eventfs: Have event files and directories default to parent uid and gid

Dongliang reported:

I found that in the latest version, the nodes of tracefs have been
changed to dynamically created.

This has caused me to encounter a problem where the gid I specified in
the mounting parameters cannot apply to all files, as in the following
situation:

/data/tmp/events # mount | grep tracefs
tracefs on /data/tmp type tracefs (rw,seclabel,relatime,gid=3012)

gid 3012 = readtracefs

/data/tmp # ls -lh
total 0
-r--r----- 1 root readtracefs 0 1970-01-01 08:00 README
-r--r----- 1 root readtracefs 0 1970-01-01 08:00 available_events

ums9621_1h10:/data/tmp/events # ls -lh
total 0
drwxr-xr-x 2 root root 0 2023-12-19 00:56 alarmtimer
drwxr-xr-x 2 root root 0 2023-12-19 00:56 asoc

It will prevent certain applications from accessing tracefs properly, I
try to avoid this issue by making the following modifications.

To fix this, have the files created default to taking the ownership of
the parent dentry unless the ownership was previously set by the user.

Link: https://lore.kernel.org/linux-trace-kernel/[e-mail]/
Link: https://lore.kernel.org/linux-trace-kernel/[e-mail]

Cc: [e-mail]
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Hongyu Jin <[e-mail]>
Fixes: 28e12c09f5aa0 ("eventfs: Save ownership and mode")
Acked-by: Masami Hiramatsu (Google) <[e-mail]>
Reported-by: Dongliang Cui <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

fs/tracefs/event_inode.c

···
 	.release	= eventfs_release,
 };
 
-static void update_inode_attr(struct inode *inode, struct eventfs_attr *attr, umode_t mode)
+static void update_inode_attr(struct dentry *dentry, struct inode *inode,
+			      struct eventfs_attr *attr, umode_t mode)
 {
 	if (!attr) {
 		inode->i_mode = mode;
···
 
 	if (attr->mode & EVENTFS_SAVE_UID)
 		inode->i_uid = attr->uid;
+	else
+		inode->i_uid = d_inode(dentry->d_parent)->i_uid;
 
 	if (attr->mode & EVENTFS_SAVE_GID)
 		inode->i_gid = attr->gid;
+	else
+		inode->i_gid = d_inode(dentry->d_parent)->i_gid;
 }
 
 /**
···
 		return eventfs_failed_creating(dentry);
 
 	/* If the user updated the directory's attributes, use them */
-	update_inode_attr(inode, attr, mode);
+	update_inode_attr(dentry, inode, attr, mode);
 
 	inode->i_op = &eventfs_file_inode_operations;
 	inode->i_fop = fop;
···
 		return eventfs_failed_creating(dentry);
 
 	/* If the user updated the directory's attributes, use them */
-	update_inode_attr(inode, &ei->attr, S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO);
+	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_fop = &eventfs_file_operations;