[PATCH] D133361: [BPF] Attribute btf_decl_tag("ctx") for structs

[Alexei Starovoitov via Phabricator via cfe-commits]

ast added a comment.

In D133361#4629081 <https://reviews.llvm.org/D133361#4629081>, @eddyz87 wrote:

> Note on current propagation logic: whenever there is an expression E of type T, where T is a struct annotated with btf_decl_tag("ctx"), all usages of  E are traversed recursively visiting `getelementptr` and calls to `preserve_{struct,union,array}_index`, the latter are replaced by `getelementptr`. E.g.:
>
>   #define __pai __attribute__((preserve_access_context));
>   #define __ctx __attribute__((btf_decl_tag("ctx")))
>   struct bar { int a; int b; } __pai;
>   struct foo {
>     struct bar b;
>   } __ctx __pai;
>   ... struct foo f; ...
>   ... f.b.bb ...
>
> The call to `preserve_struct_index` generated for `bb` in `f.b.bb` would be replaced by `getelementptr` and later by `getelementptr.and.load`.
> However, context structures don't have nested structures at the moment. The list of context structures is:
>
> - __sk_buff
> - bpf_cgroup_dev_ctx
> - bpf_sk_lookup
> - bpf_sock
> - bpf_sock_addr
> - bpf_sock_ops
> - bpf_sockopt
> - bpf_sysctl
> - sk_msg_md
> - sk_reuseport_md
> - xdp_md

Right. Such recursive propagation of PAI is necessary. For btf_tag we cannot do it. Always propagating it won't be correct.
New preserve_const_field_offset would need to be propagated too and we actually have nested unions.
__bpf_md_ptr is such example. btf_tag wouldn't propagate into that union, but attr(preserve_const_field_offset) should.


Repository:
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  https://reviews.llvm.org/D133361/new/

https://reviews.llvm.org/D133361