO;
+	}
+	return 0;
+}
+
+static int vfat_sync_ipos(struct inode *dir, struct inode *inode)
+{
+	if (IS_DIRSYNC(dir))
+		return fat_sync_inode(inode);
+	mark_inode_dirty(inode);
+	return 0;
+}
+
+static int vfat_update_dotdot_de(struct inode *dir, struct inode *inode,
+				 struct buffer_head *dotdot_bh,
+				 struct msdos_dir_entry *dotdot_de)
+{
+	fat_set_start(dotdot_de, MSDOS_I(dir)->i_logstart);
+	mark_buffer_dirty_inode(dotdot_bh, inode);
+	if (IS_DIRSYNC(dir))
+		return sync_dirty_buffer(dotdot_bh);
+	return 0;
+}
+
+static void vfat_update_dir_metadata(struct inode *dir, struct timespec64 *ts)
+{
+	inode_inc_iversion(dir);
+	fat_truncate_time(dir, ts, S_CTIME | S_MTIME);
+	if (IS_DIRSYNC(dir))
+		(void)fat_sync_inode(dir);
+	else
+		mark_inode_dirty(dir);
+}
+
 static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 		       struct inode *new_dir, struct dentry *new_dentry)
 {
 	struct buffer_head *dotdot_bh;
-	struct msdos_dir_entry *dotdot_de;
+	struct msdos_dir_entry *dotdot_de = NULL;
 	struct inode *old_inode, *new_inode;
 	struct fat_slot_info old_sinfo, sinfo;
 	struct timespec64 ts;
 	loff_t new_i_pos;
-	int err, is_dir, update_dotdot, corrupt = 0;
+	int err, is_dir, corrupt = 0;
 	struct super_block *sb = old_dir->i_sb;
 
 	old_sinfo.bh = sinfo.bh = dotdot_bh = NULL;
@@ -909,15 +948,13 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 	if (err)
 		goto out;
 
-	is_dir = S_ISDIR(old_inode->i_mode);
-	update_dotdot = (is_dir && old_dir != new_dir);
-	if (update_dotdot) {
-		if (fat_get_dotdot_entry(old_inode, &dotdot_bh, &dotdot_de)) {
-			err = -EIO;
+	if (old_dir != new_dir) {
+		err = vfat_get_dotdot_de(old_inode, &dotdot_bh, &dotdot_de);
+		if (err)
 			goto out;
-		}
 	}
 
+	is_dir = S_ISDIR(old_inode->i_mode);
 	ts = current_time(old_dir);
 	if (new_inode) {
 		if (is_dir) {
@@ -938,21 +975,15 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 
 	fat_detach(old_inode);
 	fat_attach(old_inode, new_i_pos);
-	if (IS_DIRSYNC(new_dir)) {
-		err = fat_sync_inode(old_inode);
-		if (err)
-			goto error_inode;
-	} else
-		mark_inode_dirty(old_inode);
+	err = vfat_sync_ipos(new_dir, old_inode);
+	if (err)
+		goto error_inode;
 
-	if (update_dotdot) {
-		fat_set_start(dotdot_de, MSDOS_I(new_dir)->i_logstart);
-		mark_buffer_dirty_inode(dotdot_bh, old_inode);
-		if (IS_DIRSYNC(new_dir)) {
-			err = sync_dirty_buffer(dotdot_bh);
-			if (err)
-				goto error_dotdot;
-		}
+	if (dotdot_de) {
+		err = vfat_update_dotdot_de(new_dir, old_inode, dotdot_bh,
+					    dotdot_de);
+		if (err)
+			goto error_dotdot;
 		drop_nlink(old_dir);
 		if (!new_inode)
  			inc_nlink(new_dir);
@@ -962,12 +993,7 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 	old_sinfo.bh = NULL;
 	if (err)
 		goto error_dotdot;
-	inode_inc_iversion(old_dir);
-	fat_truncate_time(old_dir, &ts, S_CTIME|S_MTIME);
-	if (IS_DIRSYNC(old_dir))
-		(void)fat_sync_inode(old_dir);
-	else
-		mark_inode_dirty(old_dir);
+	vfat_update_dir_metadata(old_dir, &ts);
 
 	if (new_inode) {
 		drop_nlink(new_inode);
@@ -987,10 +1013,9 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 	/* data cluster is shared, serious corruption */
 	corrupt = 1;
 
