Fix apparent garbage in the message buffer.

While we have had a fix in place (options PRINTF_BUFR_SIZE=128) to fix
scrambled console output, the message buffer and syslog were still getting
log messages one character at a time.  While all of the characters still
made it into the log (courtesy of atomic operations), they were often
interleaved when there were multiple threads writing to the buffer at the
same time.

This fixes message buffer accesses to use buffering logic as well, so that
strings that are less than PRINTF_BUFR_SIZE will be put into the message
buffer atomically.  So now dmesg output should look the same as console
output.

subr_msgbuf.c:		Convert most message buffer calls to use a new spin
			lock instead of atomic variables in some places.

			Add a new routine, msgbuf_addstr(), that adds a
			NUL-terminated string to a message buffer.  This
			takes a priority argument, which allows us to
			eliminate some races (at least in the the string
			at a time case) that are present in the
			implementation of msglogchar().  (dangling and
			lastpri are static variables, and are subject to
			races when multiple callers are present.)

			msgbuf_addstr() also allows the caller to request
			that carriage returns be stripped out of the
			string.  This matches the behavior of msglogchar(),
			but in testing so far it doesn't appear that any
			newlines are being stripped out.  So the carriage
			return removal functionality may be a candidate
			for removal later on if further analysis shows
			that it isn't necessary.

subr_prf.c:		Add a new msglogstr() routine that calls
			msgbuf_logstr().

			Rename putcons() to putbuf().  This now handles
			buffered output to the message log as well as
			the console.  Also, remove the logic in putcons()
			(now putbuf()) that added a carriage return before
			a newline.  The console path was the only path that
			needed it, and cnputc() (called by cnputs())
			already adds a carriage return.  So this
			duplication resulted in kernel-generated console
			output lines ending in '\r''\r''\n'.

			Refactor putchar() to handle the new buffering
			scheme.

			Add buffering to log().

			Change log_console() to use msglogstr() instead of
			msglogchar().  Don't add extra newlines by default
			in log_console().  Hide that behavior behind a
			tunable/sysctl (kern.log_console_add_linefeed) for
			those who would like the old behavior.  The old
			behavior led to the insertion of extra newlines
			for log output for programs that print out a
			string, and then a trailing newline on a separate
			write.  (This is visible with dmesg -a.)

msgbuf.h:		Add a prototype for msgbuf_addstr().

			Add three new fields to struct msgbuf, msg_needsnl,
			msg_lastpri and msg_lock.  The first two are needed
			for log message functionality previously handled
			by msglogchar().  (Which is still active if
			buffering isn't enabled.)

			Include sys/lock.h and sys/mutex.h for the new
			mutex.

Reviewed by:	gibbs

1 files changed, 171 insertions, 18 deletions

diff --git a/sys/kern/subr_msgbuf.c b/sys/kern/subr_msgbuf.c
index 14cd39d..c146ab0 100644
--- a/sys/kern/subr_msgbuf.c
+++ b/sys/kern/subr_msgbuf.c
@@ -31,8 +31,16 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/lock.h>
+#include <sys/mutex.h>
 #include <sys/msgbuf.h>
 
+/*
+ * Maximum number conversion buffer length: uintmax_t in base 2, plus <>
+ * around the priority, and a terminating NUL.
+ */
+#define	MAXPRIBUF	(sizeof(intmax_t) * NBBY + 3)
+
 /* Read/write sequence numbers are modulo a multiple of the buffer size. */
 #define SEQMOD(size) ((size) * 16)
 
@@ -51,6 +59,9 @@ msgbuf_init(struct msgbuf *mbp, void *ptr, int size)
 	mbp->msg_seqmod = SEQMOD(size);
 	msgbuf_clear(mbp);
 	mbp->msg_magic = MSG_MAGIC;
+	mbp->msg_lastpri = -1;
+	mbp->msg_needsnl = 0;
+	mtx_init(&mbp->msg_lock, "msgbuf", NULL, MTX_SPIN);
 }
 
 /*
@@ -80,6 +91,11 @@ msgbuf_reinit(struct msgbuf *mbp, void *ptr, int size)
 		}
 		msgbuf_clear(mbp);
 	}
+
+	mbp->msg_lastpri = -1;
+	/* Assume that the old message buffer didn't end in a newline. */
+	mbp->msg_needsnl = 1;
+	mtx_init(&mbp->msg_lock, "msgbuf", NULL, MTX_SPIN);
 }
 
