[PATCH v2 0/5] tracefs/eventfs: Fix inconsistent permissions

[PATCH v2 0/5] tracefs/eventfs: Fix inconsistent permissions

[Steven Rostedt]

The tracefs and eventfs permissions are created dynamically based
on what the mount point inode has or the instances directory inode has.
But the way it worked had some inconsistencies that could lead to
security issues as the file system is not behaving like admins would
expect.

The files and directories could ignore the remount option that changes
the gid or uid ownerships, leaving files susceptable to access that
is not expected. This happens if a file had its value changed previously
and then a remount changed all the files permissions. The one that
was changed previously would not be affected.

This change set resolves these inconsistencies.

This also fixes the test_ownership.tc test as it would pass on the
first time it is run, but fail on the second time, because of the
inconsistant state of the permissions. Now you can run that test
multiple times and it will always pass.

Changes since v1: https://lore.kernel.org/linux-trace-kernel/20240502030024.062275408@goodmis.org/

- Testing showed that taking a mutex when freeing the tracefs_inode
  caused a lockdep splat as it can happen in the RCU softirq context.
  Convert the mutex to a spinlock for adding and removing the node
  from the link list, and free the node via call_rcu() so that the
  iteration of the list only needs to be protected by rcu_read_lock().


Steven Rostedt (Google) (5):
      tracefs: Reset permissions on remount if permissions are options
      tracefs: Still use mount point as default permissions for instances
      eventfs: Do not differentiate the toplevel events directory
      eventfs: Do not treat events directory different than other directories
      eventfs: Have "events" directory get permissions from its parent

----
 fs/tracefs/event_inode.c | 102 ++++++++++++++++++++++++++++-------------------
 fs/tracefs/inode.c       |  77 +++++++++++++++++++++++++++++++++--
 fs/tracefs/internal.h    |  14 ++++---
 3 files changed, 144 insertions(+), 49 deletions(-)

[PATCH v2 1/5] tracefs: Reset permissions on remount if permissions are options

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

There's an inconsistency with the way permissions are handled in tracefs.
Because the permissions are generated when accessed, they default to the
root inode's permission if they were never set by the user. If the user
sets the permissions, then a flag is set and the permissions are saved via
the inode (for tracefs files) or an internal attribute field (for
eventfs).

But if a remount happens that specify the permissions, all the files that
were not changed by the user gets updated, but the ones that were are not.
If the user were to remount the file system with a given permission, then
all files and directories within that file system should be updated.

This can cause security issues if a file's permission was updated but the
admin forgot about it. They could incorrectly think that remounting with
permissions set would update all files, but miss some.

For example:

 # cd /sys/kernel/tracing
 # chgrp 1002 current_tracer
 # ls -l
[..]
 -rw-r-----  1 root root 0 May  1 21:25 buffer_size_kb
 -rw-r-----  1 root root 0 May  1 21:25 buffer_subbuf_size_kb
 -r--r-----  1 root root 0 May  1 21:25 buffer_total_size_kb
 -rw-r-----  1 root lkp  0 May  1 21:25 current_tracer
 -rw-r-----  1 root root 0 May  1 21:25 dynamic_events
 -r--r-----  1 root root 0 May  1 21:25 dyn_ftrace_total_info
 -r--r-----  1 root root 0 May  1 21:25 enabled_functions

Where current_tracer now has group "lkp".

 # mount -o remount,gid=1001 .
 # ls -l
 -rw-r-----  1 root tracing 0 May  1 21:25 buffer_size_kb
 -rw-r-----  1 root tracing 0 May  1 21:25 buffer_subbuf_size_kb
 -r--r-----  1 root tracing 0 May  1 21:25 buffer_total_size_kb
 -rw-r-----  1 root lkp     0 May  1 21:25 current_tracer
 -rw-r-----  1 root tracing 0 May  1 21:25 dynamic_events
 -r--r-----  1 root tracing 0 May  1 21:25 dyn_ftrace_total_info
 -r--r-----  1 root tracing 0 May  1 21:25 enabled_functions

Everything changed but the "current_tracer".

