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

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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 <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250612035037.2207911-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Yonghong Song
2025-06-12 19:07:51 -07:00
committed by Alexei Starovoitov
parent 4fc012daf9
commit 96fcf7e7a7
2 changed files with 14 additions and 9 deletions
Download Patch File Download Diff File Expand all files Collapse all files
tools/testing/selftests/bpf/progs/test_sockmap_change_tail.c
+7 -2
View File
@@ -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
+7 -7
View File
@@ -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);