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