Add a new link list that keeps track of all the tracefs_inodes which has
the permission flags that tell if the file/dir should use the root inode's
permission or not. Then on remount, clear all the flags so that the
default behavior of using the root inode's permission is done for all
files and directories.

Cc: stable@vger.kernel.org
Fixes: 8186fff7ab649 ("tracefs/eventfs: Use root and instance inodes as default ownership")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 29 +++++++++++++++++++++++
 fs/tracefs/inode.c       | 50 +++++++++++++++++++++++++++++++++++++++-
 fs/tracefs/internal.h    |  7 +++++-
 3 files changed, 84 insertions(+), 2 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index f5510e26f0f6..a754c6725a52 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -301,6 +301,35 @@ static const struct file_operations eventfs_file_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+/*
+ * On a remount of tracefs, if UID or GID options are set, then
+ * the mount point inode permissions should be used.
+ * Reset the saved permission flags appropriately.
+ */
+void eventfs_remount(struct tracefs_inode *ti, bool update_uid, bool update_gid)
+{
+	struct eventfs_inode *ei = ti->private;
+
+	if (!ei)
+		return;
+
+	if (update_uid)
+		ei->attr.mode &= ~EVENTFS_SAVE_UID;
+
+	if (update_gid)
+		ei->attr.mode &= ~EVENTFS_SAVE_GID;
+
+	if (!ei->entry_attrs)
+		return;
+
+	for (int i = 0; i < ei->nr_entries; i++) {
+		if (update_uid)
+			ei->entry_attrs[i].mode &= ~EVENTFS_SAVE_UID;
+		if (update_gid)
+			ei->entry_attrs[i].mode &= ~EVENTFS_SAVE_GID;
+	}
+}
+
 /* Return the evenfs_inode of the "events" directory */
 static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
 {
diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 5545e6bf7d26..5f2417df1c43 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -30,20 +30,47 @@ static struct vfsmount *tracefs_mount;
 static int tracefs_mount_count;
 static bool tracefs_registered;
 
+/*
+ * Keep track of all tracefs_inodes in order to update their
+ * flags if necessary on a remount.
+ */
+static DEFINE_SPINLOCK(tracefs_inode_lock);
+static LIST_HEAD(tracefs_inodes);
+
 static struct inode *tracefs_alloc_inode(struct super_block *sb)
 {
 	struct tracefs_inode *ti;
+	unsigned long flags;
 
 	ti = kmem_cache_alloc(tracefs_inode_cachep, GFP_KERNEL);
 	if (!ti)
 		return NULL;
 
+	spin_lock_irqsave(&tracefs_inode_lock, flags);
+	list_add_rcu(&ti->list, &tracefs_inodes);
+	spin_unlock_irqrestore(&tracefs_inode_lock, flags);
+
 	return &ti->vfs_inode;
 }
 
+static void tracefs_free_inode_rcu(struct rcu_head *rcu)
+{
+	struct tracefs_inode *ti;
+
+	ti = container_of(rcu, struct tracefs_inode, rcu);
+	kmem_cache_free(tracefs_inode_cachep, ti);
+}
+
 static void tracefs_free_inode(struct inode *inode)
 {
-	kmem_cache_free(tracefs_inode_cachep, get_tracefs(inode));
+	struct tracefs_inode *ti = get_tracefs(inode);
+	unsigned long flags;
+
+	spin_lock_irqsave(&tracefs_inode_lock, flags);
+	list_del_rcu(&ti->list);
+	spin_unlock_irqrestore(&tracefs_inode_lock, flags);
+
+	call_rcu(&ti->rcu, tracefs_free_inode_rcu);
 }
 
 static ssize_t default_read_file(struct file *file, char __user *buf,
@@ -313,6 +340,8 @@ static int tracefs_apply_options(struct super_block *sb, bool remount)
 	struct tracefs_fs_info *fsi = sb->s_fs_info;
 	struct inode *inode = d_inode(sb->s_root);
 	struct tracefs_mount_opts *opts = &fsi->mount_opts;
+	struct tracefs_inode *ti;
+	bool update_uid, update_gid;
 	umode_t tmp_mode;
 
