[ Upstream commit e549b39a0a ]
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: 5640b6d894 ("selftests/bpf: fix "metadata marker" getting overwritten by the netstack")
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20240506145023.214248-1-mschmidt@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
c3ae972a22