bpf: fix accessing bpf_sysctl.file_pos on s390
"ctx:file_pos sysctl:read write ok" fails on s390 with "Read value != nux". This is because verifier rewrites a complete 32-bit bpf_sysctl.file_pos update to a partial update of the first 32 bits of 64-bit *bpf_sysctl_kern.ppos, which is not correct on big-endian systems. Fix by using an offset on big-endian systems. Ditto for bpf_sysctl.file_pos reads. Currently the test does not detect a problem there, since it expects to see 0, which it gets with high probability in error cases, so change it to seek to offset 3 and expect 3 in bpf_sysctl.file_pos. Fixes: e1550bfe ("bpf: Add file_pos field to bpf_sysctl ctx") Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com> Acked-by: Yonghong Song <yhs@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20190816105300.49035-1-iii@linux.ibm.com/
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