 	/*
@@ -332,6 +361,25 @@ static int tracefs_apply_options(struct super_block *sb, bool remount)
 	if (!remount || opts->opts & BIT(Opt_gid))
 		inode->i_gid = opts->gid;
 
+	if (remount && (opts->opts & BIT(Opt_uid) || opts->opts & BIT(Opt_gid))) {
+
+		update_uid = opts->opts & BIT(Opt_uid);
+		update_gid = opts->opts & BIT(Opt_gid);
+
+		rcu_read_lock();
+		list_for_each_entry_rcu(ti, &tracefs_inodes, list) {
+			if (update_uid)
+				ti->flags &= ~TRACEFS_UID_PERM_SET;
+
+			if (update_gid)
+				ti->flags &= ~TRACEFS_GID_PERM_SET;
+
+			if (ti->flags & TRACEFS_EVENT_INODE)
+				eventfs_remount(ti, update_uid, update_gid);
+		}
+		rcu_read_unlock();
+	}
+
 	return 0;
 }
 
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index 15c26f9aaad4..29f0c999975b 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -11,8 +11,12 @@ enum {
 };
 
 struct tracefs_inode {
-	struct inode            vfs_inode;
+	union {
+		struct inode            vfs_inode;
+		struct rcu_head		rcu;
+	};
 	/* The below gets initialized with memset_after(ti, 0, vfs_inode) */
+	struct list_head	list;
 	unsigned long           flags;
 	void                    *private;
 };
@@ -73,6 +77,7 @@ struct dentry *tracefs_end_creating(struct dentry *dentry);
 struct dentry *tracefs_failed_creating(struct dentry *dentry);
 struct inode *tracefs_get_inode(struct super_block *sb);
 
+void eventfs_remount(struct tracefs_inode *ti, bool update_uid, bool update_gid);
 void eventfs_d_release(struct dentry *dentry);
 
 #endif /* _TRACEFS_INTERNAL_H */
-- 
2.43.0

Re: [PATCH v2 1/5] tracefs: Reset permissions on remount if permissions are options

[Steven Rostedt]

On Thu, 02 May 2024 11:15:48 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> +/*
> + * On a remount of tracefs, if UID or GID options are set, then
> + * the mount point inode permissions should be used.
> + * Reset the saved permission flags appropriately.
> + */
> +void eventfs_remount(struct tracefs_inode *ti, bool update_uid, bool update_gid)
> +{
> +	struct eventfs_inode *ei = ti->private;

And I just realized there's a race here too :-p

I need to set ti->private = NULL before freeing the ei, and probably free
the ei via RCU.

-- Steve


> +
> +	if (!ei)
> +		return;
> +
> +	if (update_uid)
> +		ei->attr.mode &= ~EVENTFS_SAVE_UID;
> +
> +	if (update_gid)
> +		ei->attr.mode &= ~EVENTFS_SAVE_GID;
> +
> +	if (!ei->entry_attrs)
> +		return;
> +
> +	for (int i = 0; i < ei->nr_entries; i++) {
> +		if (update_uid)
> +			ei->entry_attrs[i].mode &= ~EVENTFS_SAVE_UID;
> +		if (update_gid)
> +			ei->entry_attrs[i].mode &= ~EVENTFS_SAVE_GID;
> +	}
> +}
> +

[PATCH v2 2/5] tracefs: Still use mount point as default permissions for instances

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

If the instances directory's permissions were never change, then have it
and its children use the mount point permissions as the default.

Currently, the permissions of instance directories are determined by the
instance directory's permissions itself. But if the tracefs file system is
remounted and changes the permissions, the instance directory and its
children should use the new permission.

But because both the instance directory and its children use the instance
directory's inode for permissions, it misses the update.

To demonstrate this:

  # cd /sys/kernel/tracing/
  # mkdir instances/foo
  # ls -ld instances/foo
 drwxr-x--- 5 root root 0 May  1 19:07 instances/foo
  # ls -ld instances
 drwxr-x--- 3 root root 0 May  1 18:57 instances
  # ls -ld current_tracer
 -rw-r----- 1 root root 0 May  1 18:57 current_tracer

  # mount -o remount,gid=1002 .
  # ls -ld instances
 drwxr-x--- 3 root root 0 May  1 18:57 instances
  # ls -ld instances/foo/
 drwxr-x--- 5 root root 0 May  1 19:07 instances/foo/
  # ls -ld current_tracer
 -rw-r----- 1 root lkp 0 May  1 18:57 current_tracer

Notice that changing the group id to that of "lkp" did not affect the
instances directory nor its children. It should have been:

  # ls -ld current_tracer
 -rw-r----- 1 root root 0 May  1 19:19 current_tracer
  # ls -ld instances/foo/
 drwxr-x--- 5 root root 0 May  1 19:25 instances/foo/
  # ls -ld instances
 drwxr-x--- 3 root root 0 May  1 19:19 instances

  # mount -o remount,gid=1002 .
  # ls -ld current_tracer
 -rw-r----- 1 root lkp 0 May  1 19:19 current_tracer
  # ls -ld instances
 drwxr-x--- 3 root lkp 0 May  1 19:19 instances
  # ls -ld instances/foo/
 drwxr-x--- 5 root lkp 0 May  1 19:25 instances/foo/

Where all files were updated by the remount gid update.

Cc: stable@vger.kernel.org
Fixes: 8186fff7ab649 ("tracefs/eventfs: Use root and instance inodes as default ownership")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/inode.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 5f2417df1c43..e1bf65efdbb3 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -180,16 +180,39 @@ static void set_tracefs_inode_owner(struct inode *inode)
 {
 	struct tracefs_inode *ti = get_tracefs(inode);
 	struct inode *root_inode = ti->private;
+	kuid_t uid;
+	kgid_t gid;
+
+	uid = root_inode->i_uid;
+	gid = root_inode->i_gid;
+
+	/*
+	 * If the root is not the mount point, then check the root's
+	 * permissions. If it was never set, then default to the
+	 * mount point.
+	 */
+	if (root_inode != d_inode(root_inode->i_sb->s_root)) {
+		struct tracefs_inode *rti;
+
+		rti = get_tracefs(root_inode);
+		root_inode = d_inode(root_inode->i_sb->s_root);
+
+		if (!(rti->flags & TRACEFS_UID_PERM_SET))
+			uid = root_inode->i_uid;
+
+		if (!(rti->flags & TRACEFS_GID_PERM_SET))
+			gid = root_inode->i_gid;
+	}
 
 	/*
 	 * If this inode has never been referenced, then update
 	 * the permissions to the superblock.
 	 */
 	if (!(ti->flags & TRACEFS_UID_PERM_SET))
-		inode->i_uid = root_inode->i_uid;
+		inode->i_uid = uid;
 
 	if (!(ti->flags & TRACEFS_GID_PERM_SET))
-		inode->i_gid = root_inode->i_gid;
+		inode->i_gid = gid;
 }
 
 static int tracefs_permission(struct mnt_idmap *idmap,
-- 
2.43.0

[PATCH v2 3/5] eventfs: Do not differentiate the toplevel events directory

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The toplevel events directory is really no different than the events
directory of instances. Having the two be different caused
inconsistencies and made it harder to fix the permissions bugs.

