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;
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