| Message ID | 20230516214945.1013578-1-yhs@fb.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | de58ef414d8d7a0a635cd331b3b013d8216c4e60 |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf-next] selftests/bpf: Fix s390 sock_field test failure | expand |
Hello: This patch was applied to bpf/bpf-next.git (master) by Alexei Starovoitov <ast@kernel.org>: On Tue, 16 May 2023 14:49:45 -0700 you wrote: > 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. > > [...] Here is the summary with links: - [bpf-next] selftests/bpf: Fix s390 sock_field test failure https://git.kernel.org/bpf/bpf-next/c/de58ef414d8d You are awesome, thank you!
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); }
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(-)