printk: Fix calculation of length used to discard records

While tracking down a weird buffer overflow issue in a program that looked to be sane, I started double checking the length returned by syslog(SYSLOG_ACTION_READ_ALL, ...) to make sure it wasn't overflowing the buffer. Sure enough, it was. I saw this in strace:

11339 syslog(SYSLOG_ACTION_READ_ALL, "<5>[244017.708129] REISERFS (dev"..., 8192) = 8279

It turns out that the loops that calculate how much space the entries will take when they're copied don't include the newlines and prefixes that will be included in the final output since prev flags is passed as 0.

This patch properly accounts for it and fixes the overflow.

---
 kernel/printk.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -1034,6 +1034,7 @@ static int syslog_print_all(char __user
 			struct log *msg = log_from_idx(idx);
 
 			len += msg_print_text(msg, prev, true, NULL, 0);
+			prev = msg->flags;
 			idx = log_next(idx);
 			seq++;
 		}

@@ -1046,6 +1047,7 @@ static int syslog_print_all(char __user
 			struct log *msg = log_from_idx(idx);
 
 			len -= msg_print_text(msg, prev, true, NULL, 0);
+			prev = msg->flags;
 			idx = log_next(idx);
 			seq++;
 		}

Jeff Mahoney
SUSE Labs