tracing: Document tracefs gid mount option

Commit ee7f366 ("tracefs: Have new files inherit the ownership of
their parent") and commit 48b27b6 ("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 <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Ali Zahraee <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: David Howells <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Kalesh Singh <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: Kalesh Singh

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"