patches/printk-devkmsg-llseek-reset-clear-if-it-is-lost.patch

From: John Ogness <john.ogness@linutronix.de>
Date: Fri, 22 Feb 2019 23:02:44 +0100
Subject: [PATCH] printk: devkmsg: llseek: reset clear if it is lost

SEEK_DATA will seek to the last clear record. If this clear record
is no longer in the ring buffer, devkmsg_llseek() will go into an
infinite loop. Fix that by resetting the clear sequence if the old
clear record is no longer in the ring buffer.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/printk/printk.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -761,6 +761,7 @@ static loff_t devkmsg_llseek(struct file
 {
 	struct devkmsg_user *user = file->private_data;
 	loff_t ret;
+	u64 seq;
 
 	if (!user)
 		return -EBADF;
@@ -783,7 +784,7 @@ static loff_t devkmsg_llseek(struct file
 		 * changes no global state, and does not clear anything.
 		 */
 		for (;;) {
-			prb_iter_init(&user->iter, &printk_rb, NULL);
+			prb_iter_init(&user->iter, &printk_rb, &seq);
 			ret = prb_iter_seek(&user->iter, clear_seq);
 			if (ret > 0) {
 				/* seeked to clear seq */
@@ -800,6 +801,10 @@ static loff_t devkmsg_llseek(struct file
 				break;
 			}
 			/* iterator invalid, start over */
+
+			/* reset clear_seq if it is no longer available */
+			if (seq > clear_seq)
+				clear_seq = 0;
 		}
 		ret = 0;
 		break;