[PATCH] D93563: BPF: add extern func to data sections if specified

[Yonghong Song via Phabricator via llvm-commits]

yonghong-song added a comment.

For:
void foo1(int); works ?
void foo2(int) __ attribute __((section("bar"))); sort-of works, but this one goes into the same section as foo1 ?

Current llvm without this patch, the following are generated BTFs:

  BTF_KIND_FUNC (foo1) -> BTF_KIND_FUNC_PROTO (foo1)
  BTF_KIND_FUNC (foo2) -> BTF_KIND_FUNC_PROTO (foo2)

there are no data sections generated.

With this patch, the following BTFs are generated:

  BTF_KIND_FUNC (foo1) -> BTF_KIND_FUNC_PROTO (foo1)
  BTF_KIND_FUNC (foo2) -> BTF_KIND_FUNC_PROTO (foo2)
  BTF_KIND_VAR(foo2) -> BTF_KIND_FUNC(foo2)
  BTF_KIND_DATASEC(bar) -> BTF_KIND_VAR(foo2)

The above is not perfect either.
extern functions are actually extern variables, ideally we should treat them the same as
extern variables, so we probably should not generate BTF_KIND_FUNC() in the above,
we should generate

  BTF_KIND_VAR(foo1) -> BTF_KIND_FUNC_PROTO(foo1)
  BTF_KIND_VAR(foo2) -> BTF_KIND_FUNC_PROTO(foo2)
  BTF_KIND_DATASEC(.extern) -> BTF_KIND_VAR(foo1)
  BTF_KIND_DATASEC(bar) -> BTF_KIND_VAR(foo2)

So we do not generate BTF_KIND_FUNC for extern functions.

But unfortunately, we have the following in uapi,
enum btf_func_linkage {

  BTF_FUNC_STATIC = 0,
  BTF_FUNC_GLOBAL = 1,
  BTF_FUNC_EXTERN = 2,

};

But BTF_FUNC_EXTERN is not used yet, maybe we can remove this
and BTF_KIND_VAR to represent external function instead of
using BTF_KIND_FUNC? This will make things a little bit
easier to unify with section handling. Or alternatively we can
have BTF_KIND_VAR points to BTF_KIND_FUNC but it feels
a little bit redundant.

The only issue is consistency w.r.t. non-extern functions.
Currently we don't put them into any BTF_KIND_VAR.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D93563/new/

https://reviews.llvm.org/D93563