Re: [PATCH net-next v2 09/12] net-timestamp: add tx OPT_ID_TCP support for bpf case

On Tue, Oct 15, 2024 at 9:38 AM Willem de Bruijn
[off-list ref] wrote:

Jason Xing wrote:

quoted

From: Jason Xing <kernelxing@tencent.com>

We can set OPT_ID|OPT_ID_TCP before we initialize the last skb
from each sendmsg. We only set the socket once like how we use
setsockopt() with OPT_ID|OPT_ID_TCP flags.

Note: we will check if non-bpf _and_ bpf sk_tsflags have OPT_ID
flag. If either of them has been set before, we will not initialize
the key any more,

Where and how is this achieved?

Please see this patch and you will find the following codes.
+       tsflags |= (sk->sk_tsflags[SOCKETOPT_TS_REQUESTOR] |
+                   sk->sk_tsflags[BPFPROG_TS_REQUESTOR]);

But the difference/problem is that the non-bpf feature only init it
when connect() is done, but the bpf feature could do it at the
beginning of connect(). If running txtimestamp -l 1000, the former
will generate 999 for turkey while the latter 1000.

Also be aware of the subtle distinction between passing OPT_ID_TCP
along with OPT_ID or not.