printk/NMI: handle continuous lines and missing newline

Commit 4bcc595ccd80 ("printk: reinstate KERN_CONT for printing
continuation lines") added back KERN_CONT message header.  As a result
it might appear in the middle of the line when the parts are squashed
via the temporary NMI buffer.

A reasonable solution seems to be to split the text in the NNI temporary
not only by newlines but also by the message headers.

Another solution would be to filter out KERN_CONT when writing to the
temporary buffer.  But this would complicate the lockless handling.
Also it would not solve problems with a missing newline that was there
even before the KERN_CONT stuff.

This patch moves the temporary buffer handling into separate function.
I played with it and it seems that using the char pointers make the code
easier to read.

Also it prints the final newline as a continuous line.

Finally, it moves handling of the s->len overflow into the paranoid
check.  And allows to recover from the disaster.

Link: http://lkml.kernel.org/r/1478695291-12169-2-git-send-email-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: Chris Mason <clm@fb.com>
Cc: Josef Bacik <jbacik@fb.com>
Cc: David Sterba <dsterba@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/printk/nmi.c

@@ -114,16 +114,51 @@ static void printk_nmi_flush_line(const char *text, int len)
 
 }
 
-/*
- * printk one line from the temporary buffer from @start index until
- * and including the @end index.
- */
-static void printk_nmi_flush_seq_line(struct nmi_seq_buf *s,
-					int start, int end)
+/* printk part of the temporary buffer line by line */
+static int printk_nmi_flush_buffer(const char *start, size_t len)
 {
-	const char *buf = s->buffer + start;
+	const char *c, *end;
+	bool header;
+
+	c = start;
+	end = start + len;
+	header = true;
+
+	/* Print line by line. */
+	while (c < end) {
+		if (*c == '\n') {
+			printk_nmi_flush_line(start, c - start + 1);
+			start = ++c;
+			header = true;
+			continue;
+		}
+
+		/* Handle continuous lines or missing new line. */
+		if ((c + 1 < end) && printk_get_level(c)) {
+			if (header) {
+				c = printk_skip_level(c);
+				continue;
+			}
+
+			printk_nmi_flush_line(start, c - start);
+			start = c++;
+			header = true;
+			continue;
+		}
+
+		header = false;
+		c++;
+	}
 
-	printk_nmi_flush_line(buf, (end - start) + 1);
+	/* Check if there was a partial line. Ignore pure header. */
+	if (start < end && !header) {
+		static const char newline[] = KERN_CONT "\n";
+
+		printk_nmi_flush_line(start, end - start);
+		printk_nmi_flush_line(newline, strlen(newline));
+	}
+
+	return len;
 }
 
 /*
@@ -136,8 +171,8 @@ static void __printk_nmi_flush(struct irq_work *work)
 		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
 	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
 	unsigned long flags;
-	size_t len, size;
-	int i, last_i;
+	size_t len;
+	int i;
 
 	/*
 	 * The lock has two functions. First, one reader has to flush all
@@ -155,12 +190,14 @@ more:
 	/*
 	 * This is just a paranoid check that nobody has manipulated
 	 * the buffer an unexpected way. If we printed something then
-	 * @len must only increase.
+	 * @len must only increase. Also it should never overflow the
+	 * buffer size.
 	 */
-	if (i && i >= len) {
+	if ((i && i >= len) || len > sizeof(s->buffer)) {
 		const char *msg = "printk_nmi_flush: internal error\n";
 
 		printk_nmi_flush_line(msg, strlen(msg));
+		len = 0;
 	}
 
 	if (!len)
@@ -168,22 +205,7 @@ more:
 
 	/* Make sure that data has been written up to the @len */
 	smp_rmb();
-
-	size = min(len, sizeof(s->buffer));
-	last_i = i;
-
-	/* Print line by line. */
-	for (; i < size; i++) {
-		if (s->buffer[i] == '\n') {
-			printk_nmi_flush_seq_line(s, last_i, i);
-			last_i = i + 1;
-		}
-	}
-	/* Check if there was a partial line. */
-	if (last_i < size) {
-		printk_nmi_flush_seq_line(s, last_i, size - 1);
-		printk_nmi_flush_line("\n", strlen("\n"));
-	}
+	i += printk_nmi_flush_buffer(s->buffer + i, len - i);
 
 	/*
 	 * Check that nothing has got added in the meantime and truncate