selftests/bpf: fix lots of silly mistakes pointed out by compiler

Once we enable -Wall for BPF sources, compiler will complain about lots
of unused variables, variables that are set but never read, etc.

Fix all these issues first before enabling -Wall in Makefile.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230309054015.4068562-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Use __NR_prlimit64 instead of __NR_getrlimit in user_ringbuf test

After commit 80d7da1cac62 ("asm-generic: Drop getrlimit and setrlimit
syscalls from default list"), new architectures won't need to include
getrlimit and setrlimit, they are superseded with prlimit64.

In order to maintain compatibility for the new architectures, such as
LoongArch which does not define __NR_getrlimit, it is better to use
__NR_prlimit64 instead of __NR_getrlimit in user_ringbuf test to fix
the following build error:

TEST-OBJ [test_progs] user_ringbuf.test.o
tools/testing/selftests/bpf/prog_tests/user_ringbuf.c: In function 'kick_kernel_cb':
tools/testing/selftests/bpf/prog_tests/user_ringbuf.c:593:17: error: '__NR_getrlimit' undeclared (first use in this function)
593 | syscall(__NR_getrlimit);
| ^~~~~~~~~~~~~~
tools/testing/selftests/bpf/prog_tests/user_ringbuf.c:593:17: note: each undeclared identifier is reported only once for each function it appears in
make: *** [Makefile:573: tools/testing/selftests/bpf/user_ringbuf.test.o] Error 1
make: Leaving directory 'tools/testing/selftests/bpf'

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1677235015-21717-4-git-send-email-yangtiezhu@loongson.cn

selftests/bpf: Make bpf_user_ringbuf_drain() selftest callback return 1

In commit 1bfe26fb0827 ("bpf: Add verifier support for custom callback
return range"), the verifier was updated to require callbacks to BPF
helpers to explicitly specify the range of values that can be returned.
bpf_user_ringbuf_drain() was merged after this in commit 205715673844
("bpf: Add bpf_user_ringbuf_drain() helper"), and this change in default
behavior was missed. This patch updates the BPF_MAP_TYPE_USER_RINGBUF
selftests to also return 1 from a bpf_user_ringbuf_drain() callback so
as to properly test this going forward.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221012232015.1510043-3-void@manifault.com

selftests/bpf: Add selftests validating the user ringbuf

This change includes selftests that validate the expected behavior and
APIs of the new BPF_MAP_TYPE_USER_RINGBUF map type.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220920000100.477320-5-void@manifault.com