tracefs: Fix mount options and add selftest

Fix tracefs mount options:

The commit 78ff64081949 ("vfs: Convert tracefs to use the new mount API") broke the gid setting when set by fstab or other mount utility. It is ignored when it is set. Fix the code so that it recognises the option again and will honor the settings on mount at boot up.

Update the internal documentation and create a selftest to make sure it doesn't break again in the future.

git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git
tracefs/fixes

Head SHA1: 8b55572e51805184353ee7d587c720a51818fb82

Kalesh Singh (3):
tracing: Fix tracefs mount options
tracing: Document tracefs gid mount option tracing/selftests: Add tracefs mount options test

----

 fs/tracefs/inode.c                                 |  12 ++-
 kernel/trace/trace.c                               |   4 +
 .../ftrace/test.d/00basic/mount_options.tc         | 101 +++++++++++++++++++++
 .../ftrace/test.d/00basic/test_ownership.tc        |  16 +---
 tools/testing/selftests/ftrace/test.d/functions    |  25 +++++
 5 files changed, 142 insertions(+), 16 deletions(-)
 create mode 100644 tools/testing/selftests/ftrace/test.d/00basic/mount_options.tc

From: Kalesh Singh <[email protected]>

Commit 78ff64081949 ("vfs: Convert tracefs to use the new mount API") converted tracefs to use the new mount APIs caused mount options (e.g. gid=<gid>) to not take effect.

The tracefs superblock can be updated from multiple paths:
- on fs_initcall() to init_trace_printk_function_export()
- from a work queue to initialize eventfs tracer_init_tracefs_work_func()
- fsconfig() syscall to mount or remount of tracefs

The tracefs superblock root inode gets created early on in init_trace_printk_function_export().

With the new mount API, tracefs effectively uses get_tree_single() instead of the old API mount_single().

Previously, mount_single() ensured that the options are always applied to the superblock root inode:
(1) If the root inode didn't exist, call fill_super() to create it and apply the options.
(2) If the root inode exists, call reconfigure_single() which effectively calls tracefs_apply_options() to parse and apply options to the subperblock's fs_info and inode and remount eventfs (if necessary)

On the other hand, get_tree_single() effectively calls vfs_get_super() which:
(3) If the root inode doesn't exists, calls fill_super() to create it and apply the options.
(4) If the root inode already exists, updates the fs_context root with the superblock's root inode.

(4) above is always the case for tracefs mounts, since the super block's root inode will already be created by init_trace_printk_function_export().

This means that the mount options get ignored:
- Since it isn't applied to the superblock's root inode, it doesn't get inherited by the children.
- Since eventfs is initialized from a separate work queue and before call to mount with the options, and it doesn't get remounted for mount.

Ensure that the mount options are applied to the super block and eventfs is remounted to respect the mount options.

To understand this better, if fstab has the following:

tracefs /sys/kernel/tracing tracefs nosuid,nodev,noexec,gid=tracing 0 0

On boot up, permissions look like:

 # ls -l /sys/kernel/tracing/trace
 -rw-r----- 1 root root 0 Nov  1 08:37 /sys/kernel/tracing/trace

When it should look like:

 # ls -l /sys/kernel/tracing/trace
 -rw-r----- 1 root tracing 0 Nov  1 08:37 /sys/kernel/tracing/trace

---
 fs/tracefs/inode.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

index 1748dff58c3b..cfc614c638da 100644
@@ -392,6 +392,9 @@ static int tracefs_reconfigure(struct fs_context *fc)
 	struct tracefs_fs_info *sb_opts = sb->s_fs_info;
 	struct tracefs_fs_info *new_opts = fc->s_fs_info;
 
+	if (!new_opts)
+		return 0;
+
 	sync_filesystem(sb);
 	/* structure copy of new mount options to sb */
 	*sb_opts = *new_opts;

