[PATCH v2 0/3] fat: add support for the renameat2 RENAME_EXCHANGE flag

[PATCH v2 0/3] fat: add support for the renameat2 RENAME_EXCHANGE flag

[Javier Martinez Canillas]

Hello,

This series add 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 in patch #2 and finally patch #3 adds some kselftests to test it.

This is a v2 that addresses issues pointed out in the first version that
was posted as an RFC:

https://lore.kernel.org/lkml/20220519092343.2776414-1-javierm@redhat.com/

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

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

 MAINTAINERS                                   |   1 +
 fs/fat/namei_vfat.c                           | 193 +++++++++++++++++-
 tools/testing/selftests/Makefile              |   1 +
 .../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, 316 insertions(+), 7 deletions(-)
 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 v2 1/3] 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 v2 2/3] 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 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 | 174 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 173 insertions(+), 1 deletion(-)

diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 88ccb2ee3537..97caec8c5207 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -1017,13 +1017,185 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
 	goto out;
 }
 
+/* Helpers for vfat_rename_exchange() */
+
+static int vfat_get_dotdot_info(struct inode *inode, struct buffer_head **dotdot_bh,
+				struct msdos_dir_entry **dotdot_de)
+{
+	if (!S_ISDIR(inode->i_mode))
+		return 0;
+
+	return fat_get_dotdot_entry(inode, dotdot_bh, dotdot_de);
+}
+
+static void vfat_exchange_dentries(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 int vfat_sync_after_exchange(struct inode *dir, struct inode *inode)
+{
+	int err = 0;
+
+	if (IS_DIRSYNC(dir))
+		err = fat_sync_inode(inode);
+	else
+		mark_inode_dirty(inode);
+
+	return err;
+}
+
+static int vfat_update_dotdot_info(struct buffer_head *dotdot_bh, struct msdos_dir_entry *dotdot_de,
+				   struct inode *dir, struct inode *inode)
+{
+	int err = 0;
+
+	fat_set_start(dotdot_de, MSDOS_I(dir)->i_logstart);
+	mark_buffer_dirty_inode(dotdot_bh, inode);
+
+	if (IS_DIRSYNC(dir))
+		err = sync_dirty_buffer(dotdot_bh);
+
+	return err;
+}
+
+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_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_info(old_inode, &old_dotdot_bh, &old_dotdot_de);
+		if (err)
+			goto out;
+
+		err = vfat_get_dotdot_info(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;
+
+	/* exchange the two dentries */
+	vfat_exchange_dentries(old_inode, new_inode, old_i_pos, new_i_pos);
+
+	err = vfat_sync_after_exchange(old_dir, new_inode);
+	if (err)
+		goto error_exchange;
+
+	err = vfat_sync_after_exchange(new_dir, old_inode);
+	if (err)
+		goto error_exchange;
+
+	/* update ".." directory entry info */
+	if (old_dotdot_de) {
+		err = vfat_update_dotdot_info(old_dotdot_bh, old_dotdot_de, new_dir, old_inode);
+		if (err)
+			goto error_old_dotdot;
+
+		drop_nlink(old_dir);
+		inc_nlink(new_dir);
+	}
+
+	if (new_dotdot_de) {
+		err = vfat_update_dotdot_info(new_dotdot_bh, new_dotdot_de, old_dir, new_inode);
+		if (err)
+			goto error_new_dotdot;
+
+		drop_nlink(new_dir);
+		inc_nlink(old_dir);
+	}
+
+	/* update inode version and timestamps */
+	inode_inc_iversion(old_inode);
+	inode_inc_iversion(new_inode);
+
+	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:
+	/* data cluster is shared, serious corruption */
+	corrupt = 1;
+
+	if (new_dotdot_de) {
+		corrupt |= vfat_update_dotdot_info(new_dotdot_bh, new_dotdot_de,
+						   new_dir, new_inode);
+	}
+
+error_old_dotdot:
+	/* data cluster is shared, serious corruption */
+	corrupt = 1;
+
+	if (old_dotdot_de) {
+		corrupt |= vfat_update_dotdot_info(old_dotdot_bh, old_dotdot_de,
+						   old_dir, old_inode);
+	}
+
+error_exchange:
+	vfat_exchange_dentries(old_inode, new_inode, new_i_pos, old_i_pos);
+
+	if (corrupt) {
+		corrupt |= fat_sync_inode(old_inode);
+		corrupt |= fat_sync_inode(new_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 v2 3/3] 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>
---

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/Makefile        |  7 ++
 .../testing/selftests/filesystems/fat/config  |  2 +
 .../filesystems/fat/rename_exchange.c         | 37 +++++++++
 .../filesystems/fat/run_fat_tests.sh          | 82 +++++++++++++++++++
 6 files changed, 130 insertions(+)
 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/Makefile b/tools/testing/selftests/filesystems/fat/Makefile
new file mode 100644
index 000000000000..93ee73c16828
--- /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 -I../../../../usr/include/
+
+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 v2 3/3] selftests/filesystems: add a vfat RENAME_EXCHANGE test

[Muhammad Usama Anjum]

On 5/24/22 12:36 PM, Javier Martinez Canillas wrote:
> diff --git a/tools/testing/selftests/filesystems/fat/Makefile b/tools/testing/selftests/filesystems/fat/Makefile
> new file mode 100644
> index 000000000000..93ee73c16828
> --- /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
Please create a .gitignore file and add rename_exchange to it.

> +CFLAGS += -O2 -g -Wall -I../../../../usr/include/
Build may fail if kernel on which this test is being build has old
headers. Always use $(KHDR_INCLUDES) instead of
"-I../../../../usr/include/" to use the kernel headers from the kernel
source instead of the natively installed headers.

-- 
Muhammad Usama Anjum

Re: [PATCH v2 3/3] selftests/filesystems: add a vfat RENAME_EXCHANGE test

[Javier Martinez Canillas]

Hello Muhammad,

Thanks a lot for your feedback.

On 5/24/22 16:54, Muhammad Usama Anjum wrote:
> On 5/24/22 12:36 PM, Javier Martinez Canillas wrote:
>> diff --git a/tools/testing/selftests/filesystems/fat/Makefile b/tools/testing/selftests/filesystems/fat/Makefile
>> new file mode 100644
>> index 000000000000..93ee73c16828
>> --- /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
> Please create a .gitignore file and add rename_exchange to it.
>

Sorry, you mentioned that in the first revision but I missed.

Yes, I'll do it that on v3.
 
>> +CFLAGS += -O2 -g -Wall -I../../../../usr/include/
> Build may fail if kernel on which this test is being build has old
> headers. Always use $(KHDR_INCLUDES) instead of
> "-I../../../../usr/include/" to use the kernel headers from the kernel
> source instead of the natively installed headers.
> 

Sure, I'll do that as well.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat