Merge branch 'ignore-additional-fields-in-the-struct_ops-maps-in-an-updated-version'

Kui-Feng Lee says:

====================
Ignore additional fields in the struct_ops maps in an updated version.

According to an offline discussion, it would be beneficial to
implement a backward-compatible method for struct_ops types with
additional fields that are not present in older kernels.

This patchset accepts additional fields of a struct_ops map with all
zero values even if these fields are not in the corresponding type in
the kernel. This provides a way to be backward compatible. User space
programs can use the same map on a machine running an old kernel by
clearing fields that do not exist in the kernel.

For example, in a test case, it adds an additional field "zeroed" that
doesn't exist in struct bpf_testmod_ops of the kernel.

    struct bpf_testmod_ops___zeroed {
    	int (*test_1)(void);
    	void (*test_2)(int a, int b);
    	int (*test_maybe_null)(int dummy, struct task_struct *task);
    	int zeroed;
    };

    SEC(".struct_ops.link")
    struct bpf_testmod_ops___zeroed testmod_zeroed = {
    	.test_1 = (void *)test_1,
    	.test_2 = (void *)test_2_v2,
    };

Here, it doesn't assign a value to "zeroed" of testmod_zeroed, and by
default the value of this field will be zero. So, the map will be
accepted by libbpf, but libbpf will skip the "zeroed" field. However,
if the "zeroed" field is assigned to any value other than "0", libbpf
will reject to load this map.
---
Changes from v1:

 - Fix the issue about function pointer fields.

 - Change a warning message, and add an info message for skipping
   fields.

 - Add a small demo of additional arguments that are not in the
   function pointer prototype in the kernel.

v1: https://lore.kernel.org/all/20240312183245.341141-1-thinker.li@gmail.com/

Kui-Feng Lee (3):
  libbpf: Skip zeroed or null fields if not found in the kernel type.
  selftests/bpf: Ensure libbpf skip all-zeros fields of struct_ops maps.
  selftests/bpf: Accept extra arguments if they are not used.

 tools/lib/bpf/libbpf.c                        |  24 +++-
 .../bpf/prog_tests/test_struct_ops_module.c   | 103 ++++++++++++++++++
 .../bpf/progs/struct_ops_extra_arg.c          |  49 +++++++++
 .../selftests/bpf/progs/struct_ops_module.c   |  16 ++-
 4 files changed, 186 insertions(+), 6 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_extra_arg.c
====================

Link: https://lore.kernel.org/r/20240313214139.685112-1-thinker.li@gmail.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>