linux-kernel - [PATCH v2 1/4] printk/NMI: Handle continuous lines and missing newline

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Message-Id: <1478695291-12169-2-git-send-email-pmladek@suse.com>
Date:   Wed,  9 Nov 2016 13:41:28 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, Chris Mason <clm@...com>,
        Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
        linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>
Subject: [PATCH v2 1/4] printk/NMI: Handle continuous lines and missing newline

The commit 4bcc595ccd80decb4245 ("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.

Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 kernel/printk/nmi.c | 78 ++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 50 insertions(+), 28 deletions(-)

diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 16bab471c7e2..26cd42d459d3 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -113,16 +113,51 @@ 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;
 }
 
 /*
@@ -135,8 +170,8 @@ 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
@@ -154,12 +189,14 @@ static void __printk_nmi_flush(struct irq_work *work)
 	/*
 	 * 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)
@@ -167,22 +204,7 @@ static void __printk_nmi_flush(struct irq_work *work)
 
 	/* 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
-- 
1.8.5.6