printk: report dropping of messages from logbuf

If the log ring buffer becomes full, we silently overwrite old messages
with new data. console_unlock will detect this case and fast-forward the
console_* pointers to skip over the corrupted data, but nothing will be
reported to the user.

This patch hijacks the first valid log message after detecting that we
dropped messages and prefixes it with a note detailing how many messages
were dropped. For long (~1000 char) messages, this will result in some
truncation of the real message, but given that we're dropping things
anyway, that doesn't seem to be the end of the world.

Signed-off-by: Will Deacon <[e-mail]>
Acked-by: Peter Zijlstra <[e-mail]>
Cc: Kay Sievers <[e-mail]>
Cc: Jan Kara <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/printk/printk.c

···
 		}
 
 		if (console_seq < log_first_seq) {
+			len = sprintf(text, "** %u printk messages dropped ** ",
+				      (unsigned)(log_first_seq - console_seq));
+
 			/* messages are gone, move to first one */
 			console_seq = log_first_seq;
 			console_idx = log_first_idx;
 			console_prev = 0;
+		} else {
+			len = 0;
 		}
 skip:
 		if (console_seq == log_next_seq)
···
 		}
 
 		level = msg->level;
-		len = msg_print_text(msg, console_prev, false,
-				     text, sizeof(text));
+		len += msg_print_text(msg, console_prev, false,
+				      text + len, sizeof(text) - len);
 		console_idx = log_next(console_idx);
 		console_seq++;
 		console_prev = msg->flags;