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
zero.

This patch properly accounts for it and fixes the overflow.

CC: stable@kernel.org
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/printk.c b/kernel/printk.c
index 6a76ab9d4476466b5ba3b3e37d38aadbb2e02644..66a2ea37b5763b450929a5f955e3bc81ad130a1c 100644 (file)
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1034,6 +1034,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
                        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 *buf, int size, bool clear)
                        struct log *msg = log_from_idx(idx);
 
                        len -= msg_print_text(msg, prev, true, NULL, 0);
+                       prev = msg->flags;
                        idx = log_next(idx);
                        seq++;
                }