 /*
@@ -110,25 +126,140 @@ msgbuf_getcount(struct msgbuf *mbp)
 }
 
 /*
- * Append a character to a message buffer.  This function can be
- * considered fully reentrant so long as the number of concurrent
- * callers is less than the number of characters in the buffer.
- * However, the message buffer is only guaranteed to be consistent
- * for reading when there are no callers in this function.
+ * Add a character into the message buffer, and update the checksum and
+ * sequence number.
+ *
+ * The caller should hold the message buffer spinlock.
+ */
+static inline void
+msgbuf_do_addchar(struct msgbuf *mbp, u_int *seq, int c)
+{
+	u_int pos;
+
+	/* Make sure we properly wrap the sequence number. */
+	pos = MSGBUF_SEQ_TO_POS(mbp, *seq);
+
+	mbp->msg_cksum += (u_int)c -
+	    (u_int)(u_char)mbp->msg_ptr[pos];
+
+	mbp->msg_ptr[pos] = c;
+
+	*seq = MSGBUF_SEQNORM(mbp, *seq + 1);
+}
+
+/*
+ * Append a character to a message buffer.
  */
 void
 msgbuf_addchar(struct msgbuf *mbp, int c)
 {
-	u_int new_seq, pos, seq;
-
-	do {
-		seq = mbp->msg_wseq;
-		new_seq = MSGBUF_SEQNORM(mbp, seq + 1);
-	} while (atomic_cmpset_rel_int(&mbp->msg_wseq, seq, new_seq) == 0);
-	pos = MSGBUF_SEQ_TO_POS(mbp, seq);
-	atomic_add_int(&mbp->msg_cksum, (u_int)(u_char)c -
-	    (u_int)(u_char)mbp->msg_ptr[pos]);
-	mbp->msg_ptr[pos] = c;
+	mtx_lock_spin(&mbp->msg_lock);
+
+	msgbuf_do_addchar(mbp, &mbp->msg_wseq, c);
+
+	mtx_unlock_spin(&mbp->msg_lock);
+}
+
+/*
+ * Append a NUL-terminated string with a priority to a message buffer.
+ * Filter carriage returns if the caller requests it.
+ *
+ * XXX The carriage return filtering behavior is present in the
+ * msglogchar() API, however testing has shown that we don't seem to send
+ * carriage returns down this path.  So do we still need it?
+ */
+void
+msgbuf_addstr(struct msgbuf *mbp, int pri, char *str, int filter_cr)
+{
+	u_int seq;
+	size_t len, prefix_len;
+	char prefix[MAXPRIBUF];
+	int nl, i;
+
+	len = strlen(str);
+	prefix_len = 0;
+	nl = 0;
+
+	/* If we have a zero-length string, no need to do anything. */
+	if (len == 0)
+		return;
+
+	mtx_lock_spin(&mbp->msg_lock);
+
+	/*
+	 * If this is true, we may need to insert a new priority sequence,
+	 * so prepare the prefix.
+	 */
+	if (pri != -1)
+		prefix_len = sprintf(prefix, "<%d>", pri);
+
+	/*
+	 * Starting write sequence number.
+	 */
+	seq = mbp->msg_wseq;
+
+	/*
+	 * Whenever there is a change in priority, we have to insert a
+	 * newline, and a priority prefix if the priority is not -1.  Here
+	 * we detect whether there was a priority change, and whether we
+	 * did not end with a newline.  If that is the case, we need to
+	 * insert a newline before this string.
+	 */
+	if (mbp->msg_lastpri != pri && mbp->msg_needsnl != 0) {
+
+		msgbuf_do_addchar(mbp, &seq, '\n');
+		mbp->msg_needsnl = 0;
+	}
+
+	for (i = 0; i < len; i++) {
+		/*
+		 * If we just had a newline, and the priority is not -1
+		 * (and therefore prefix_len != 0), then we need a priority
+		 * prefix for this line.
+		 */
+		if (mbp->msg_needsnl == 0 && prefix_len != 0) {
+			int j;
+
+			for (j = 0; j < prefix_len; j++)
+				msgbuf_do_addchar(mbp, &seq, prefix[j]);
+		}
+
+		/*
+		 * Don't copy carriage returns if the caller requested
+		 * filtering.
+		 * 
+		 * XXX This matches the behavior of msglogchar(), but is it
+		 * necessary?  Testing has shown that we don't seem to get
+		 * carriage returns here.
+		 */
+		if ((filter_cr != 0) && (str[i] == '\r'))
+			continue;
+
+		/*
+		 * Clear this flag if we see a newline.  This affects whether
+		 * we need to insert a new prefix or insert a newline later.
+		 */
+		if (str[i] == '\n')
+			mbp->msg_needsnl = 0;
+		else
+			mbp->msg_needsnl = 1;
+
+		msgbuf_do_addchar(mbp, &seq, str[i]);
+	}
+	/*
+	 * Update the write sequence number for the actual number of
+	 * characters we put in the message buffer.  (Depends on whether
+	 * carriage returns are filtered.)
+	 */
+	mbp->msg_wseq = seq;
+
+	/*
+	 * Set the last priority.
+	 */
+	mbp->msg_lastpri = pri;
+
+	mtx_unlock_spin(&mbp->msg_lock);
+
 }
 
 /*
@@ -141,14 +272,21 @@ msgbuf_getchar(struct msgbuf *mbp)
 	u_int len, wseq;
 	int c;
 
+	mtx_lock_spin(&mbp->msg_lock);
+
 	wseq = mbp->msg_wseq;
 	len = MSGBUF_SEQSUB(mbp, wseq, mbp->msg_rseq);
-	if (len == 0)
+	if (len == 0) {
+		mtx_unlock_spin(&mbp->msg_lock);
 		return (-1);
+	}
 	if (len > mbp->msg_size)
 		mbp->msg_rseq = MSGBUF_SEQNORM(mbp, wseq - mbp->msg_size);
 	c = (u_char)mbp->msg_ptr[MSGBUF_SEQ_TO_POS(mbp, mbp->msg_rseq)];
 	mbp->msg_rseq = MSGBUF_SEQNORM(mbp, mbp->msg_rseq + 1);
+
+	mtx_unlock_spin(&mbp->msg_lock);
+
 	return (c);
 }
 
@@ -161,10 +299,14 @@ msgbuf_getbytes(struct msgbuf *mbp, char *buf, int buflen)
 {
 	u_int len, pos, wseq;
 
+	mtx_lock_spin(&mbp->msg_lock);
+
 	wseq = mbp->msg_wseq;
 	len = MSGBUF_SEQSUB(mbp, wseq, mbp->msg_rseq);
-	if (len == 0)
+	if (len == 0) {
+		mtx_unlock_spin(&mbp->msg_lock);
 		return (0);
+	}
 	if (len > mbp->msg_size) {
 		mbp->msg_rseq = MSGBUF_SEQNORM(mbp, wseq - mbp->msg_size);
 		len = mbp->msg_size;
@@ -175,6 +317,9 @@ msgbuf_getbytes(struct msgbuf *mbp, char *buf, int buflen)
 
 	bcopy(&mbp->msg_ptr[pos], buf, len);
 	mbp->msg_rseq = MSGBUF_SEQNORM(mbp, mbp->msg_rseq + len);
+
+	mtx_unlock_spin(&mbp->msg_lock);
+
 	return (len);
 }
 
@@ -193,16 +338,21 @@ msgbuf_peekbytes(struct msgbuf *mbp, char *buf, int buflen, u_int *seqp)
 {
 	u_int len, pos, wseq;
 
+	mtx_lock_spin(&mbp->msg_lock);
+
 	if (buf == NULL) {
 		/* Just initialise *seqp. */
 		*seqp = MSGBUF_SEQNORM(mbp, mbp->msg_wseq - mbp->msg_size);
+		mtx_unlock_spin(&mbp->msg_lock);
 		return (0);
 	}
 
 	wseq = mbp->msg_wseq;
 	len = MSGBUF_SEQSUB(mbp, wseq, *seqp);
-	if (len == 0)
+	if (len == 0) {
+		mtx_unlock_spin(&mbp->msg_lock);
 		return (0);
+	}
 	if (len > mbp->msg_size) {
 		*seqp = MSGBUF_SEQNORM(mbp, wseq - mbp->msg_size);
 		len = mbp->msg_size;
@@ -212,6 +362,9 @@ msgbuf_peekbytes(struct msgbuf *mbp, char *buf, int buflen, u_int *seqp)
 	len = min(len, (u_int)buflen);
 	bcopy(&mbp->msg_ptr[MSGBUF_SEQ_TO_POS(mbp, *seqp)], buf, len);
 	*seqp = MSGBUF_SEQNORM(mbp, *seqp + len);
+
+	mtx_unlock_spin(&mbp->msg_lock);
+
 	return (len);
 }