[PATCH v2 0/3] Tracefs mount option fixes

[PATCH v2 0/3] Tracefs mount option fixes

[Kalesh Singh]

Hi all,

This is v2 of the series fixing the tracefs mount options.

Changes in v2:
  - Update the commit descriptions, per Eric and Steve
  - Fix ordering of the patches, per Steve

v1 can be found at:
  - https://lore.kernel.org/r/20241028214550.2099923-1-kaleshsingh@google.com/

Thanks,
Kalesh

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 +--
 .../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


base-commit: 42f7652d3eb527d03665b09edac47f85fb600924
--
2.47.0.163.g1226f6d8fa-goog

[PATCH v2 1/3] tracing: Fix tracefs mount options

[Kalesh Singh]

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()
    - form a work queue to initialize eventfs
      tracer_init_tracefs_work_func()
    - fsconfig() syscall to mount or remount of tracefs

The tracefs super block 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, called fill_super() to create it
        and apply the options.
    (2) If the root inode exists, called reconfigure_single() which
        effectively called 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 option get ignored:
    - Since it isn't applied to the super block'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.

[1] https://lore.kernel.org/r/536e99d3-345c-448b-adee-a21389d7ab4b@redhat.com/

Fixes: 78ff64081949 ("vfs: Convert tracefs to use the new mount API")
Cc: Eric Sandeen <sandeen@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---

Changes in v2:
  - Update the commit title to reflect that all mount options are
    affected not only gid, per Eric

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

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 1748dff58c3b..cfc614c638da 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -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)
-- 
2.47.0.163.g1226f6d8fa-goog

[PATCH v2 2/3] tracing: Document tracefs gid mount option

[Kalesh Singh]

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.

Cc: Eric Sandeen <sandeen@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---

Changes in v2:
  - Reference commits instead of lore links, per Steve

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

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a8f52b6527ca..2b64b3ec67d9 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -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"
--
2.47.0.163.g1226f6d8fa-goog

[PATCH v2 3/3] tracing/selftests: Add tracefs mount options test

[Kalesh Singh]

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.

Cc: Eric Sandeen <sandeen@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 .../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

diff --git a/tools/testing/selftests/ftrace/test.d/00basic/mount_options.tc b/tools/testing/selftests/ftrace/test.d/00basic/mount_options.tc
new file mode 100644
index 000000000000..35e8d47d6072
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/00basic/mount_options.tc
@@ -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
diff --git a/tools/testing/selftests/ftrace/test.d/00basic/test_ownership.tc b/tools/testing/selftests/ftrace/test.d/00basic/test_ownership.tc
index 094419e190c2..e71cc3ad0bdf 100644
--- a/tools/testing/selftests/ftrace/test.d/00basic/test_ownership.tc
+++ b/tools/testing/selftests/ftrace/test.d/00basic/test_ownership.tc
@@ -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`
diff --git a/tools/testing/selftests/ftrace/test.d/functions b/tools/testing/selftests/ftrace/test.d/functions
index 779f3e62ec90..84d6a9c7ad67 100644
--- a/tools/testing/selftests/ftrace/test.d/functions
+++ b/tools/testing/selftests/ftrace/test.d/functions
@@ -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
-- 
2.47.0.163.g1226f6d8fa-goog