Re: [PATCH bpf-next] selftests/bpf: add btf dedup test covering module BTF dedup

[Eduard Zingerman <eddyz87@gmail.com>]

Alan Maguire <alan.maguire@oracle.com> writes:

> Recently issues were observed with module BTF deduplication failures
> [1].  Add a dedup selftest that ensures that core kernel types are
> referenced from split BTF as base BTF types.  To do this use bpf_testmod
> functions which utilize core kernel types, specifically
>
> ssize_t
> bpf_testmod_test_write(struct file *file, struct kobject *kobj,
>                        struct bin_attribute *bin_attr,
>                        char *buf, loff_t off, size_t len);
>
> __bpf_kfunc struct sock *bpf_kfunc_call_test3(struct sock *sk);
>
> __bpf_kfunc void bpf_kfunc_call_test_pass_ctx(struct __sk_buff *skb);
>
> For each of these ensure that the types they reference -
> struct file, struct kobject, struct bin_attr etc - are in base BTF.
> Note that because bpf_testmod.ko is built with distilled base BTF
> the associated reference types - i.e. the PTR that points at a
> "struct file" - will be in split BTF.  As a result the test resolves
> typedef and pointer references and verifies the pointed-at or
> typedef'ed type is in base BTF.  Because we use BTF from
> /sys/kernel/btf/bpf_testmod relocation has occurred for the
> referenced types and they will be base - not distilled base - types.
>
> For large-scale dedup issues, we see such types appear in split BTF and
> as a result this test fails.  Hence it is proposed as a test which will
> fail when large-scale dedup issues have occurred.
>
> [1] https://lore.kernel.org/dwarves/CAADnVQL+-LiJGXwxD3jEUrOonO-fX0SZC8496dVzUXvfkB7gYQ@mail.gmail.com/
>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---

The test passes for LLVM and fails for gcc 14, when using pahole w/o
fixes discussed in [1].

Acked-by: Eduard Zingerman <eddyz87@gmail.com>