selftests/bpf: Move sys_pidfd_open() into task_local_storage_helpers.h

sys_pidfd_open() is defined twice in both test_bprm_opts.c and
test_local_storage.c, so move it to a common header file. And it will be
used in map_tests as well.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20220901061938.3789460-4-houtao@huaweicloud.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

bpf: Add tests for bpf_bprm_opts_set helper

The test forks a child process, updates the local storage to set/unset
the securexec bit.

The BPF program in the test attaches to bprm_creds_for_exec which checks
the local storage of the current task to set the secureexec bit on the
binary parameters (bprm).

The child then execs a bash command with the environment variable
TMPDIR set in the envp. The bash command returns a different exit code
based on its observed value of the TMPDIR variable.

Since TMPDIR is one of the variables that is ignored by the dynamic
loader when the secureexec bit is set, one should expect the
child execution to not see this value when the secureexec bit is set.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20201117232929.2156341-2-kpsingh@chromium.org