[PATCH] Wrong printk return value

[Guillaume Chazarain]

What's the true meaning of the printk return value?
Should it include the prioty prefix length of 3? and what about the timing
information? In both cases it was broken:

strace -e write echo 1 > /dev/kmsg
=> write(1, "1\n", 2)                      = 5

strace -e write echo "<1>1" > /dev/kmsg
=> write(1, "<1>1\n", 5)                   = 8

The returned length was "length of input string + 3", I made it "length
of printed string including any prefix".

Successful printk calls can have a return value different than the input
length (priority prefix, timing), so /bin/echo will still think there is 
an error.
So, to avoid breaking programs that assume write(buff, len) != len is an
error, /dev/kmsg should return the buffer length when the call succeeds.

The only drawback is that now it's no more possible to use /dev/kmsg to
check the printk return value :-(

--
Guillaume

--- linux-2.6.13-rc5/kernel/printk.c
+++ linux-2.6.13-rc5/kernel/printk.c
@@ -553,7 +553,7 @@
 				   p[1] <= '7' && p[2] == '>') {
 					loglev_char = p[1];
 					p += 3;
-					printed_len += 3;
+					printed_len -= 3;
 				} else {
 					loglev_char = default_message_loglevel
 						+ '0';
@@ -568,7 +568,7 @@
 
 				for (tp = tbuf; tp < tbuf + tlen; tp++)
 					emit_log_char(*tp);
-				printed_len += tlen - 3;
+				printed_len += tlen;
 			} else {
 				if (p[0] != '<' || p[1] < '0' ||
 				   p[1] > '7' || p[2] != '>') {
@@ -576,8 +576,8 @@
 					emit_log_char(default_message_loglevel
 						+ '0');
 					emit_log_char('>');
+					printed_len += 3;
 				}
-				printed_len += 3;
 			}
 			log_level_unknown = 0;
 			if (!*p)
--- linux-2.6.13-rc5/drivers/char/mem.c
+++ linux-2.6.13-rc5/drivers/char/mem.c
@@ -819,7 +819,7 @@ static ssize_t kmsg_write(struct file * 
 			  size_t count, loff_t *ppos)
 {
 	char *tmp;
-	int ret;
+	ssize_t ret;
 
 	tmp = kmalloc(count + 1, GFP_KERNEL);
 	if (tmp == NULL)
@@ -828,6 +828,9 @@ static ssize_t kmsg_write(struct file * 
 	if (!copy_from_user(tmp, buf, count)) {
 		tmp[count] = 0;
 		ret = printk("%s", tmp);
+		if (ret > count)
+			/* printk can add a prefix */
+			ret = count;
 	}
 	kfree(tmp);
 	return ret;