printk: use also the last bytes in the ring buffer

It seems that we have newer used the last byte in the ring buffer.  In
fact, we have newer used the last 4 bytes because of padding.

First problem is in the check for free space.  The exact number of free
bytes is enough to store the length of data.

Second problem is in the check where the ring buffer is rotated.  The
left side counts the first unused index.  It is unused, so it might be
the same as the size of the buffer.

Note that the first problem has to be fixed together with the second
one.  Otherwise, the buffer is rotated even when there is enough space
on the end of the buffer.  Then the beginning of the buffer is rewritten
and valid entries get corrupted.
Signed-off-by: Petr Mladek <pmladek@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Kay Sievers <kay@vrfy.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/printk/printk.c

@@ -319,7 +319,7 @@ static void log_store(int facility, int level,
 		else
 			free = log_first_idx - log_next_idx;
 
-		if (free > size + sizeof(struct printk_log))
+		if (free >= size + sizeof(struct printk_log))
 			break;
 
 		/* drop old messages until we have enough contiuous space */
@@ -327,7 +327,7 @@ static void log_store(int facility, int level,
 		log_first_seq++;
 	}
 
-	if (log_next_idx + size + sizeof(struct printk_log) >= log_buf_len) {
+	if (log_next_idx + size + sizeof(struct printk_log) > log_buf_len) {
 		/*
 		 * This message + an additional empty header does not fit
 		 * at the end of the buffer. Add an empty header with len == 0