[PATCH v3 0/2] fscontext: do not consume log entries when returning -EMSGSIZE

[PATCH v3 0/2] fscontext: do not consume log entries when returning -EMSGSIZE

[Aleksa Sarai]

Userspace generally expects APIs that return -EMSGSIZE to allow for them
to adjust their buffer size and retry the operation. However, the
fscontext log would previously clear the message even in the -EMSGSIZE
case.

Given that it is very cheap for us to check whether the buffer is too
small before we remove the message from the ring buffer, let's just do
that instead. While we're at it, refactor some fscontext_read() into a
separate helper to make the ring buffer logic a bit easier to read.

Fixes: 007ec26cdc9f ("vfs: Implement logging through fs_context")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
Changes in v3:
- selftests: use EXPECT_STREQ()
- v2: <https://lore.kernel.org/r/20250806-fscontext-log-cleanups-v2-0-88e9d34d142f@cyphar.com>

Changes in v2:
- Refactor message fetching to fetch_message_locked() which returns
  ERR_PTR() in error cases. [Al Viro]
- v1: <https://lore.kernel.org/r/20250806-fscontext-log-cleanups-v1-0-880597d42a5a@cyphar.com>

---
Aleksa Sarai (2):
      fscontext: do not consume log entries when returning -EMSGSIZE
      selftests/filesystems: add basic fscontext log tests

 fs/fsopen.c                                    |  54 +++++-----
 tools/testing/selftests/filesystems/.gitignore |   1 +
 tools/testing/selftests/filesystems/Makefile   |   2 +-
 tools/testing/selftests/filesystems/fclog.c    | 130 +++++++++++++++++++++++++
 4 files changed, 162 insertions(+), 25 deletions(-)
---
base-commit: 66639db858112bf6b0f76677f7517643d586e575
change-id: 20250806-fscontext-log-cleanups-50f0143674ae

Best regards,
-- 
Aleksa Sarai <cyphar@cyphar.com>

[PATCH v3 1/2] fscontext: do not consume log entries when returning -EMSGSIZE

[Aleksa Sarai]

Userspace generally expects APIs that return -EMSGSIZE to allow for them
to adjust their buffer size and retry the operation. However, the
fscontext log would previously clear the message even in the -EMSGSIZE
case.

Given that it is very cheap for us to check whether the buffer is too
small before we remove the message from the ring buffer, let's just do
that instead. While we're at it, refactor some fscontext_read() into a
separate helper to make the ring buffer logic a bit easier to read.

Fixes: 007ec26cdc9f ("vfs: Implement logging through fs_context")
Cc: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org> # v5.2+
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 fs/fsopen.c | 54 ++++++++++++++++++++++++++++++------------------------
 1 file changed, 30 insertions(+), 24 deletions(-)

diff --git a/fs/fsopen.c b/fs/fsopen.c
index 1aaf4cb2afb2..538fdf814fbf 100644
--- a/fs/fsopen.c
+++ b/fs/fsopen.c
@@ -18,47 +18,53 @@
 #include "internal.h"
 #include "mount.h"
 
+static inline const char *fetch_message_locked(struct fc_log *log, size_t len,
+					       bool *need_free)
+{
+	const char *p;
+	int index;
+
+	if (unlikely(log->head == log->tail))
+		return ERR_PTR(-ENODATA);
+
+	index = log->tail & (ARRAY_SIZE(log->buffer) - 1);
+	p = log->buffer[index];
+	if (unlikely(strlen(p) > len))
+		return ERR_PTR(-EMSGSIZE);
+
+	log->buffer[index] = NULL;
+	*need_free = log->need_free & (1 << index);
+	log->need_free &= ~(1 << index);
+	log->tail++;
+
+	return p;
+}
+
 /*
  * Allow the user to read back any error, warning or informational messages.
+ * Only one message is returned for each read(2) call.
  */
 static ssize_t fscontext_read(struct file *file,
 			      char __user *_buf, size_t len, loff_t *pos)
 {
 	struct fs_context *fc = file->private_data;
-	struct fc_log *log = fc->log.log;
-	unsigned int logsize = ARRAY_SIZE(log->buffer);
 	ssize_t ret;
-	char *p;
+	const char *p;
 	bool need_free;
-	int index, n;
+	int n;
 
 	ret = mutex_lock_interruptible(&fc->uapi_mutex);
 	if (ret < 0)
 		return ret;
-
-	if (log->head == log->tail) {
-		mutex_unlock(&fc->uapi_mutex);
-		return -ENODATA;
-	}
-
-	index = log->tail & (logsize - 1);
-	p = log->buffer[index];
-	need_free = log->need_free & (1 << index);
-	log->buffer[index] = NULL;
-	log->need_free &= ~(1 << index);
-	log->tail++;
+	p = fetch_message_locked(fc->log.log, len, &need_free);
 	mutex_unlock(&fc->uapi_mutex);
+	if (IS_ERR(p))
+		return PTR_ERR(p);
 