-	if (update_dotdot) {
-		fat_set_start(dotdot_de, MSDOS_I(old_dir)->i_logstart);
-		mark_buffer_dirty_inode(dotdot_bh, old_inode);
-		corrupt |= sync_dirty_buffer(dotdot_bh);
+	if (dotdot_de) {
+		corrupt |= vfat_update_dotdot_de(old_dir, old_inode, dotdot_bh,
+						 dotdot_de);
 	}
 error_inode:
 	fat_detach(old_inode);
-- 
2.36.1

[PATCH v6 3/4] fat: add renameat2 RENAME_EXCHANGE flag support

[Javier Martinez Canillas]

The renameat2 RENAME_EXCHANGE flag allows to atomically exchange two paths
but is currently not supported by the Linux vfat filesystem driver.

Add a vfat_rename_exchange() helper function that implements this support.

The super block lock is acquired during the operation to ensure atomicity,
and in the error path actions made are reversed also with the mutex held.

It makes the operation as transactional as possible, within the limitation
impossed by vfat due not having a journal with logs to replay.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

Changes in v6:
- Simplify logic to determine if nlink have to modified (OGAWA Hirofumi).

Changes in v5:
- Only update nlink for different parent dirs and file types (OGAWA Hirofumi).

Changes in v4:
- Rebase the patch on top of OGAWA Hirofumi proposed changes.
- Drop iversion increment for old and new file inodes (OGAWA Hirofumi).

Changes in v2:
- Only update the new_dir inode version and timestamps if != old_dir
  (Alex Larsson).
- Add some helper functions to avoid duplicating code (OGAWA Hirofumi).
- Use braces for multi-lines blocks even if are one statement (OGAWA Hirofumi).
- Mention in commit message that the operation is as transactional as possible
  but within the vfat limitations of not having a journal (Colin Walters).

 fs/fat/namei_vfat.c | 123 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 122 insertions(+), 1 deletion(-)

diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 9c04053a8f1c..21620054e1c4 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -1042,13 +1042,134 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 	goto out;
 }
 
+static void vfat_exchange_ipos(struct inode *old_inode, struct inode *new_inode,
+			       loff_t old_i_pos, loff_t new_i_pos)
+{
+	fat_detach(old_inode);
+	fat_detach(new_inode);
+	fat_attach(old_inode, new_i_pos);
+	fat_attach(new_inode, old_i_pos);
+}
+
+static void vfat_move_nlink(struct inode *src, struct inode *dst)
+{
+	drop_nlink(src);
+	inc_nlink(dst);
+}
+
+static int vfat_rename_exchange(struct inode *old_dir, struct dentry *old_dentry,
+				struct inode *new_dir, struct dentry *new_dentry)
+{
+	struct buffer_head *old_dotdot_bh = NULL, *new_dotdot_bh = NULL;
+	struct msdos_dir_entry *old_dotdot_de = NULL, *new_dotdot_de = NULL;
+	struct inode *old_inode, *new_inode;
+	struct timespec64 ts = current_time(old_dir);
+	loff_t old_i_pos, new_i_pos;
+	int err, corrupt = 0;
+	struct super_block *sb = old_dir->i_sb;
+
+	old_inode = d_inode(old_dentry);
+	new_inode = d_inode(new_dentry);
+
+	/* Acquire super block lock for the operation to be atomic */
+	mutex_lock(&MSDOS_SB(sb)->s_lock);
+
+	/* if directories are not the same, get ".." info to update */
+	if (old_dir != new_dir) {
+		err = vfat_get_dotdot_de(old_inode, &old_dotdot_bh,
+					 &old_dotdot_de);
+		if (err)
+			goto out;
+
+		err = vfat_get_dotdot_de(new_inode, &new_dotdot_bh,
+					 &new_dotdot_de);
+		if (err)
+			goto out;
+	}
+
+	old_i_pos = MSDOS_I(old_inode)->i_pos;
+	new_i_pos = MSDOS_I(new_inode)->i_pos;
+
+	vfat_exchange_ipos(old_inode, new_inode, old_i_pos, new_i_pos);
+
+	err = vfat_sync_ipos(old_dir, new_inode);
+	if (err)
+		goto error_exchange;
+	err = vfat_sync_ipos(new_dir, old_inode);
+	if (err)
+		goto error_exchange;
+
+	/* update ".." directory entry info */
+	if (old_dotdot_de) {
+		err = vfat_update_dotdot_de(new_dir, old_inode, old_dotdot_bh,
+					    old_dotdot_de);
+		if (err)
+			goto error_old_dotdot;
+	}
+	if (new_dotdot_de) {
+		err = vfat_update_dotdot_de(old_dir, new_inode, new_dotdot_bh,
+					    new_dotdot_de);
+		if (err)
+			goto error_new_dotdot;
+	}
+
+	/* if cross directory and only one is a directory, adjust nlink */
+	if (!old_dotdot_de != !new_dotdot_de) {
+		if (old_dotdot_de)
+			vfat_move_nlink(old_dir, new_dir);
+		else
+			vfat_move_nlink(new_dir, old_dir);
+	}
+
+	vfat_update_dir_metadata(old_dir, &ts);
+	/* if directories are not the same, update new_dir as well */
+	if (old_dir != new_dir)
+		vfat_update_dir_metadata(new_dir, &ts);
+
+out:
+	brelse(old_dotdot_bh);
+	brelse(new_dotdot_bh);
+	mutex_unlock(&MSDOS_SB(sb)->s_lock);
+
+	return err;
+
+error_new_dotdot:
+	if (new_dotdot_de) {
+		corrupt |= vfat_update_dotdot_de(new_dir, new_inode,
+						 new_dotdot_bh, new_dotdot_de);
+	}
+
+error_old_dotdot:
+	if (old_dotdot_de) {
+		corrupt |= vfat_update_dotdot_de(old_dir, old_inode,
+						 old_dotdot_bh, old_dotdot_de);
+	}
+
+error_exchange:
+	vfat_exchange_ipos(old_inode, new_inode, new_i_pos, old_i_pos);
+	corrupt |= vfat_sync_ipos(new_dir, new_inode);
+	corrupt |= vfat_sync_ipos(old_dir, old_inode);
+
+	if (corrupt < 0) {
+		fat_fs_error(new_dir->i_sb,
+			     "%s: Filesystem corrupted (i_pos %lld, %lld)",
+			     __func__, old_i_pos, new_i_pos);
+	}
+	goto out;
+}
+
 static int vfat_rename2(struct user_namespace *mnt_userns, struct inode *old_dir,
 			struct dentry *old_dentry, struct inode *new_dir,
 			struct dentry *new_dentry, unsigned int flags)
 {
-	if (flags & ~RENAME_NOREPLACE)
+	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE))
 		return -EINVAL;
 
+	if (flags & RENAME_EXCHANGE) {
+		return vfat_rename_exchange(old_dir, old_dentry,
+					    new_dir, new_dentry);
+	}
+
 	/* VFS already handled RENAME_NOREPLACE, handle it as a normal rename */
 	return vfat_rename(old_dir, old_dentry, new_dir, new_dentry);
 }
-- 
2.36.1

[PATCH v6 4/4] selftests/filesystems: add a vfat RENAME_EXCHANGE test

[Javier Martinez Canillas]

Add a test for the renameat2 RENAME_EXCHANGE support in vfat, but split it
in a tool that just does the rename exchange and a script that is run by
the kselftests framework on `make TARGETS="filesystems/fat" kselftest`.

That way the script can be easily extended to test other file operations.

The script creates a 1 MiB disk image, that is then formated with a vfat
filesystem and mounted using a loop device. That way all file operations
are done on an ephemeral filesystem.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---

(no changes since v4)

Changes in v4:
- Add Muhammad Usama Anjum Acked-by tag.

Changes in v3:
- Add a .gitignore for the rename_exchange binary (Muhammad Usama Anjum).
- Include $(KHDR_INCLUDES) instead of hardcoding a relative path in Makefile
  (Muhammad Usama Anjum).

Changes in v2:
- Call sync to flush the page cache before checking the file contents
  (Alex Larsson).

