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:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D133361/new/
https://reviews.llvm.org/D133361
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