tracing: Document tracefs gid mount option

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: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Ali Zahraee <ahzahraee@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/20241030171928.4168869-3-kaleshsingh@google.com
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a8f52b6527ca3aad4fb52945f5d221b4973b2e23..2b64b3ec67d9f5ccdfec023c618ba759454628e1 100644 (file)
--- 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"