Re: [PATCH net-next v4 10/11] net-timestamp: export the tskey for TCP bpf extension

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

On 1/7/25 8:21 PM, Jason Xing wrote:
> Hi Martin,
> 
>>> -     bpf_skops_tx_timestamping(sk, skb, op, 2, args);
>>> +     if (sk_is_tcp(sk))
>>> +             args[2] = skb_shinfo(skb)->tskey;
>>
>> Instead of only passing one info "skb_shinfo(skb)->tskey" of a skb, pass the
>> whole skb ptr to the bpf prog. Take a look at bpf_skops_init_skb. Lets start
>> with end_offset = 0 for now so that the bpf prog won't use it to read the
>> skb->data. It can be revisited later.
>>
>>          bpf_skops_init_skb(&sock_ops, skb, 0);
>>
>> The bpf prog can use bpf_cast_to_kern_ctx() and bpf_core_cast() to get to the
>> skb_shinfo(skb). Take a look at the md_skb example in type_cast.c.
> 
> In recent days, I've been working on this part. It turns out to be
> infeasible to pass "struct __sk_buff *skb" as the second parameter in
> skops_sockopt() in patch [11/11]. I cannot find a way to acquire the
> skb itself

I didn't mean to pass skb in sock_ops_kern->args[] or pass skb to the bpf prog
"SEC("sockops") skops_sockopt(struct bpf_sock_ops *skops, struct sk_buff *skb)".
The bpf prog can only take one ctx argument which is
"struct bpf_sock_ops *skops" here.

I meant to have kernel initializing the sock_ops_kern->skb by doing
"bpf_skops_init_skb(&sock_ops, skb, 0);" before running the bpf prog.

The bpf prog can read the skb by using bpf_cast_to_kern_ctx() and bpf_core_cast().

Something like the following. I directly change the existing test_tcp_hdr_options.c.
It has not been changed to use the vmlinux.h, so I need to redefine some parts of
the sk_buff, skb_shared_info, and bpf_sock_ops_kern. Your new test should directly
include <vmlinux.h> and no need to redefine them.

Untested code:

diff --git i/tools/testing/selftests/bpf/progs/test_tcp_hdr_options.c w/tools/testing/selftests/bpf/progs/test_tcp_hdr_options.c
index 5f4e87ee949a..c98ebe71f6ba 100644
--- i/tools/testing/selftests/bpf/progs/test_tcp_hdr_options.c
+++ w/tools/testing/selftests/bpf/progs/test_tcp_hdr_options.c
@@ -12,8 +12,10 @@
  #include <linux/types.h>
  #include <bpf/bpf_helpers.h>
  #include <bpf/bpf_endian.h>
+#include <bpf/bpf_core_read.h>
  #define BPF_PROG_TEST_TCP_HDR_OPTIONS
  #include "test_tcp_hdr_options.h"
+#include "bpf_kfuncs.h"
  
  #ifndef sizeof_field
  #define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))
@@ -348,9 +350,63 @@ static int current_mss_opt_len(struct bpf_sock_ops *skops)
  	return CG_OK;
  }
  
+struct sk_buff {
+	unsigned int		end;
+	unsigned char		*head;
+} __attribute__((preserve_access_index));
+
+struct skb_shared_info {
+	__u8		flags;
+	__u8		meta_len;
+	__u8		nr_frags;
+	__u8		tx_flags;
+	unsigned short	gso_size;
+	unsigned short	gso_segs;
+	unsigned int	gso_type;
+	__u32		tskey;
+} __attribute__((preserve_access_index));
+
+struct bpf_sock_ops_kern {
+	struct	sock *sk;
+	union {
+		__u32 args[4];
+		__u32 reply;
+		__u32 replylong[4];
+	};
+	struct sk_buff	*syn_skb;
+	struct sk_buff	*skb;
+	void	*skb_data_end;
+	__u8	op;
+	__u8	is_fullsock;
+	__u8	remaining_opt_len;
+	__u64	temp;			/* temp and everything after is not
+					 * initialized to 0 before calling
+					 * the BPF program. New fields that
+					 * should be initialized to 0 should
+					 * be inserted before temp.
+					 * temp is scratch storage used by
+					 * sock_ops_convert_ctx_access
+					 * as temporary storage of a register.
+					 */
+} __attribute__((preserve_access_index));
+
  static int handle_hdr_opt_len(struct bpf_sock_ops *skops)
  {
  	__u8 tcp_flags = skops_tcp_flags(skops);
+	struct bpf_sock_ops_kern *skops_kern;
+	struct skb_shared_info *shared_info;
+	struct sk_buff *skb;
+
+	skops_kern = bpf_cast_to_kern_ctx(skops);
+	skb = skops_kern->skb;
+
+	if (skb) {
+		shared_info = bpf_core_cast(skb->head + skb->end, struct skb_shared_info);
+		/* printk as an example. don't do that in selftests. */
+		bpf_printk("tskey %u gso_size %u gso_segs %u gso_type %u flags %x\n",
+			   shared_info->tskey, shared_info->gso_size,
+			   shared_info->gso_segs, shared_info->gso_type, shared_info->flags);
+	}
  
  	if ((tcp_flags & TCPHDR_SYNACK) == TCPHDR_SYNACK)
  		return synack_opt_len(skops);