Make all events directories act the same.

Cc: stable@vger.kernel.org
Fixes: 8186fff7ab649 ("tracefs/eventfs: Use root and instance inodes as default ownership")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 29 ++++++++---------------------
 fs/tracefs/internal.h    |  7 +++----
 2 files changed, 11 insertions(+), 25 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index a754c6725a52..2971609946e5 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -68,7 +68,6 @@ enum {
 	EVENTFS_SAVE_MODE	= BIT(16),
 	EVENTFS_SAVE_UID	= BIT(17),
 	EVENTFS_SAVE_GID	= BIT(18),
-	EVENTFS_TOPLEVEL	= BIT(19),
 };
 
 #define EVENTFS_MODE_MASK	(EVENTFS_SAVE_MODE - 1)
@@ -232,14 +231,10 @@ static int eventfs_set_attr(struct mnt_idmap *idmap, struct dentry *dentry,
 	return ret;
 }
 
-static void update_top_events_attr(struct eventfs_inode *ei, struct super_block *sb)
+static void update_events_attr(struct eventfs_inode *ei, struct super_block *sb)
 {
 	struct inode *root;
 
-	/* Only update if the "events" was on the top level */
-	if (!ei || !(ei->attr.mode & EVENTFS_TOPLEVEL))
-		return;
-
 	/* Get the tracefs root inode. */
 	root = d_inode(sb->s_root);
 	ei->attr.uid = root->i_uid;
@@ -252,10 +247,10 @@ static void set_top_events_ownership(struct inode *inode)
 	struct eventfs_inode *ei = ti->private;
 
 	/* The top events directory doesn't get automatically updated */
-	if (!ei || !ei->is_events || !(ei->attr.mode & EVENTFS_TOPLEVEL))
+	if (!ei || !ei->is_events)
 		return;
 
-	update_top_events_attr(ei, inode->i_sb);
+	update_events_attr(ei, inode->i_sb);
 
 	if (!(ei->attr.mode & EVENTFS_SAVE_UID))
 		inode->i_uid = ei->attr.uid;
@@ -284,7 +279,7 @@ static int eventfs_permission(struct mnt_idmap *idmap,
 	return generic_permission(idmap, inode, mask);
 }
 
-static const struct inode_operations eventfs_root_dir_inode_operations = {
+static const struct inode_operations eventfs_dir_inode_operations = {
 	.lookup		= eventfs_root_lookup,
 	.setattr	= eventfs_set_attr,
 	.getattr	= eventfs_get_attr,
@@ -352,7 +347,7 @@ static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
 		// Walk upwards until you find the events inode
 	} while (!ei->is_events);
 
-	update_top_events_attr(ei, dentry->d_sb);
+	update_events_attr(ei, dentry->d_sb);
 
 	return ei;
 }
@@ -458,7 +453,7 @@ static struct dentry *lookup_dir_entry(struct dentry *dentry,
 	update_inode_attr(dentry, inode, &ei->attr,
 			  S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO);
 
-	inode->i_op = &eventfs_root_dir_inode_operations;
+	inode->i_op = &eventfs_dir_inode_operations;
 	inode->i_fop = &eventfs_file_operations;
 
 	/* All directories will have the same inode number */
@@ -838,14 +833,6 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	uid = d_inode(dentry->d_parent)->i_uid;
 	gid = d_inode(dentry->d_parent)->i_gid;
 
-	/*
-	 * If the events directory is of the top instance, then parent
-	 * is NULL. Set the attr.mode to reflect this and its permissions will
-	 * default to the tracefs root dentry.
-	 */
-	if (!parent)
-		ei->attr.mode = EVENTFS_TOPLEVEL;
-
 	/* This is used as the default ownership of the files and directories */
 	ei->attr.uid = uid;
 	ei->attr.gid = gid;
@@ -854,13 +841,13 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	INIT_LIST_HEAD(&ei->list);
 
 	ti = get_tracefs(inode);
-	ti->flags |= TRACEFS_EVENT_INODE | TRACEFS_EVENT_TOP_INODE;
+	ti->flags |= TRACEFS_EVENT_INODE;
 	ti->private = ei;
 
 	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
 	inode->i_uid = uid;
 	inode->i_gid = gid;
-	inode->i_op = &eventfs_root_dir_inode_operations;
+	inode->i_op = &eventfs_dir_inode_operations;
 	inode->i_fop = &eventfs_file_operations;
 
 	dentry->d_fsdata = get_ei(ei);
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index 29f0c999975b..f704d8348357 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -4,10 +4,9 @@
 
 enum {
 	TRACEFS_EVENT_INODE		= BIT(1),
-	TRACEFS_EVENT_TOP_INODE		= BIT(2),
-	TRACEFS_GID_PERM_SET		= BIT(3),
-	TRACEFS_UID_PERM_SET		= BIT(4),
-	TRACEFS_INSTANCE_INODE		= BIT(5),
+	TRACEFS_GID_PERM_SET		= BIT(2),
+	TRACEFS_UID_PERM_SET		= BIT(3),
+	TRACEFS_INSTANCE_INODE		= BIT(4),
 };
 
 struct tracefs_inode {
-- 
2.43.0

[PATCH v2 4/5] eventfs: Do not treat events directory different than other directories

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

Treat the events directory the same as other directories when it comes to
permissions. The events directory was considered different because it's
dentry is persistent, whereas the other directory dentries are created
when accessed. But the way tracefs now does its ownership by using the
root dentry's permissions as the default permissions, the events directory
can get out of sync when a remount is performed setting the group and user
permissions.

Remove the special case for the events directory on setting the
attributes. This allows the updates caused by remount to work properly as
well as simplifies the code.

Cc: stable@vger.kernel.org
Fixes: 8186fff7ab649 ("tracefs/eventfs: Use root and instance inodes as default ownership")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 16 +---------------
 1 file changed, 1 insertion(+), 15 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 2971609946e5..ff4ee3dad56a 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -199,21 +199,7 @@ static int eventfs_set_attr(struct mnt_idmap *idmap, struct dentry *dentry,
 	 * determined by the parent directory.
 	 */
 	if (dentry->d_inode->i_mode & S_IFDIR) {
-		/*
-		 * The events directory dentry is never freed, unless its
-		 * part of an instance that is deleted. It's attr is the
-		 * default for its child files and directories.
-		 * Do not update it. It's not used for its own mode or ownership.
-		 */
-		if (ei->is_events) {
-			/* But it still needs to know if it was modified */
-			if (iattr->ia_valid & ATTR_UID)
-				ei->attr.mode |= EVENTFS_SAVE_UID;
-			if (iattr->ia_valid & ATTR_GID)
-				ei->attr.mode |= EVENTFS_SAVE_GID;
-		} else {
-			update_attr(&ei->attr, iattr);
-		}
+		update_attr(&ei->attr, iattr);
 
 	} else {
 		name = dentry->d_name.name;
-- 
2.43.0

[PATCH v2 5/5] eventfs: Have "events" directory get permissions from its parent

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The events directory gets its permissions from the root inode. But this
can cause an inconsistency if the instances directory changes its
permissions, as the permissions of the created directories under it should
inherit the permissions of the instances directory when directories under
it are created.

Currently the behavior is:

 # cd /sys/kernel/tracing
 # chgrp 1002 instances
 # mkdir instances/foo
 # ls -l instances/foo
[..]
 -r--r-----  1 root lkp  0 May  1 18:55 buffer_total_size_kb
 -rw-r-----  1 root lkp  0 May  1 18:55 current_tracer
 -rw-r-----  1 root lkp  0 May  1 18:55 error_log
 drwxr-xr-x  1 root root 0 May  1 18:55 events
 --w-------  1 root lkp  0 May  1 18:55 free_buffer
 drwxr-x---  2 root lkp  0 May  1 18:55 options
 drwxr-x--- 10 root lkp  0 May  1 18:55 per_cpu
 -rw-r-----  1 root lkp  0 May  1 18:55 set_event

All the files and directories under "foo" has the "lkp" group except the
"events" directory. That's because its getting its default value from the
mount point instead of its parent.