 MAINTAINERS                                   |  1 +
 tools/testing/selftests/Makefile              |  1 +
 .../selftests/filesystems/fat/.gitignore      |  2 +
 .../selftests/filesystems/fat/Makefile        |  7 ++
 .../testing/selftests/filesystems/fat/config  |  2 +
 .../filesystems/fat/rename_exchange.c         | 37 +++++++++
 .../filesystems/fat/run_fat_tests.sh          | 82 +++++++++++++++++++
 7 files changed, 132 insertions(+)
 create mode 100644 tools/testing/selftests/filesystems/fat/.gitignore
 create mode 100644 tools/testing/selftests/filesystems/fat/Makefile
 create mode 100644 tools/testing/selftests/filesystems/fat/config
 create mode 100644 tools/testing/selftests/filesystems/fat/rename_exchange.c
 create mode 100755 tools/testing/selftests/filesystems/fat/run_fat_tests.sh

diff --git a/MAINTAINERS b/MAINTAINERS
index 4fdbbd6c1984..158771bb7755 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20841,6 +20841,7 @@ M:	OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
 S:	Maintained
 F:	Documentation/filesystems/vfat.rst
 F:	fs/fat/
+F:	tools/testing/selftests/filesystems/fat/
 
 VFIO DRIVER
 M:	Alex Williamson <alex.williamson@redhat.com>
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 0aedcd76cf0f..fc59ad849a90 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -16,6 +16,7 @@ TARGETS += exec
 TARGETS += filesystems
 TARGETS += filesystems/binderfs
 TARGETS += filesystems/epoll
+TARGETS += filesystems/fat
 TARGETS += firmware
 TARGETS += fpu
 TARGETS += ftrace
diff --git a/tools/testing/selftests/filesystems/fat/.gitignore b/tools/testing/selftests/filesystems/fat/.gitignore
new file mode 100644
index 000000000000..b89920ed841c
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fat/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+rename_exchange
diff --git a/tools/testing/selftests/filesystems/fat/Makefile b/tools/testing/selftests/filesystems/fat/Makefile
new file mode 100644
index 000000000000..902033f6ef09
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fat/Makefile
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+
+TEST_PROGS := run_fat_tests.sh
+TEST_GEN_PROGS_EXTENDED := rename_exchange
+CFLAGS += -O2 -g -Wall $(KHDR_INCLUDES)
+
+include ../../lib.mk
diff --git a/tools/testing/selftests/filesystems/fat/config b/tools/testing/selftests/filesystems/fat/config
new file mode 100644
index 000000000000..6cf95e787a17
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fat/config
@@ -0,0 +1,2 @@
+CONFIG_BLK_DEV_LOOP=y
+CONFIG_VFAT_FS=y
diff --git a/tools/testing/selftests/filesystems/fat/rename_exchange.c b/tools/testing/selftests/filesystems/fat/rename_exchange.c
new file mode 100644
index 000000000000..e488ad354fce
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fat/rename_exchange.c
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Program that atomically exchanges two paths using
+ * the renameat2() system call RENAME_EXCHANGE flag.
+ *
+ * Copyright 2022 Red Hat Inc.
+ * Author: Javier Martinez Canillas <javierm@redhat.com>
+ */
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+void print_usage(const char *program)
+{
+	printf("Usage: %s [oldpath] [newpath]\n", program);
+	printf("Atomically exchange oldpath and newpath\n");
+}
+
+int main(int argc, char *argv[])
+{
+	int ret;
+
+	if (argc != 3) {
+		print_usage(argv[0]);
+		exit(EXIT_FAILURE);
+	}
+
+	ret = renameat2(AT_FDCWD, argv[1], AT_FDCWD, argv[2], RENAME_EXCHANGE);
+	if (ret) {
+		perror("rename exchange failed");
+		exit(EXIT_FAILURE);
+	}
+
+	exit(EXIT_SUCCESS);
+}
diff --git a/tools/testing/selftests/filesystems/fat/run_fat_tests.sh b/tools/testing/selftests/filesystems/fat/run_fat_tests.sh
new file mode 100755
index 000000000000..7f35dc3d15df
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fat/run_fat_tests.sh
@@ -0,0 +1,82 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Run filesystem operations tests on an 1 MiB disk image that is formatted with
+# a vfat filesystem and mounted in a temporary directory using a loop device.
+#
+# Copyright 2022 Red Hat Inc.
+# Author: Javier Martinez Canillas <javierm@redhat.com>
+
+set -e
+set -u
+set -o pipefail
+
+BASE_DIR="$(dirname $0)"
+TMP_DIR="$(mktemp -d /tmp/fat_tests_tmp.XXXX)"
+IMG_PATH="${TMP_DIR}/fat.img"
+MNT_PATH="${TMP_DIR}/mnt"
+
+cleanup()
+{
+    mountpoint -q "${MNT_PATH}" && unmount_image
+    rm -rf "${TMP_DIR}"
+}
+trap cleanup SIGINT SIGTERM EXIT
+
+create_loopback()
+{
+    touch "${IMG_PATH}"
+    chattr +C "${IMG_PATH}" >/dev/null 2>&1 || true
+
+    truncate -s 1M "${IMG_PATH}"
+    mkfs.vfat "${IMG_PATH}" >/dev/null 2>&1
+}
+
+mount_image()
+{
+    mkdir -p "${MNT_PATH}"
+    sudo mount -o loop "${IMG_PATH}" "${MNT_PATH}"
+}
+
+rename_exchange_test()
+{
+    local rename_exchange="${BASE_DIR}/rename_exchange"
+    local old_path="${MNT_PATH}/old_file"
+    local new_path="${MNT_PATH}/new_file"
+
+    echo old | sudo tee "${old_path}" >/dev/null 2>&1
+    echo new | sudo tee "${new_path}" >/dev/null 2>&1
+    sudo "${rename_exchange}" "${old_path}" "${new_path}" >/dev/null 2>&1
+    sudo sync -f "${MNT_PATH}"
+    grep new "${old_path}" >/dev/null 2>&1
+    grep old "${new_path}" >/dev/null 2>&1
+}
+
+rename_exchange_subdir_test()
+{
+    local rename_exchange="${BASE_DIR}/rename_exchange"
+    local dir_path="${MNT_PATH}/subdir"
+    local old_path="${MNT_PATH}/old_file"
+    local new_path="${dir_path}/new_file"
+
+    sudo mkdir -p "${dir_path}"
+    echo old | sudo tee "${old_path}" >/dev/null 2>&1
+    echo new | sudo tee "${new_path}" >/dev/null 2>&1
+    sudo "${rename_exchange}" "${old_path}" "${new_path}" >/dev/null 2>&1
+    sudo sync -f "${MNT_PATH}"
+    grep new "${old_path}" >/dev/null 2>&1
+    grep old "${new_path}" >/dev/null 2>&1
+}
+
+unmount_image()
+{
+    sudo umount "${MNT_PATH}" &> /dev/null
+}
+
+create_loopback
+mount_image
+rename_exchange_test
+rename_exchange_subdir_test
+unmount_image
+
+exit 0
-- 
2.36.1

Re: [PATCH v6 0/4] fat: add support for the renameat2 RENAME_EXCHANGE flag

[OGAWA Hirofumi]

Javier Martinez Canillas <javierm@redhat.com> writes:

> The series adds support for the renameat2 system call RENAME_EXCHANGE flag
> (which allows to atomically replace two paths) to the vfat filesystem code.
>
> There are many use cases for this, but we are particularly interested in
> making possible for vfat filesystems to be part of OSTree [0] deployments.
>
> Currently OSTree relies on symbolic links to make the deployment updates
> an atomic transactional operation. But RENAME_EXCHANGE could be used [1]
> to achieve a similar level of robustness when using a vfat filesystem.
>
> Patch #1 is just a preparatory patch to introduce the RENAME_EXCHANGE
> support, patch #2 moves some code blocks in vfat_rename() to a set of
> helper functions, that can be reused by tvfat_rename_exchange() that's
> added by patch #3 and finally patch #4 adds some kselftests to test it.
>
> This is a v6 that addresses issues pointed out in v5:
>
> https://lkml.org/lkml/2022/6/9/361
>
> [0]: https://github.com/ostreedev/ostree
> [1]: https://github.com/ostreedev/ostree/issues/1649

Looks good this patchset.

Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Thanks.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>