Subject: sock_get_timestamp() ktime_to_timeval returns -2?

From: "Andrew Brampton" <[email-masked]>
Date: Fri, 4 Apr 2008 19:38:42 +0100

Hi,
I'm using the following piece of code to record the received time of my 
packets.

struct timeval tv = {0,0};
if ( ioctl(s, SIOCGSTAMP, &tv) )
   return 0;

When I use UDP this is all great, but when I use TCP this stops working. I 
have since found out that I can't use this for TCP packets[1], but the 
reason I'm writing this email is because ioctl returns zero when using TCP 
and tv has a odd value in it. tv.tv_sec = -2, and tv.tv_usec = 999999.

Now I assume -2 is some kind of error code, so I dug into the linux code to 
try and figure out what it means. The ioctl eventually calls 
sock_get_timestamp() which in turn calls ktime_to_timeval. I can see in 
sock_get_timestamp() that tv_sec is compared to -1 and ioctl returns an 
error, however I can not find where tv_sec is set to -2. If -2 is another 
error code then it should be checked inside sock_get_timestamp().

So I'm wondering if this is a bug in the kernel somewhere, or should I just 
expect ioctl to fail yet return 0? I have not included a test app, but if 
you want I'll be happy to code a short app to show this problem, but the 
critital lines are the ioctl call and that a TCP socket is used. This occurs 
on both the 2.6.22-3-amd64 kernel as well as on 2.6.24.

Thanks
Andrew

[1] http://www.ussg.iu.edu/hypermail/linux/kernel/0301.2/1248.html