On 11/1/24 6:32 AM, Willem de Bruijn wrote:
In udp/raw/..., I don't know how likely is the user space having "cork->tx_flags
& SKBTX_ANY_TSTAMP" set but has neither "READ_ONCE(sk->sk_tsflags) &
SOF_TIMESTAMPING_OPT_ID" nor "cork->flags & IPCORK_TS_OPT_ID" set.
This is not something to rely on. OPT_ID was added relatively recently.
Older applications, or any that just use the most straightforward API,
will not set this.
Good point that the OPT_ID per cmsg is very new.
The datagram support on SOF_TIMESTAMPING_OPT_ID in sk->sk_tsflags had
been there for quite some time now. Is it a safe assumption that
most applications doing udp tx timestamping should have
the SOF_TIMESTAMPING_OPT_ID set to be useful?
If it is
unlikely, may be we can just disallow bpf prog from directly setting
skb_shinfo(skb)->tskey for this particular skb.
For all other cases, in __ip[6]_append_data, directly call a bpf prog and also
pass the kernel decided tskey to the bpf prog.
The kernel passed tskey could be 0 (meaning the user space has not used it). The
bpf prog can give one for the kernel to use. The bpf prog can store the
sk_tskey_bpf in the bpf_sk_storage now. Meaning no need to add one to the struct
sock. The bpf prog does not have to start from 0 (e.g. start from U32_MAX
instead) if it helps.
If the kernel passed tskey is not 0, the bpf prog can just use that one
(assuming the user space is doing something sane, like the value in
SCM_TS_OPT_ID won't be jumping back and front between 0 to U32_MAX). I hope this
is very unlikely also (?) but the bpf prog can probably detect this and choose
to ignore this sk.
If an applications uses OPT_ID, it is unlikely that they will toggle
the feature on and off on a per-packet basis. So in the common case
the program could use the user-set counter or use its own if userspace
does not enable the feature. In the rare case that an application does
intermittently set an OPT_ID, the numbering would be erratic. This
does mean that an actively malicious application could mess with admin
measurements.
All make sense. Given it is reasonable to assume the user space should either
has SOF_TIMESTAMPING_OPT_ID always on or always off. When it is off, the bpf
prog can directly provide its own tskey to be used in shinfo->tskey. The bpf
prog can generate the id itself without using the sk->sk_tskey, e.g. store an
atomic int in the bpf_sk_storage.
