[PATCH printk-rework 01/14] printk: limit second loop of syslog_print_all
From: John Ogness
Date: Thu Feb 18 2021 - 05:06:03 EST
The second loop of syslog_print_all() subtracts lengths that were
added in the first loop. With commit b031a684bfd0 ("printk: remove
logbuf_lock writer-protection of ringbuffer") it is possible that
records are (over)written during syslog_print_all(). This allows the
possibility of the second loop subtracting lengths that were never
added in the first loop.
This situation can result in syslog_print_all() filling the buffer
starting from a later record, even though there may have been room
to fit the earlier record(s) as well.
Fixes: b031a684bfd0 ("printk: remove logbuf_lock writer-protection of ringbuffer")
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
kernel/printk/printk.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c7239d169bbe..411787b900ac 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1495,6 +1495,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
struct printk_info info;
unsigned int line_count;
struct printk_record r;
+ u64 max_seq;
char *text;
int len = 0;
u64 seq;
@@ -1513,9 +1514,15 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
prb_for_each_info(clear_seq, prb, seq, &info, &line_count)
len += get_record_print_text_size(&info, line_count, true, time);
+ /*
+ * Set an upper bound for the next loop to avoid subtracting lengths
+ * that were never added.
+ */
+ max_seq = seq;
+
/* move first record forward until length fits into the buffer */
prb_for_each_info(clear_seq, prb, seq, &info, &line_count) {
- if (len <= size)
+ if (len <= size || info.seq >= max_seq)
break;
len -= get_record_print_text_size(&info, line_count, true, time);
}
--
2.20.1