libbpf: better fix for handling nulled-out struct_ops program

Previous attempt to fix the handling of nulled-out (from skeleton)
struct_ops program is working well only if struct_ops program is defined
as non-autoloaded by default (i.e., has SEC("?struct_ops") annotation,
with question mark).

Unfortunately, that fix is incomplete due to how
bpf_object_adjust_struct_ops_autoload() is marking referenced or
non-referenced struct_ops program as autoloaded (or not). Because
bpf_object_adjust_struct_ops_autoload() is run after
bpf_map__init_kern_struct_ops() step, which sets program slot to NULL,
such programs won't be considered "referenced", and so its autoload
property won't be changed.

This all sounds convoluted and it is, but the desire is to have as
natural behavior (as far as struct_ops usage is concerned) as possible.

This fix is redoing the original fix but makes it work for
autoloaded-by-default struct_ops programs as well. We achieve this by
forcing prog->autoload to false if prog was declaratively set for some
struct_ops map, but then nulled-out from skeleton (programmatically).
This achieves desired effect of not autoloading it. If such program is
still referenced somewhere else (different struct_ops map or different
callback field), it will get its autoload property adjusted by
bpf_object_adjust_struct_ops_autoload() later.

We also fix selftest, which accidentally used SEC("?struct_ops")
annotation. It was meant to use autoload-by-default program from the
very beginning.

Fixes: f973fccd43d3 ("libbpf: handle nulled-out program in struct_ops correctly")
Cc: Kui-Feng Lee <thinker.li@gmail.com>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240501041706.3712608-1-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

selftests/bpf: validate nulled-out struct_ops program is handled properly

Add a selftests validating that it's possible to have some struct_ops
callback set declaratively, then disable it (by setting to NULL)
programmatically. Libbpf should detect that such program should
not be loaded. Otherwise, it will unnecessarily fail the loading
when the host kernel does not have the type information.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240428030954.3918764-2-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

selftests/bpf: Make sure libbpf doesn't enforce the signature of a func pointer.

The verifier in the kernel ensures that the struct_ops operators behave
correctly by checking that they access parameters and context
appropriately. The verifier will approve a program as long as it correctly
accesses the context/parameters, regardless of its function signature. In
contrast, libbpf should not verify the signature of function pointers and
functions to enable flexibility in loading various implementations of an
operator even if the signature of the function pointer does not match those
in the implementations or the kernel.

With this flexibility, user space applications can adapt to different
kernel versions by loading a specific implementation of an operator based
on feature detection.

This is a follow-up of the commit c911fc61a7ce ("libbpf: Skip zeroed or
null fields if not found in the kernel type.")

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240404232342.991414-1-thinker.li@gmail.com

selftests/bpf: Ensure libbpf skip all-zeros fields of struct_ops maps.

A new version of a type may have additional fields that do not exist in
older versions. Previously, libbpf would reject struct_ops maps with a new
version containing extra fields when running on a machine with an old
kernel. However, we have updated libbpf to ignore these fields if their
values are all zeros or null in order to provide backward compatibility.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240313214139.685112-3-thinker.li@gmail.com

selftests/bpf: Test struct_ops map definition with type suffix

Extend struct_ops_module test case to check if it is possible to use
'___' suffixes for struct_ops type specification.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20240306104529.6453-5-eddyz87@gmail.com

selftests/bpf: Test if shadow types work correctly.

Change the values of fields, including scalar types and function pointers,
and check if the struct_ops map works as expected.

The test changes the field "test_2" of "testmod_1" from the pointer to
test_2() to pointer to test_3() and the field "data" to 13. The function
test_2() and test_3() both compute a new value for "test_2_result", but in
different way. By checking the value of "test_2_result", it ensures the
struct_ops map works as expected with changes through shadow types.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240229064523.2091270-6-thinker.li@gmail.com

selftests/bpf: Suppress warning message of an unused variable.

"r" is used to receive the return value of test_2 in bpf_testmod.c, but it
is not actually used. So, we remove "r" and change the return type to
"void".

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401300557.z5vzn8FM-lkp@intel.com/
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240204061204.1864529-1-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

selftests/bpf: test case for register_bpf_struct_ops().

Create a new struct_ops type called bpf_testmod_ops within the bpf_testmod
module. When a struct_ops object is registered, the bpf_testmod module will
invoke test_2 from the module.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240119225005.668602-15-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>