-	ret = -EMSGSIZE;
 	n = strlen(p);
-	if (n > len)
-		goto err_free;
-	ret = -EFAULT;
-	if (copy_to_user(_buf, p, n) != 0)
-		goto err_free;
+	if (copy_to_user(_buf, p, n))
+		n = -EFAULT;
 	ret = n;
-
-err_free:
 	if (need_free)
 		kfree(p);
 	return ret;

-- 
2.50.1

[PATCH v3 2/2] selftests/filesystems: add basic fscontext log tests

[Aleksa Sarai]

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 tools/testing/selftests/filesystems/.gitignore |   1 +
 tools/testing/selftests/filesystems/Makefile   |   2 +-
 tools/testing/selftests/filesystems/fclog.c    | 130 +++++++++++++++++++++++++
 3 files changed, 132 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/filesystems/.gitignore b/tools/testing/selftests/filesystems/.gitignore
index fcbdb1297e24..64ac0dfa46b7 100644
--- a/tools/testing/selftests/filesystems/.gitignore
+++ b/tools/testing/selftests/filesystems/.gitignore
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 dnotify_test
 devpts_pts
+fclog
 file_stressor
 anon_inode_test
 kernfs_test
diff --git a/tools/testing/selftests/filesystems/Makefile b/tools/testing/selftests/filesystems/Makefile
index 73d4650af1a5..85427d7f19b9 100644
--- a/tools/testing/selftests/filesystems/Makefile
+++ b/tools/testing/selftests/filesystems/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
 CFLAGS += $(KHDR_INCLUDES)
-TEST_GEN_PROGS := devpts_pts file_stressor anon_inode_test kernfs_test
+TEST_GEN_PROGS := devpts_pts file_stressor anon_inode_test kernfs_test fclog
 TEST_GEN_PROGS_EXTENDED := dnotify_test
 
 include ../lib.mk
diff --git a/tools/testing/selftests/filesystems/fclog.c b/tools/testing/selftests/filesystems/fclog.c
new file mode 100644
index 000000000000..912a8b755c3b
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fclog.c
@@ -0,0 +1,130 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Author: Aleksa Sarai <cyphar@cyphar.com>
+ * Copyright (C) 2025 SUSE LLC.
+ */
+
+#include <assert.h>
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mount.h>
+
+#include "../kselftest_harness.h"
+
+#define ASSERT_ERRNO(expected, _t, seen)				\
+	__EXPECT(expected, #expected,					\
+		({__typeof__(seen) _tmp_seen = (seen);			\
+		  _tmp_seen >= 0 ? _tmp_seen : -errno; }), #seen, _t, 1)
+
+#define ASSERT_ERRNO_EQ(expected, seen) \
+	ASSERT_ERRNO(expected, ==, seen)
+
+#define ASSERT_SUCCESS(seen) \
+	ASSERT_ERRNO(0, <=, seen)
+
+FIXTURE(ns)
+{
+	int host_mntns;
+};
+
+FIXTURE_SETUP(ns)
+{
+	/* Stash the old mntns. */
+	self->host_mntns = open("/proc/self/ns/mnt", O_RDONLY|O_CLOEXEC);
+	ASSERT_SUCCESS(self->host_mntns);
+
+	/* Create a new mount namespace and make it private. */
+	ASSERT_SUCCESS(unshare(CLONE_NEWNS));
+	ASSERT_SUCCESS(mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL));
+}
+
+FIXTURE_TEARDOWN(ns)
+{
+	ASSERT_SUCCESS(setns(self->host_mntns, CLONE_NEWNS));
+	ASSERT_SUCCESS(close(self->host_mntns));
+}
+
+TEST_F(ns, fscontext_log_enodata)
+{
+	int fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+	ASSERT_SUCCESS(fsfd);
+
+	/* A brand new fscontext has no log entries. */
+	char buf[128] = {};
+	for (int i = 0; i < 16; i++)
+		ASSERT_ERRNO_EQ(-ENODATA, read(fsfd, buf, sizeof(buf)));
+
+	ASSERT_SUCCESS(close(fsfd));
+}
+
+TEST_F(ns, fscontext_log_errorfc)
+{
+	int fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+	ASSERT_SUCCESS(fsfd);
+
+	ASSERT_ERRNO_EQ(-EINVAL, fsconfig(fsfd, FSCONFIG_SET_STRING, "invalid-arg", "123", 0));
+
+	char buf[128] = {};
+	ASSERT_SUCCESS(read(fsfd, buf, sizeof(buf)));
+	EXPECT_STREQ("e tmpfs: Unknown parameter 'invalid-arg'\n", buf);
+
+	/* The message has been consumed. */
+	ASSERT_ERRNO_EQ(-ENODATA, read(fsfd, buf, sizeof(buf)));
+	ASSERT_SUCCESS(close(fsfd));
+}
+
+TEST_F(ns, fscontext_log_errorfc_after_fsmount)
+{
+	int fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+	ASSERT_SUCCESS(fsfd);
+
+	ASSERT_ERRNO_EQ(-EINVAL, fsconfig(fsfd, FSCONFIG_SET_STRING, "invalid-arg", "123", 0));
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0));
+	int mfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOEXEC | MOUNT_ATTR_NOSUID);
+	ASSERT_SUCCESS(mfd);
+	ASSERT_SUCCESS(move_mount(mfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH));
+
+	/*
+	 * The fscontext log should still contain data even after
+	 * FSCONFIG_CMD_CREATE and fsmount().
+	 */
+	char buf[128] = {};
+	ASSERT_SUCCESS(read(fsfd, buf, sizeof(buf)));
+	EXPECT_STREQ("e tmpfs: Unknown parameter 'invalid-arg'\n", buf);
+
+	/* The message has been consumed. */
+	ASSERT_ERRNO_EQ(-ENODATA, read(fsfd, buf, sizeof(buf)));
+	ASSERT_SUCCESS(close(fsfd));
+}
+
+TEST_F(ns, fscontext_log_emsgsize)
+{
+	int fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+	ASSERT_SUCCESS(fsfd);
+
+	ASSERT_ERRNO_EQ(-EINVAL, fsconfig(fsfd, FSCONFIG_SET_STRING, "invalid-arg", "123", 0));
+
+	char buf[128] = {};
+	/*
+	 * Attempting to read a message with too small a buffer should not
+	 * result in the message getting consumed.
+	 */
+	ASSERT_ERRNO_EQ(-EMSGSIZE, read(fsfd, buf, 0));
+	ASSERT_ERRNO_EQ(-EMSGSIZE, read(fsfd, buf, 1));
+	for (int i = 0; i < 16; i++)
+		ASSERT_ERRNO_EQ(-EMSGSIZE, read(fsfd, buf, 16));
+
+	ASSERT_SUCCESS(read(fsfd, buf, sizeof(buf)));
+	EXPECT_STREQ("e tmpfs: Unknown parameter 'invalid-arg'\n", buf);
+
+	/* The message has been consumed. */
+	ASSERT_ERRNO_EQ(-ENODATA, read(fsfd, buf, sizeof(buf)));
+	ASSERT_SUCCESS(close(fsfd));
+}
+
+TEST_HARNESS_MAIN

-- 
2.50.1

Re: [PATCH v3 1/2] fscontext: do not consume log entries when returning -EMSGSIZE

[Al Viro]

On Thu, Aug 07, 2025 at 03:55:23AM +1000, Aleksa Sarai wrote:

> -		goto err_free;
> -	ret = -EFAULT;
> -	if (copy_to_user(_buf, p, n) != 0)
> -		goto err_free;
> +	if (copy_to_user(_buf, p, n))
> +		n = -EFAULT;
>  	ret = n;
> -
> -err_free:
>  	if (need_free)
>  		kfree(p);
>  	return ret;

Minor nit: seeing that there's only one path to that return, I would
rather turn it into
	return n;
and dropped the assignment to ret a few lines above.  Anyway, that's
trivially done when applying...

Anyway, who's carrying fscontext-related stuff this cycle?  I've got
a short series in that area, but there won't be much from me around
there - a plenty of tree-in-dcache stuff, quite a bit of mount-related
work, etc., but not a lot around the options-parsing machinery.

