kmsg - export "continuation record" flag to /dev/kmsg

In some cases we are forced to store individual records for a continuation
line print.

Export a flag to allow the external re-construction of the line. The flag
allows us to apply a similar logic externally which is used internally when
the console, /proc/kmsg or the syslog() output is printed.

  $ cat /dev/kmsg
  4,165,0,-;Free swap  = 0kB
  4,166,0,-;Total swap = 0kB
  6,167,0,c;[
  4,168,0,+;0
  4,169,0,+;1
  4,170,0,+;2
  4,171,0,+;3
  4,172,0,+;]
  6,173,0,-;[0 1 2 3 ]
  6,174,0,-;Console: colour VGA+ 80x25
  6,175,0,-;console [tty0] enabled

Signed-off-by: Kay Sievers <kay@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 21 insertions, 2 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 6c3d5bf14da2..a41106e19077 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -361,6 +361,7 @@ static void log_store(int facility, int level,
 struct devkmsg_user {
         u64 seq;
         u32 idx;
+        enum log_flags prev;
         struct mutex lock;
         char buf[8192];
 };
@@ -426,6 +427,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
         struct log *msg;
         u64 ts_usec;
         size_t i;
+        char cont = '-';
         size_t len;
         ssize_t ret;
 
@@ -463,8 +465,25 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
         msg = log_from_idx(user->idx);
         ts_usec = msg->ts_nsec;
         do_div(ts_usec, 1000);
-        len = sprintf(user->buf, "%u,%llu,%llu;",
-                      (msg->facility << 3) | msg->level, user->seq, ts_usec);
+
+        /*
+         * If we couldn't merge continuation line fragments during the print,
+         * export the stored flags to allow an optional external merge of the
+         * records. Merging the records isn't always neccessarily correct, like
+         * when we hit a race during printing. In most cases though, it produces
+         * better readable output. 'c' in the record flags mark the first
+         * fragment of a line, '+' the following.
+         */
+        if (msg->flags & LOG_CONT && !(user->prev & LOG_CONT))
+                cont = 'c';
+        else if ((msg->flags & LOG_CONT) ||
+                 ((user->prev & LOG_CONT) && !(msg->flags & LOG_PREFIX)))
+                cont = '+';
+
+        len = sprintf(user->buf, "%u,%llu,%llu,%c;",
+                      (msg->facility << 3) | msg->level,
+                      user->seq, ts_usec, cont);
+        user->prev = msg->flags;
 
         /* escape non-printable characters */
         for (i = 0; i < msg->text_len; i++) {