pstore/ftrace: Allow immediate recording

Without a module param knob there was no way to enable pstore ftrace
recording early enough to debug hangs happening during the boot process
before userspace is up enough to enable it via the regular debugfs
knobs.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210610082134.20636-1-u.kleine-koenig@pengutronix.de

Author: Uwe Kleine-König

fs/pstore/ftrace.c

@@ -64,20 +64,12 @@ static struct ftrace_ops pstore_ftrace_ops __read_mostly = {
 static DEFINE_MUTEX(pstore_ftrace_lock);
 static bool pstore_ftrace_enabled;
 
-static ssize_t pstore_ftrace_knob_write(struct file *f, const char __user *buf,
-					size_t count, loff_t *ppos)
+static int pstore_set_ftrace_enabled(bool on)
 {
-	u8 on;
 	ssize_t ret;
 
-	ret = kstrtou8_from_user(buf, count, 2, &on);
-	if (ret)
-		return ret;
-
-	mutex_lock(&pstore_ftrace_lock);
-
-	if (!on ^ pstore_ftrace_enabled)
-		goto out;
+	if (on == pstore_ftrace_enabled)
+		return 0;
 
 	if (on) {
 		ftrace_ops_set_global_filter(&pstore_ftrace_ops);
@@ -89,15 +81,30 @@ static ssize_t pstore_ftrace_knob_write(struct file *f, const char __user *buf,
 	if (ret) {
 		pr_err("%s: unable to %sregister ftrace ops: %zd\n",
 		       __func__, on ? "" : "un", ret);
-		goto err;
+	} else {
+		pstore_ftrace_enabled = on;
 	}
 
-	pstore_ftrace_enabled = on;
-out:
-	ret = count;
-err:
+	return ret;
+}
+
+static ssize_t pstore_ftrace_knob_write(struct file *f, const char __user *buf,
+					size_t count, loff_t *ppos)
+{
+	u8 on;
+	ssize_t ret;
+
+	ret = kstrtou8_from_user(buf, count, 2, &on);
+	if (ret)
+		return ret;
+
+	mutex_lock(&pstore_ftrace_lock);
+	ret = pstore_set_ftrace_enabled(on);
 	mutex_unlock(&pstore_ftrace_lock);
 
+	if (ret == 0)
+		ret = count;
+
 	return ret;
 }
 
@@ -117,13 +124,20 @@ static const struct file_operations pstore_knob_fops = {
 
 static struct dentry *pstore_ftrace_dir;
 
+static bool record_ftrace;
+module_param(record_ftrace, bool, 0400);
+MODULE_PARM_DESC(record_ftrace,
+		 "enable ftrace recording immediately (default: off)");
+
 void pstore_register_ftrace(void)
 {
 	if (!psinfo->write)
 		return;
 
 	pstore_ftrace_dir = debugfs_create_dir("pstore", NULL);
 
+	pstore_set_ftrace_enabled(record_ftrace);
+
 	debugfs_create_file("record_ftrace", 0600, pstore_ftrace_dir, NULL,
 			    &pstore_knob_fops);
 }