[bpf-next,2/6] selftests/xsk: Add tail adjustment functionality to XDP

Commit Message

Tushar Vyavahare Feb. 20, 2025, 8:41 a.m. UTC

Introduce a new function, xsk_xdp_adjust_tail, within the XDP program to
adjust the tail of packets. This function utilizes bpf_xdp_adjust_tail to
modify the packet size dynamically based on the 'count' variable.

If the adjustment fails, the packet is dropped using XDP_DROP to ensure
processing of only correctly modified packets.

Signed-off-by: Tushar Vyavahare <[e-mail]>
---
 .../selftests/bpf/progs/xsk_xdp_progs.c       | 48 +++++++++++++++++++
 tools/testing/selftests/bpf/xsk_xdp_common.h  |  1 +
 2 files changed, 49 insertions(+)

Comments

Stanislav Fomichev Feb. 20, 2025, 5:46 p.m. UTC | #1

On 02/20, Tushar Vyavahare wrote:
> Introduce a new function, xsk_xdp_adjust_tail, within the XDP program to
> adjust the tail of packets. This function utilizes bpf_xdp_adjust_tail to
> modify the packet size dynamically based on the 'count' variable.
> 
> If the adjustment fails, the packet is dropped using XDP_DROP to ensure
> processing of only correctly modified packets.
> 
> Signed-off-by: Tushar Vyavahare <[e-mail]>

Any reason not to combine patches 2..5 into a single one? I looked
through each one briefly and it's a bit hard to follow when trying
to put everything together..

Patch

diff --git a/tools/testing/selftests/bpf/progs/xsk_xdp_progs.c b/tools/testing/selftests/bpf/progs/xsk_xdp_progs.c
index ccde6a4c6319..2e8e2faf17e0 100644
--- a/tools/testing/selftests/bpf/progs/xsk_xdp_progs.c
+++ b/tools/testing/selftests/bpf/progs/xsk_xdp_progs.c
@@ -4,6 +4,8 @@ 
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include <linux/if_ether.h>
+#include <linux/ip.h>
+#include <linux/errno.h>
 #include "xsk_xdp_common.h"
 
 struct {
@@ -70,4 +72,50 @@  SEC("xdp") int xsk_xdp_shared_umem(struct xdp_md *xdp)
 	return bpf_redirect_map(&xsk, idx, XDP_DROP);
 }
 
+SEC("xdp.frags") int xsk_xdp_adjust_tail(struct xdp_md *xdp)
+{
+	__u32 buff_len, curr_buff_len;
+	int ret;
+
+	buff_len = bpf_xdp_get_buff_len(xdp);
+	if (buff_len == 0)
+		return XDP_DROP;
+
+	ret = bpf_xdp_adjust_tail(xdp, count);
+	if (ret < 0) {
+		/* Handle unsupported cases */
+		if (ret == -EOPNOTSUPP) {
+			/* Set count to -EOPNOTSUPP to indicate to userspace that this case is
+			 * unsupported
+			 */
+			count = -EOPNOTSUPP;
+			return bpf_redirect_map(&xsk, 0, XDP_DROP);
+		}
+
+		return XDP_DROP;
+	}
+
+	curr_buff_len = bpf_xdp_get_buff_len(xdp);
+	if (curr_buff_len != buff_len + count)
+		return XDP_DROP;
+
+	if (curr_buff_len > buff_len) {
+		__u32 *pkt_data = (void *)(long)xdp->data;
+		__u32 len, words_to_end, seq_num;
+
+		len = curr_buff_len - PKT_HDR_ALIGN;
+		words_to_end = len / sizeof(*pkt_data) - 1;
+		seq_num = words_to_end;
+
+		/* Convert sequence number to network byte order. Store this in the last 4 bytes of
+		 * the packet. Use 'count' to determine the position at the end of the packet for
+		 * storing the sequence number.
+		 */
+		seq_num = __constant_htonl(words_to_end);
+		bpf_xdp_store_bytes(xdp, curr_buff_len - count, &seq_num, sizeof(seq_num));
+	}
+
+	return bpf_redirect_map(&xsk, 0, XDP_DROP);
+}
+
 char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/xsk_xdp_common.h b/tools/testing/selftests/bpf/xsk_xdp_common.h
index 5a6f36f07383..45810ff552da 100644
--- a/tools/testing/selftests/bpf/xsk_xdp_common.h
+++ b/tools/testing/selftests/bpf/xsk_xdp_common.h
@@ -4,6 +4,7 @@ 
 #define XSK_XDP_COMMON_H_
 
 #define MAX_SOCKETS 2
+#define PKT_HDR_ALIGN (sizeof(struct ethhdr) + 2) /* Just to align the data in the packet */
 
 struct xdp_info {
 	__u64 count;