Cast operation has a higher precedence than addition. The code here wants to zero the 2nd half of the 64-bit metadata, but due to a pointer arithmetic mistake, it writes the zero at offset 16 instead.
Just adding parentheses around "data + 4" would fix this, but I think this will be slightly better readable with array syntax.
I was unable to test this with tools/testing/selftests/bpf/vmtest.sh, because my glibc is newer than glibc in the provided VM image. So I just checked the difference in the compiled code. objdump -S tools/testing/selftests/bpf/xdp_do_redirect.test.o: - *((__u32 *)data) = 0x42; /* metadata test value */ + ((__u32 *)data)[0] = 0x42; /* metadata test value */ be7: 48 8d 85 30 fc ff ff lea -0x3d0(%rbp),%rax bee: c7 00 42 00 00 00 movl $0x42,(%rax) - *((__u32 *)data + 4) = 0; + ((__u32 *)data)[1] = 0; bf4: 48 8d 85 30 fc ff ff lea -0x3d0(%rbp),%rax - bfb: 48 83 c0 10 add $0x10,%rax + bfb: 48 83 c0 04 add $0x4,%rax bff: c7 00 00 00 00 00 movl $0x0,(%rax)
Fixes: 5640b6d89434 ("selftests/bpf: fix "metadata marker" getting overwritten by the netstack") Signed-off-by: Michal Schmidt mschmidt@redhat.com --- tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c b/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c index 498d3bdaa4b0..bad0ea167be7 100644 --- a/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c +++ b/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c @@ -107,8 +107,8 @@ void test_xdp_do_redirect(void) .attach_point = BPF_TC_INGRESS);
memcpy(&data[sizeof(__u64)], &pkt_udp, sizeof(pkt_udp)); - *((__u32 *)data) = 0x42; /* metadata test value */ - *((__u32 *)data + 4) = 0; + ((__u32 *)data)[0] = 0x42; /* metadata test value */ + ((__u32 *)data)[1] = 0;
skel = test_xdp_do_redirect__open(); if (!ASSERT_OK_PTR(skel, "skel"))
Michal Schmidt mschmidt@redhat.com writes:
Cast operation has a higher precedence than addition. The code here wants to zero the 2nd half of the 64-bit metadata, but due to a pointer arithmetic mistake, it writes the zero at offset 16 instead.
Just adding parentheses around "data + 4" would fix this, but I think this will be slightly better readable with array syntax.
I was unable to test this with tools/testing/selftests/bpf/vmtest.sh, because my glibc is newer than glibc in the provided VM image. So I just checked the difference in the compiled code. objdump -S tools/testing/selftests/bpf/xdp_do_redirect.test.o:
- *((__u32 *)data) = 0x42; /* metadata test value */
- ((__u32 *)data)[0] = 0x42; /* metadata test value */ be7: 48 8d 85 30 fc ff ff lea -0x3d0(%rbp),%rax bee: c7 00 42 00 00 00 movl $0x42,(%rax)
- *((__u32 *)data + 4) = 0;
- ((__u32 *)data)[1] = 0; bf4: 48 8d 85 30 fc ff ff lea -0x3d0(%rbp),%rax
bfb: 48 83 c0 10 add $0x10,%rax
bfb: 48 83 c0 04 add $0x4,%rax bff: c7 00 00 00 00 00 movl $0x0,(%rax)Fixes: 5640b6d89434 ("selftests/bpf: fix "metadata marker" getting overwritten by the netstack") Signed-off-by: Michal Schmidt mschmidt@redhat.com
Reviewed-by: Toke Høiland-Jørgensen toke@redhat.com
Hello:
This patch was applied to bpf/bpf-next.git (master) by Andrii Nakryiko andrii@kernel.org:
On Mon, 6 May 2024 16:50:22 +0200 you wrote:
Cast operation has a higher precedence than addition. The code here wants to zero the 2nd half of the 64-bit metadata, but due to a pointer arithmetic mistake, it writes the zero at offset 16 instead.
Just adding parentheses around "data + 4" would fix this, but I think this will be slightly better readable with array syntax.
[...]
Here is the summary with links: - [net] selftests/bpf: fix pointer arithmetic in test_xdp_do_redirect https://git.kernel.org/bpf/bpf-next/c/e549b39a0ab8
You are awesome, thank you!
From: Michal Schmidt mschmidt@redhat.com Date: Mon, 6 May 2024 16:50:22 +0200
Cast operation has a higher precedence than addition. The code here wants to zero the 2nd half of the 64-bit metadata, but due to a pointer arithmetic mistake, it writes the zero at offset 16 instead.
Just adding parentheses around "data + 4" would fix this, but I think this will be slightly better readable with array syntax.
I was unable to test this with tools/testing/selftests/bpf/vmtest.sh, because my glibc is newer than glibc in the provided VM image. So I just checked the difference in the compiled code. objdump -S tools/testing/selftests/bpf/xdp_do_redirect.test.o:
- *((__u32 *)data) = 0x42; /* metadata test value */
- ((__u32 *)data)[0] = 0x42; /* metadata test value */ be7: 48 8d 85 30 fc ff ff lea -0x3d0(%rbp),%rax bee: c7 00 42 00 00 00 movl $0x42,(%rax)
- *((__u32 *)data + 4) = 0;
- ((__u32 *)data)[1] = 0; bf4: 48 8d 85 30 fc ff ff lea -0x3d0(%rbp),%rax
bfb: 48 83 c0 10 add $0x10,%rax
bfb: 48 83 c0 04 add $0x4,%rax bff: c7 00 00 00 00 00 movl $0x0,(%rax)Fixes: 5640b6d89434 ("selftests/bpf: fix "metadata marker" getting overwritten by the netstack") Signed-off-by: Michal Schmidt mschmidt@redhat.com
tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c b/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c index 498d3bdaa4b0..bad0ea167be7 100644 --- a/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c +++ b/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c @@ -107,8 +107,8 @@ void test_xdp_do_redirect(void) .attach_point = BPF_TC_INGRESS); memcpy(&data[sizeof(__u64)], &pkt_udp, sizeof(pkt_udp));
- *((__u32 *)data) = 0x42; /* metadata test value */
- *((__u32 *)data + 4) = 0;
- ((__u32 *)data)[0] = 0x42; /* metadata test value */
- ((__u32 *)data)[1] = 0;
Uff sorry. I was a bit in hurry to fix BPF CI and did this braino :z I know pointer arithms obviously :D I'd give a Rev-by, but it was applied already. Thanks!
skel = test_xdp_do_redirect__open(); if (!ASSERT_OK_PTR(skel, "skel"))
Thanks, Olek
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