From: Yonghong Song <yhs@fb.com>
To: <bpf@vger.kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>, <kernel-team@fb.com>,
Martin KaFai Lau <martin.lau@kernel.org>,
Ilya Leoshkevich <iii@linux.ibm.com>
Subject: [PATCH bpf-next] selftests/bpf: Fix s390 sock_field test failure
Date: Tue, 16 May 2023 14:49:45 -0700 [thread overview]
Message-ID: <20230516214945.1013578-1-yhs@fb.com> (raw)
llvm patch [1] enabled cross-function optimization for func arguments
(ArgumentPromotion) at -O2 level. And this caused s390 sock_fields
test failure ([2]). The failure is gone right now as patch [1] was
reverted in [3]. But it is possible that patch [3] will be reverted
again and then the test failure in [2] will show up again. So it is
desirable to fix the failure regardless.
The following is an analysis why sock_field test fails with
llvm patch [1].
The main problem is in
static __noinline bool sk_dst_port__load_word(struct bpf_sock *sk)
{
__u32 *word = (__u32 *)&sk->dst_port;
return word[0] == bpf_htons(0xcafe);
}
static __noinline bool sk_dst_port__load_half(struct bpf_sock *sk)
{
__u16 *half = (__u16 *)&sk->dst_port;
return half[0] == bpf_htons(0xcafe);
}
...
int read_sk_dst_port(struct __sk_buff *skb)
{
...
sk = skb->sk;
...
if (!sk_dst_port__load_word(sk))
RET_LOG();
if (!sk_dst_port__load_half(sk))
RET_LOG();
...
}
Through some cross-function optimization by ArgumentPromotion
optimization, the compiler does:
static __noinline bool sk_dst_port__load_word(__u32 word_val)
{
return word_val == bpf_htons(0xcafe);
}
static __noinline bool sk_dst_port__load_half(__u16 half_val)
{
return half_val == bpf_htons(0xcafe);
}
...
int read_sk_dst_port(struct __sk_buff *skb)
{
...
sk = skb->sk;
...
__u32 *word = (__u32 *)&sk->dst_port;
__u32 word_val = word[0];
...
if (!sk_dst_port__load_word(word_val))
RET_LOG();
__u16 half_val = word_val >> 16;
if (!sk_dst_port__load_half(half_val))
RET_LOG();
...
}
In current uapi bpf.h, we have
struct bpf_sock {
...
__be16 dst_port; /* network byte order */
__u16 :16; /* zero padding */
...
};
But the old kernel (e.g., 5.6) we have
struct bpf_sock {
...
__u32 dst_port; /* network byte order */
...
};
So for backward compatability reason, 4-byte load of
dst_port is converted to 2-byte load internally.
Specifically, 'word_val = word[0]' is replaced by 2-byte load
by the verifier and this caused the trouble for later
sk_dst_port__load_half() where half_val becomes 0.
Typical usr program won't have such a code pattern tiggering
the above bug, so let us fix the test failure with source
code change. Adding an empty asm volatile statement seems
enough to prevent undesired transformation.
[1] https://reviews.llvm.org/D148269
[2] https://lore.kernel.org/bpf/e7f2c5e8-a50c-198d-8f95-388165f1e4fd@meta.com/
[3] https://reviews.llvm.org/rG141be5c062ecf22bd287afffd310e8ac4711444a
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
tools/testing/selftests/bpf/progs/test_sock_fields.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/tools/testing/selftests/bpf/progs/test_sock_fields.c b/tools/testing/selftests/bpf/progs/test_sock_fields.c
index bbad3c2d9aa5..f75e531bf36f 100644
--- a/tools/testing/selftests/bpf/progs/test_sock_fields.c
+++ b/tools/testing/selftests/bpf/progs/test_sock_fields.c
@@ -265,7 +265,10 @@ static __noinline bool sk_dst_port__load_word(struct bpf_sock *sk)
static __noinline bool sk_dst_port__load_half(struct bpf_sock *sk)
{
- __u16 *half = (__u16 *)&sk->dst_port;
+ __u16 *half;
+
+ asm volatile ("");
+ half = (__u16 *)&sk->dst_port;
return half[0] == bpf_htons(0xcafe);
}
--
2.34.1
next reply other threads:[~2023-05-16 21:50 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-16 21:49 Yonghong Song [this message]
2023-05-17 4:00 ` [PATCH bpf-next] selftests/bpf: Fix s390 sock_field test failure patchwork-bot+netdevbpf
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