selftests/bpf: Fix two net related test failures with 64K page size

When running BPF selftests on arm64 with a 64K page size, I encountered
the following two test failures:
  sockmap_basic/sockmap skb_verdict change tail:FAIL
  tc_change_tail:FAIL

With further debugging, I identified the root cause in the following
kernel code within __bpf_skb_change_tail():

    u32 max_len = BPF_SKB_MAX_LEN;
    u32 min_len = __bpf_skb_min_len(skb);
    int ret;

    if (unlikely(flags || new_len > max_len || new_len < min_len))
        return -EINVAL;

With a 4K page size, new_len = 65535 and max_len = 16064, the function
returns -EINVAL. However, With a 64K page size, max_len increases to
261824, allowing execution to proceed further in the function. This is
because BPF_SKB_MAX_LEN scales with the page size and larger page sizes
result in higher max_len values.

Updating the new_len parameter in both tests based on actual kernel
page size resolved both failures.

Signed-off-by: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Yonghong Song

tools/testing/selftests/bpf/progs/test_sockmap_change_tail.c

@@ -1,8 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2024 ByteDance */
-#include <linux/bpf.h>
+#include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
 
+#ifndef PAGE_SIZE
+#define PAGE_SIZE __PAGE_SIZE
+#endif
+#define BPF_SKB_MAX_LEN (PAGE_SIZE << 2)
+
 struct {
 	__uint(type, BPF_MAP_TYPE_SOCKMAP);
 	__uint(max_entries, 1);
@@ -31,7 +36,7 @@ int prog_skb_verdict(struct __sk_buff *skb)
 		change_tail_ret = bpf_skb_change_tail(skb, skb->len + 1, 0);
 		return SK_PASS;
 	} else if (data[0] == 'E') { /* Error */
-		change_tail_ret = bpf_skb_change_tail(skb, 65535, 0);
+		change_tail_ret = bpf_skb_change_tail(skb, BPF_SKB_MAX_LEN, 0);
 		return SK_PASS;
 	}
 	return SK_PASS;

tools/testing/selftests/bpf/progs/test_tc_change_tail.c

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: GPL-2.0
-#include <linux/bpf.h>
+#include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
-#include <linux/if_ether.h>
-#include <linux/in.h>
-#include <linux/ip.h>
-#include <linux/udp.h>
-#include <linux/pkt_cls.h>
+
+#ifndef PAGE_SIZE
+#define PAGE_SIZE __PAGE_SIZE
+#endif
+#define BPF_SKB_MAX_LEN (PAGE_SIZE << 2)
 
 long change_tail_ret = 1;
 
@@ -94,7 +94,7 @@ int change_tail(struct __sk_buff *skb)
 			bpf_skb_change_tail(skb, len, 0);
 		return TCX_PASS;
 	} else if (payload[0] == 'E') { /* Error */
-		change_tail_ret = bpf_skb_change_tail(skb, 65535, 0);
+		change_tail_ret = bpf_skb_change_tail(skb, BPF_SKB_MAX_LEN, 0);
 		return TCX_PASS;
 	} else if (payload[0] == 'Z') { /* Zero */
 		change_tail_ret = bpf_skb_change_tail(skb, 0, 0);