Have the "events" directory make its default value based on its parent's
permissions. That now gives:

 # ls -l instances/foo
[..]
 -rw-r-----  1 root lkp 0 May  1 21:16 buffer_subbuf_size_kb
 -r--r-----  1 root lkp 0 May  1 21:16 buffer_total_size_kb
 -rw-r-----  1 root lkp 0 May  1 21:16 current_tracer
 -rw-r-----  1 root lkp 0 May  1 21:16 error_log
 drwxr-xr-x  1 root lkp 0 May  1 21:16 events
 --w-------  1 root lkp 0 May  1 21:16 free_buffer
 drwxr-x---  2 root lkp 0 May  1 21:16 options
 drwxr-x--- 10 root lkp 0 May  1 21:16 per_cpu
 -rw-r-----  1 root lkp 0 May  1 21:16 set_event

Cc: stable@vger.kernel.org
Fixes: 8186fff7ab649 ("tracefs/eventfs: Use root and instance inodes as default ownership")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index ff4ee3dad56a..3ea32159d556 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -37,6 +37,7 @@ static DEFINE_MUTEX(eventfs_mutex);
 
 struct eventfs_root_inode {
 	struct eventfs_inode		ei;
+	struct inode			*parent_inode;
 	struct dentry			*events_dir;
 };
 
@@ -219,12 +220,23 @@ static int eventfs_set_attr(struct mnt_idmap *idmap, struct dentry *dentry,
 
 static void update_events_attr(struct eventfs_inode *ei, struct super_block *sb)
 {
-	struct inode *root;
+	struct eventfs_root_inode *rei;
+	struct inode *parent;
+
+	rei = get_root_inode(ei);
+
+	/* Use the parent inode permissions unless root set its permissions */
+	parent = rei->parent_inode;
 
-	/* Get the tracefs root inode. */
-	root = d_inode(sb->s_root);
-	ei->attr.uid = root->i_uid;
-	ei->attr.gid = root->i_gid;
+	if (rei->ei.attr.mode & EVENTFS_SAVE_UID)
+		ei->attr.uid = rei->ei.attr.uid;
+	else
+		ei->attr.uid = parent->i_uid;
+
+	if (rei->ei.attr.mode & EVENTFS_SAVE_GID)
+		ei->attr.gid = rei->ei.attr.gid;
+	else
+		ei->attr.gid = parent->i_gid;
 }
 
 static void set_top_events_ownership(struct inode *inode)
@@ -810,6 +822,7 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	// Note: we have a ref to the dentry from tracefs_start_creating()
 	rei = get_root_inode(ei);
 	rei->events_dir = dentry;
+	rei->parent_inode = d_inode(dentry->d_sb->s_root);
 
 	ei->entries = entries;
 	ei->nr_entries = size;
@@ -819,10 +832,15 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	uid = d_inode(dentry->d_parent)->i_uid;
 	gid = d_inode(dentry->d_parent)->i_gid;
 
-	/* This is used as the default ownership of the files and directories */
 	ei->attr.uid = uid;
 	ei->attr.gid = gid;
 
+	/*
+	 * When the "events" directory is created, it takes on the
+	 * permissions of its parent. But can be reset on remount.
+	 */
+	ei->attr.mode |= EVENTFS_SAVE_UID | EVENTFS_SAVE_GID;
+
 	INIT_LIST_HEAD(&ei->children);
 	INIT_LIST_HEAD(&ei->list);
 
-- 
2.43.0