@@ -478,14 +481,17 @@ static int tracefs_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_op = &tracefs_super_operations;
 	sb->s_d_op = &tracefs_dentry_operations;
 
-	tracefs_apply_options(sb, false);
-
 	return 0;
 }
 
 static int tracefs_get_tree(struct fs_context *fc)
 {
-	return get_tree_single(fc, tracefs_fill_super);
+	int err = get_tree_single(fc, tracefs_fill_super);
+
+	if (err)
+		return err;
+
+	return tracefs_reconfigure(fc);
 }
 
 static void tracefs_free_fc(struct fs_context *fc)

2.45.2

From: Kalesh Singh <[email protected]>

Commit ee7f3666995d ("tracefs: Have new files inherit the ownership of their parent") and commit 48b27b6b5191 ("tracefs: Set all files to the same group ownership as the mount option") introduced a new gid mount option that allows specifying a group to apply to all entries in tracefs.

Document this in the tracing readme.

---
 kernel/trace/trace.c | 4 ++++
 1 file changed, 4 insertions(+)

index a8f52b6527ca..2b64b3ec67d9 100644
@@ -5501,6 +5501,10 @@ static const struct file_operations tracing_iter_fops = {
 
 static const char readme_msg[] =
 	"tracing mini-HOWTO:\n\n"
+	"By default tracefs removes all OTH file permission bits.\n"
+	"When mounting tracefs an optional group id can be specified\n"
+	"which adds the group to every directory and file in tracefs:\n\n"
+	"\t e.g. mount -t tracefs [-o [gid=<gid>]] nodev /sys/kernel/tracing\n\n"
 	"# echo 0 > tracing_on : quick way to disable tracing\n"
 	"# echo 1 > tracing_on : quick way to re-enable tracing\n\n"
 	" Important files:\n"

2.45.2

From: Kalesh Singh <[email protected]>

Add a selftest to check that the tracefs gid mount option is applied correctly.

./ftracetest test.d/00basic/mount_options.tc

Use the new readme string "[gid=<gid>] as a requirement and also update test_ownership.tc requirements to use this.

---
 .../ftrace/test.d/00basic/mount_options.tc    | 101 ++++++++++++++++++
 .../ftrace/test.d/00basic/test_ownership.tc   |  16 +--
 .../testing/selftests/ftrace/test.d/functions |  25 +++++
 3 files changed, 129 insertions(+), 13 deletions(-)
 create mode 100644 tools/testing/selftests/ftrace/test.d/00basic/mount_options.tc