Christian, do you have any plans around that area?

Re: [PATCH v3 1/2] fscontext: do not consume log entries when returning -EMSGSIZE

[Aleksa Sarai]

[-- Attachment #1: Type: text/plain, Size: 1188 bytes --]

On 2025-08-06, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Thu, Aug 07, 2025 at 03:55:23AM +1000, Aleksa Sarai wrote:
> 
> > -		goto err_free;
> > -	ret = -EFAULT;
> > -	if (copy_to_user(_buf, p, n) != 0)
> > -		goto err_free;
> > +	if (copy_to_user(_buf, p, n))
> > +		n = -EFAULT;
> >  	ret = n;
> > -
> > -err_free:
> >  	if (need_free)
> >  		kfree(p);
> >  	return ret;
> 
> Minor nit: seeing that there's only one path to that return, I would
> rather turn it into
> 	return n;
> and dropped the assignment to ret a few lines above.  Anyway, that's
> trivially done when applying...

It felt odd to use "return ret;" at the start and switch to "return n;"
at the end, but feel free to change it when applying.

> Anyway, who's carrying fscontext-related stuff this cycle?  I've got
> a short series in that area, but there won't be much from me around
> there - a plenty of tree-in-dcache stuff, quite a bit of mount-related
> work, etc., but not a lot around the options-parsing machinery.
> 
> Christian, do you have any plans around that area?

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

[-- Attachment #2: signature.asc --]
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Re: [PATCH v3 1/2] fscontext: do not consume log entries when returning -EMSGSIZE

[Al Viro]

On Thu, Aug 07, 2025 at 12:46:17PM +1000, Aleksa Sarai wrote:

> It felt odd to use "return ret;" at the start and switch to "return n;"
> at the end, but feel free to change it when applying.

s/ret/err/ would take care of that, as well as clarifying the intent -
not that "mutex_lock_interruptible() returns 0 or error" hadn't been
the basic common knowledge...

Anyway, that's really nitpicking at this point.

Re: [PATCH v3 1/2] fscontext: do not consume log entries when returning -EMSGSIZE

[Christian Brauner]

On Wed, Aug 06, 2025 at 08:07:51PM +0100, Al Viro wrote:
> On Thu, Aug 07, 2025 at 03:55:23AM +1000, Aleksa Sarai wrote:
> 
> > -		goto err_free;
> > -	ret = -EFAULT;
> > -	if (copy_to_user(_buf, p, n) != 0)
> > -		goto err_free;
> > +	if (copy_to_user(_buf, p, n))
> > +		n = -EFAULT;
> >  	ret = n;
> > -
> > -err_free:
> >  	if (need_free)
> >  		kfree(p);
> >  	return ret;
> 
> Minor nit: seeing that there's only one path to that return, I would
> rather turn it into
> 	return n;
> and dropped the assignment to ret a few lines above.  Anyway, that's
> trivially done when applying...
> 
> Anyway, who's carrying fscontext-related stuff this cycle?  I've got
> a short series in that area, but there won't be much from me around
> there - a plenty of tree-in-dcache stuff, quite a bit of mount-related
> work, etc., but not a lot around the options-parsing machinery.
> 
> Christian, do you have any plans around that area?

I've got a tree for that already and have applied related stuff there.
I've fixed up the comments from this thread.

Re: [PATCH v3 0/2] fscontext: do not consume log entries when returning -EMSGSIZE

[Christian Brauner]

On Thu, 07 Aug 2025 03:55:22 +1000, Aleksa Sarai wrote:
> Userspace generally expects APIs that return -EMSGSIZE to allow for them
> to adjust their buffer size and retry the operation. However, the
> fscontext log would previously clear the message even in the -EMSGSIZE
> case.
> 
> Given that it is very cheap for us to check whether the buffer is too
> small before we remove the message from the ring buffer, let's just do
> that instead. While we're at it, refactor some fscontext_read() into a
> separate helper to make the ring buffer logic a bit easier to read.
> 
> [...]

Applied to the vfs-6.18.mount branch of the vfs/vfs.git tree.
Patches in the vfs-6.18.mount branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-6.18.mount

[1/2] fscontext: do not consume log entries when returning -EMSGSIZE
      https://git.kernel.org/vfs/vfs/c/b78c4328c498
[2/2] selftests/filesystems: add basic fscontext log tests
      https://git.kernel.org/vfs/vfs/c/d70b6ceebd29