[PATCH 1/3] clear_warn_once: expand debugfs to include read support

[Paul Gortmaker]

The existing clear_warn_once variable is write-only; used as per the
documentation to reset the warn_once to as-booted state with:

	echo 1 > /sys/kernel/debug/clear_warn_once

The objective is to expand on that functionality, which requires the
debugfs variable to be read/write and not just write-only.

Here, we deal with the debugfs boilerplate associated with converting
it from write-only to read-write, in order to factor that out for easier
review, and for what may be a possible future useful bisect point.

Existing functionality is unchanged - the only difference is that we
have tied in a variable that lets you now read the variable and see
the last value written.

Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 kernel/panic.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 332736a72a58..1d425970a50c 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -654,21 +654,36 @@ EXPORT_SYMBOL(__warn_printk);
 
 /* Support resetting WARN*_ONCE state */
 
-static int clear_warn_once_set(void *data, u64 val)
+static u64 warn_once_reset;
+
+static void do_clear_warn_once(void)
 {
 	generic_bug_clear_once();
 	memset(__start_once, 0, __end_once - __start_once);
+}
+
+static int warn_once_get(void *data, u64 *val)
+{
+	*val = warn_once_reset;
+	return 0;
+}
+
+static int warn_once_set(void *data, u64 val)
+{
+	warn_once_reset = val;
+
+	do_clear_warn_once();
 	return 0;
 }
 
-DEFINE_DEBUGFS_ATTRIBUTE(clear_warn_once_fops, NULL, clear_warn_once_set,
-			 "%lld\n");
+DEFINE_DEBUGFS_ATTRIBUTE(clear_warn_once_fops, warn_once_get, warn_once_set,
+			 "%llu\n");
 
 static __init int register_warn_debugfs(void)
 {
 	/* Don't care about failure */
-	debugfs_create_file_unsafe("clear_warn_once", 0200, NULL, NULL,
-				   &clear_warn_once_fops);
+	debugfs_create_file_unsafe("clear_warn_once", 0600, NULL,
+				   &warn_once_reset, &clear_warn_once_fops);
 	return 0;
 }
 
-- 
2.25.1