selftests/bpf: Fix a compile error for BPF_F_BPRM_SECUREEXEC

When CONFIG_BPF_LSM is not configured, running bpf selftesting will show
BPF_F_BPRM_SECUREEXEC undefined error for bprm_opts.c.

The problem is that bprm_opts.c includes vmliunx.h. The vmlinux.h is
generated by "bpftool btf dump file ./vmlinux format c". On the other
hand, BPF_F_BPRM_SECUREEXEC is defined in include/uapi/linux/bpf.h
and used only in bpf_lsm.c. When CONFIG_BPF_LSM is not set, bpf_lsm
will not be compiled, so vmlinux.h will not include definition of
BPF_F_BPRM_SECUREEXEC.

Ideally, we want to compile bpf selftest regardless of the configuration
setting, so change the include file from vmlinux.h to bpf.h.

Signed-off-by: Jiang Wang <jiang.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20201224011242.585967-1-jiang.wang@bytedance.com

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