Re: [PATCH dwarves 2/2] btf_encoder: generate BTF_KIND_TAG from llvm annotations

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sun, Sep 19, 2021 at 5:36 PM Yonghong Song <yhs@fb.com> wrote:
>
> The following is an example with latest upstream clang:
>   $ cat t.c
>   #define __tag1 __attribute__((btf_tag("tag1")))
>   #define __tag2 __attribute__((btf_tag("tag2")))
>
>   struct t {
>           int a:1 __tag1;
>           int b __tag2;
>   } __tag1 __tag2;
>
>   int g __tag1 __attribute__((section(".data..percpu")));
>
>   int __tag1 foo(struct t *a1, int a2 __tag2) {
>     return a1->b + a2 + g;
>   }
>
>   $ clang -O2 -g -c t.c
>   $ pahole -JV t.o
>   Found per-CPU symbol 'g' at address 0x0
>   Found 1 per-CPU variables!
>   Found 1 functions!
>   File t.o:
>   [1] INT int size=4 nr_bits=32 encoding=SIGNED
>   [2] PTR (anon) type_id=3
>   [3] STRUCT t size=8
>         a type_id=1 bitfield_size=1 bits_offset=0
>         b type_id=1 bitfield_size=0 bits_offset=32
>   [4] TAG tag1 type_id=3 component_idx=0
>   [5] TAG tag2 type_id=3 component_idx=1
>   [6] TAG tag1 type_id=3 component_idx=-1
>   [7] TAG tag2 type_id=3 component_idx=-1
>   [8] FUNC_PROTO (anon) return=1 args=(2 a1, 1 a2)
>   [9] FUNC foo type_id=8
>   [10] TAG tag2 type_id=9 component_idx=1
>   [11] TAG tag1 type_id=9 component_idx=-1
>   search cu 't.c' for percpu global variables.
>   Variable 'g' from CU 't.c' at address 0x0 encoded
>   [12] VAR g type=1 linkage=1
>   [13] TAG tag1 type_id=12 component_idx=-1
>   [14] DATASEC .data..percpu size=4 vlen=1
>         type=12 offset=0 size=4
>   $ ...
>
> With additional option --skip_encoding_btf_tag, pahole doesn't
> generate BTF_KIND_TAGs any more.
>   $ pahole -JV --skip_encoding_btf_tag t.o
>   Found per-CPU symbol 'g' at address 0x0
>   Found 1 per-CPU variables!
>   Found 1 functions!
>   File t.o:
>   [1] INT int size=4 nr_bits=32 encoding=SIGNED
>   [2] PTR (anon) type_id=3
>   [3] STRUCT t size=8
>         a type_id=1 bitfield_size=1 bits_offset=0
>         b type_id=1 bitfield_size=0 bits_offset=32
>   [4] FUNC_PROTO (anon) return=1 args=(2 a1, 1 a2)
>   [5] FUNC foo type_id=4
>   search cu 't.c' for percpu global variables.
>   Variable 'g' from CU 't.c' at address 0x0 encoded
>   [6] VAR g type=1 linkage=1
>   [7] DATASEC .data..percpu size=4 vlen=1
>         type=6 offset=0 size=4
>   $ ...
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  btf_encoder.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>

[...]

> @@ -1244,6 +1266,10 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, struct
>                         goto out;
>                 }
>
> +               list_for_each_entry(annot, &var->annots, node) {
> +                       btf_encoder__add_tag(encoder, annot->value, id, annot->component_idx);

check errors?

> +               }
> +
>                 /*
>                  * add a BTF_VAR_SECINFO in encoder->percpu_secinfo, which will be added into
>                  * encoder->types later when we add BTF_VAR_DATASEC.
> @@ -1359,6 +1385,7 @@ void btf_encoder__delete(struct btf_encoder *encoder)
>  int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu)
>  {
>         uint32_t type_id_off = btf__get_nr_types(encoder->btf);
> +       struct llvm_annotation *annot;
>         uint32_t core_id;
>         struct function *fn;
>         struct tag *pos;
> @@ -1396,6 +1423,20 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu)
>                 encoder->has_index_type = true;
>         }
>
> +       cu__for_each_type(cu, core_id, pos) {
> +               struct namespace *ns;
> +               int btf_type_id;
> +
> +               if (pos->tag != DW_TAG_structure_type && pos->tag != DW_TAG_union_type)
> +                       continue;
> +
> +               btf_type_id = type_id_off + core_id;
> +               ns = tag__namespace(pos);
> +               list_for_each_entry(annot, &ns->annots, node) {
> +                       btf_encoder__add_tag(encoder, annot->value, btf_type_id, annot->component_idx);

same, this can fail

> +               }
> +       }
> +
>         cu__for_each_function(cu, core_id, fn) {
>                 int btf_fnproto_id, btf_fn_id;
>                 const char *name;
> @@ -1436,6 +1477,10 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu)
>                         printf("error: failed to encode function '%s'\n", function__name(fn));
>                         goto out;
>                 }
> +
> +               list_for_each_entry(annot, &fn->annots, node) {
> +                       btf_encoder__add_tag(encoder, annot->value, btf_fn_id, annot->component_idx);

and here as well

> +               }
>         }
>
>         if (!encoder->skip_encoding_vars)
> --
> 2.30.2
>