Merge tag 'vfs-6.18-rc1.mount' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs mount updates from Christian Brauner:
 "This contains some work around mount api handling:

   - Output the warning message for mnt_too_revealing() triggered during
     fsmount() to the fscontext log. This makes it possible for the
     mount tool to output appropriate warnings on the command line.

     For example, with the newest fsopen()-based mount(8) from
     util-linux, the error messages now look like:

       # mount -t proc proc /tmp
       mount: /tmp: fsmount() failed: VFS: Mount too revealing.
              dmesg(1) may have more information after failed mount system call.

   - Do not consume fscontext log entries when returning -EMSGSIZE

     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.

   - Drop an unused argument from do_remount()"

* tag 'vfs-6.18-rc1.mount' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  vfs: fs/namespace.c: remove ms_flags argument from do_remount
  selftests/filesystems: add basic fscontext log tests
  fscontext: do not consume log entries when returning -EMSGSIZE
  vfs: output mount_too_revealing() errors to fscontext
  docs/vfs: Remove mentions to the old mount API helpers
  fscontext: add custom-prefix log helpers
  fs: Remove mount_bdev
  fs: Remove mount_nodev

3 files changed, 132 insertions, 1 deletions

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