[PATCH 1/2] printk: Make continuation flags from /dev/kmsg useful again

[James Byrne]

Commit 5aa068ea4082 ("printk: remove games with previous record flags")
abolished the practice of setting the log flag to 'c' for the first
continuation line and '+' for subsequent lines. Since this change all
continuation lines are flagged with 'c' and '+' is never used.

>From the point of view of a consumer of /dev/kmsg, the behaviour now
is not very useful because you cannot join lines together based on the
'c' flag since you do not know whether a line starting with 'c' was
actually a continuation of the previous line or not.

This commit changes the flag field emitted in /dev/kmsg to expose the
log buffer flags in a more useful way:

- If LOG_NEWLINE=1 and LOG_CONT=0 the flag will be '-', meaning this is
a single self-contained line.
- If LOG_NEWLINE=0 and LOG_CONT=0 the flag will be 'c', meaning that
this is potentially the start of a sequence of continuation lines.
- If LOG_NEWLINE=0 and LOG_CONT=1 the flag will be '+', meaning that
this is the middle of a sequence of continuation lines.
- If LOG_NEWLINE=1 and LOG_CONT=1 the flag will be '*', meaning that
this is the end of a sequence of continuation lines.

This allows a consumer to concatenate continuations in a straightforward
manner.

Signed-off-by: James Byrne <james.byrne@xxxxxxxxxxxxxxxxx>
---

 Documentation/ABI/testing/dev-kmsg | 14 ++++++++------
 kernel/printk/printk.c             | 19 +++++++++++++++++--
 2 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
index f307506eb54c..6326deeaf5e3 100644
--- a/Documentation/ABI/testing/dev-kmsg
+++ b/Documentation/ABI/testing/dev-kmsg
@@ -90,12 +90,14 @@ Description:	The /dev/kmsg character device node provides userspace access
 		  +sound:card0 - subsystem:devname
 
 		The flags field carries '-' by default. A 'c' indicates a
-		fragment of a line. Note, that these hints about continuation
-		lines are not necessarily correct, and the stream could be
-		interleaved with unrelated messages, but merging the lines in
-		the output usually produces better human readable results. A
-		similar logic is used internally when messages are printed to
-		the console, /proc/kmsg or the syslog() syscall.
+		fragment of a line. Following fragments are flagged with '+'
+		and the final fragment in a sequence is flagged with '*'. Note
+		that these hints about continuation lines are not necessarily
+		correct, and the stream could be interleaved with unrelated
+		messages, but merging the lines in the output usually produces
+		better human readable results. A similar logic is used
+		internally when messages are printed to the console, /proc/kmsg
+		or the syslog() syscall.
 
 		By default, kernel tries to avoid fragments by concatenating
 		when it can and fragments are rare; however, when extended
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ca65327a6de8..a3db7f5e56d9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -715,7 +715,7 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
 				    struct printk_log *msg, u64 seq)
 {
 	u64 ts_usec = msg->ts_nsec;
-	char caller[20];
+	char flag, caller[20];
 #ifdef CONFIG_PRINTK_CALLER
 	u32 id = msg->caller_id;
 
@@ -727,9 +727,24 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
 
 	do_div(ts_usec, 1000);
 
+	switch (msg->flags & (LOG_CONT|LOG_NEWLINE)) {
+	case LOG_CONT|LOG_NEWLINE:
+		flag = '*';
+		break;
+	case LOG_CONT:
+		flag = '+';
+		break;
+	case LOG_NEWLINE:
+		flag = '-';
+		break;
+	default:
+		flag = 'c';
+		break;
+	}
+
 	return scnprintf(buf, size, "%u,%llu,%llu,%c%s;",
 			 (msg->facility << 3) | msg->level, seq, ts_usec,
-			 msg->flags & LOG_CONT ? 'c' : '-', caller);
+			 flag, caller);
 }
 
 static ssize_t msg_print_ext_body(char *buf, size_t size,
-- 
2.24.0