new file mode 100644
index 000000000000..35e8d47d6072
@@ -0,0 +1,101 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: Test tracefs GID mount option
+# requires: "[gid=<gid>]":README
+
+fail() {
+	local msg="$1"
+
+	echo "FAILED: $msg"
+	exit_fail
+}
+
+find_alternate_gid() {
+	local original_gid="$1"
+	tac /etc/group | grep -v ":$original_gid:" | head -1 | cut -d: -f3
+}
+
+mount_tracefs_with_options() {
+	local mount_point="$1"
+	local options="$2"
+
+	mount -t tracefs -o "$options" nodev "$mount_point"
+
+	setup
+}
+
+unmount_tracefs() {
+	local mount_point="$1"
+
+	# Need to make sure the mount isn't busy so that we can umount it
+	(cd $mount_point; finish_ftrace;)
+
+	cleanup
+}
+
+create_instance() {
+	local mount_point="$1"
+	local instance="$mount_point/instances/$(mktemp -u test-XXXXXX)"
+
+	mkdir "$instance"
+	echo "$instance"
+}
+
+remove_instance() {
+	local instance="$1"
+
+	rmdir "$instance"
+}
+
+check_gid() {
+	local mount_point="$1"
+	local expected_gid="$2"
+
+	echo "Checking permission group ..."
+
+	cd "$mount_point"
+
+	for file in "." "events" "events/sched" "events/sched/sched_switch" "events/sched/sched_switch/enable"; do
+		local gid=`stat -c "%g" $file`
+		if [ "$gid" -ne "$expected_gid" ]; then
+			cd - # Return to the previous working directory (tracefs root)
+			fail "$(realpath $file): Expected group $expected_gid; Got group $gid"
+		fi
+	done
+
+	cd - # Return to the previous working directory (tracefs root)
+}
+
+test_gid_mount_option() {
+	local mount_point=$(get_mount_point)
+	local mount_options=$(get_mnt_options "$mount_point")
+	local original_group=$(stat -c "%g" .)
+	local other_group=$(find_alternate_gid "$original_group")
+
+	# Set up mount options with new GID for testing
+	local new_options=`echo "$mount_options" | sed -e "s/gid=[0-9]*/gid=$other_group/"`
+	if [ "$new_options" = "$mount_options" ]; then
+		new_options="$mount_options,gid=$other_group"
+		mount_options="$mount_options,gid=$original_group"
+	fi
+
+	# Unmount existing tracefs instance and mount with new GID
+	unmount_tracefs "$mount_point"
+	mount_tracefs_with_options "$mount_point" "$new_options"
+
+	check_gid "$mount_point" "$other_group"
+
+	# Check that files created after the mount inherit the GID
+	local instance=$(create_instance "$mount_point")
+	check_gid "$instance" "$other_group"
+	remove_instance "$instance"
+
+	# Unmount and remount with the original GID
+	unmount_tracefs "$mount_point"
+	mount_tracefs_with_options "$mount_point" "$mount_options"
+	check_gid "$mount_point" "$original_group"
+}
+
+test_gid_mount_option
+
+exit 0

index 094419e190c2..e71cc3ad0bdf 100644
@@ -1,24 +1,14 @@
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 # description: Test file and directory ownership changes for eventfs
+# requires: "[gid=<gid>]":README
 
 original_group=`stat -c "%g" .`
 original_owner=`stat -c "%u" .`
 
-mount_point=`stat -c '%m' .`
+local mount_point=$(get_mount_point)
 
-# If stat -c '%m' does not work (e.g. busybox) or failed, try to use the
-# current working directory (which should be a tracefs) as the mount point.
-if [ ! -d "$mount_point" ]; then
-	if mount | grep -qw $PWD ; then
-		mount_point=$PWD
-	else
-		# If PWD doesn't work, that is an environmental problem.
-		exit_unresolved
-	fi
-fi
-
-mount_options=`mount | grep "$mount_point" | sed -e 's/.*(\(.*\)).*/\1/'`
+mount_options=$(get_mnt_options "$mount_point")
 
 # find another owner and group that is not the original
 other_group=`tac /etc/group | grep -v ":$original_group:" | head -1 | cut -d: -f3`

index 779f3e62ec90..84d6a9c7ad67 100644
@@ -193,3 +193,28 @@ ftrace_errlog_check() { # err-prefix command-with-error-pos-by-^ command-file
     # "  Command: " and "^\n" => 13
     test $(expr 13 + $pos) -eq $N
 }
+
+# Helper to get the tracefs mount point
+get_mount_point() {
+	local mount_point=`stat -c '%m' .`
+
+	# If stat -c '%m' does not work (e.g. busybox) or failed, try to use the
+	# current working directory (which should be a tracefs) as the mount point.
+	if [ ! -d "$mount_point" ]; then
+		if mount | grep -qw "$PWD"; then
+			mount_point=$PWD
+		else
+			# If PWD doesn't work, that is an environmental problem.
+			exit_unresolved
+		fi
+	fi
+	echo "$mount_point"
+}
+
+# Helper function to retrieve mount options for a given mount point
+get_mnt_options() {
+	local mnt_point="$1"
+	local opts=$(mount | grep -m1 "$mnt_point" | sed -e 's/.*(\(.*\)).*/\1/')
+
+	echo "$opts"
+}
\ No newline at end of file

2.45.2