Re: [PATCH bpf-next v9 06/12] bpf: support SCM_TSTAMP_SCHED of SO_TIMESTAMPING

[Martin KaFai Lau <martin.lau@linux.dev>]

On 2/8/25 2:32 AM, Jason Xing wrote:
> Support SCM_TSTAMP_SCHED case. Introduce SKBTX_BPF used as
> an indicator telling us whether the skb should be traced
> by the bpf prog.

The BPF side does not exactly support SCM_TSTAMP_SCHED as a report value.

What this patch does is:

Add a new sock_ops callback, BPF_SOCK_OPS_TS_SCHED_OPT_CB. This callback will 
occur at the same timestamping point as the user space's SCM_TSTAMP_SCHED. The
BPF program can use it to get the same SCM_TSTAMP_SCHED timestamp without 
modifying the user-space application.

A new SKBTX_BPF flag is added to mark skb_shinfo(skb)->tx_flags, ensuring that 
the new BPF timestamping and the current user space's SO_TIMESTAMPING do not 
interfere with each other.

I would remove most of the SO_TIMESTAMPING comments from the commit messages. 
The timestamping points are the same but there is not much overlapping on the 
API side.

Subject could be:
bpf: Add BPF_SOCK_OPS_TS_SCHED_OPT_CB callback

[ The same probably for patch 7-9. ]

> 
> Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
> ---
>   include/linux/skbuff.h         |  6 +++++-
>   include/uapi/linux/bpf.h       |  4 ++++
>   net/core/dev.c                 |  3 ++-
>   net/core/skbuff.c              | 20 ++++++++++++++++++++
>   tools/include/uapi/linux/bpf.h |  4 ++++
>   5 files changed, 35 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index bb2b751d274a..52f6e033e704 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -489,10 +489,14 @@ enum {
>   
>   	/* generate software time stamp when entering packet scheduling */
>   	SKBTX_SCHED_TSTAMP = 1 << 6,
> +
> +	/* used for bpf extension when a bpf program is loaded */
> +	SKBTX_BPF = 1 << 7,
>   };
>   
>   #define SKBTX_ANY_SW_TSTAMP	(SKBTX_SW_TSTAMP    | \
> -				 SKBTX_SCHED_TSTAMP)
> +				 SKBTX_SCHED_TSTAMP | \
> +				 SKBTX_BPF)
>   #define SKBTX_ANY_TSTAMP	(SKBTX_HW_TSTAMP | \
>   				 SKBTX_HW_TSTAMP_USE_CYCLES | \
>   				 SKBTX_ANY_SW_TSTAMP)
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 6116eb3d1515..30d2c078966b 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7032,6 +7032,10 @@ enum {
>   					 * by the kernel or the
>   					 * earlier bpf-progs.
>   					 */
> +	BPF_SOCK_OPS_TS_SCHED_OPT_CB,	/* Called when skb is passing through
> +					 * dev layer when SK_BPF_CB_TX_TIMESTAMPING
> +					 * feature is on.
> +					 */
>   };
>   
>   /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect
> diff --git a/net/core/dev.c b/net/core/dev.c
> index afa2282f2604..d57946c96511 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4500,7 +4500,8 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
>   	skb_reset_mac_header(skb);
>   	skb_assert_len(skb);
>   
> -	if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_SCHED_TSTAMP))
> +	if (unlikely(skb_shinfo(skb)->tx_flags &
> +		     (SKBTX_SCHED_TSTAMP | SKBTX_BPF)))
>   		__skb_tstamp_tx(skb, NULL, NULL, skb->sk, SCM_TSTAMP_SCHED);
>   
>   	/* Disable soft irqs for various locks below. Also
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 46530d516909..6f55eb90a632 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5555,6 +5555,23 @@ static bool skb_tstamp_tx_report_so_timestamping(struct sk_buff *skb,
>   	return false;
>   }
>   
> +static void skb_tstamp_tx_report_bpf_timestamping(struct sk_buff *skb,
> +						  struct sock *sk,
> +						  int tstype)
> +{
> +	int op;
> +
> +	switch (tstype) {
> +	case SCM_TSTAMP_SCHED:
> +		op = BPF_SOCK_OPS_TS_SCHED_OPT_CB;
> +		break;
> +	default:
> +		return;
> +	}
> +
> +	bpf_skops_tx_timestamping(sk, skb, op);
> +}
> +
>   void __skb_tstamp_tx(struct sk_buff *orig_skb,
>   		     const struct sk_buff *ack_skb,
>   		     struct skb_shared_hwtstamps *hwtstamps,
> @@ -5567,6 +5584,9 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
>   	if (!sk)
>   		return;
>   
> +	if (skb_shinfo(orig_skb)->tx_flags & SKBTX_BPF)
> +		skb_tstamp_tx_report_bpf_timestamping(orig_skb, sk, tstype);
> +
>   	if (!skb_tstamp_tx_report_so_timestamping(orig_skb, tstype, sw))
>   		return;
>   
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 70366f74ef4e..eed91b7296b7 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -7025,6 +7025,10 @@ enum {
>   					 * by the kernel or the
>   					 * earlier bpf-progs.
>   					 */
> +	BPF_SOCK_OPS_TS_SCHED_OPT_CB,	/* Called when skb is passing through
> +					 * dev layer when SK_BPF_CB_TX_TIMESTAMPING
> +					 * feature is on.
> +					 */
>   };
>   
>   /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect