Re: [PATCH v3 bpf-next 0/9] Libbpf-side __arg_ctx fallback support

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-01-03 at 17:38 -0800, Andrii Nakryiko wrote:
> Support __arg_ctx global function argument tag semantics even on older kernels
> that don't natively support it through btf_decl_tag("arg:ctx").
> 
> Patches #2-#6 are preparatory work to allow to postpone BTF loading into the
> kernel until after all the BPF program relocations (including global func
> appending to main programs) are done. Patch #4 is perhaps the most important
> and establishes pre-created stable placeholder FDs, so that relocations can
> embed valid map FDs into ldimm64 instructions.
> 
> Once BTF is done after relocation, what's left is to adjust BTF information to
> have each main program's copy of each used global subprog to point to its own
> adjusted FUNC -> FUNC_PROTO type chain (if they use __arg_ctx) in such a way
> as to satisfy type expectations of BPF verifier regarding the PTR_TO_CTX
> argument definition. See patch #8 for details.
> 
> Patch #8 adds few more __arg_ctx use cases (edge cases like multiple arguments
> having __arg_ctx, etc) to test_global_func_ctx_args.c, to make it simple to
> validate that this logic indeed works on old kernels. It does. But just to be
> 100% sure patch #9 adds a test validating that libbpf uploads func_info with
> properly modified BTF data.
> 
> v2->v3:
>   - drop renaming patch (Alexei, Eduard);
>   - use memfd_create() instead of /dev/null for placeholder FD (Eduard);
>   - add one more test for validating BTF rewrite logic (Eduard);
>   - fixed wrong -errno usage, reshuffled some BTF rewrite bits (Eduard);
> v1->v2:
>   - do internal functions renaming in patch #1 (Alexei);
>   - extract cloning of FUNC -> FUNC_PROTO information into separate function
>     (Alexei);

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

Thank you for adding the test in patch #9.