On 13/03/2023 02:17, Eduard Zingerman wrote:
> "btf:type_tag" is an DW_TAG_LLVM_annotation object that encodes
> btf_type_tag attributes in DWARF. Contrary to existing "btf_type_tag"
> it allows to associate such attributes with non-pointer types.
> When "btf:type_tag" is attached to a type it applies to this type.
>
> For example the following C code:
>
> struct echo {
> int __attribute__((btf_type_tag("__c"))) c;
> }
>
> Produces the following DWARF:
>
> 0x29: DW_TAG_structure_type
> DW_AT_name ("echo")
>
> 0x40: DW_TAG_member
> DW_AT_name ("c")
> DW_AT_type (0x8c "int")
>
> 0x8c: DW_TAG_base_type
> DW_AT_name ("int")
> DW_AT_encoding (DW_ATE_signed)
> DW_AT_byte_size (0x04)
>
> 0x90: DW_TAG_LLVM_annotation
> DW_AT_name ("btf:type_tag")
> DW_AT_const_value ("__c")
>
> Meaning that type 0x8c is an `int` with type tag `__c`.
> Corresponding BTF looks as follows:
>
> [1] STRUCT 'echo'
> ...
> 'c' type_id=8 bits_offset=128
> [4] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> [8] TYPE_TAG '__c' type_id=4
>
> Void pointers with type tag annotations are represented as objects
> of unspecified type with name "void", here is an example:
>
> struct st {
> void __attribute__((btf_type_tag("__d"))) *d;
> }
>
> And corresponding DWARF:
>
> 0x29: DW_TAG_structure_type
> DW_AT_name ("st")
>
> 0x49: DW_TAG_member
> DW_AT_name ("d")
> DW_AT_type (0xa6 "void *")
>
> 0xa6: DW_TAG_pointer_type
> DW_AT_type (0xaf "void")
>
> 0xaf: DW_TAG_unspecified_type
> DW_AT_name ("void")
>
> 0xb1: DW_TAG_LLVM_annotation
> DW_AT_name ("btf:type_tag")
> DW_AT_const_value ("__d")
>
> This commit adds support for DW_TAG_LLVM_annotation "btf:type_tag"
> attached to the following entities:
> - base types;
> - arrays;
> - pointers;
> - structs
> - unions;
> - enums;
> - typedefs.
>
> This is achieved via the following modifications:
> - Types `struct btf_type_tag_type` and `struct llvm_annotation` are
> consolidated as a single type `struct llvm_annotation`, in order to
> reside in a single `annots` list associated with struct/union/enum
> or typedef.
> - `struct unspecified_type` is added to handle `void *` types annotated
> with `btf:type_tag`.
> - `struct tag` is extended with `annots` list.
> - DWARF load phase is modified to fill `annots` fields for the above
> mentioned types.
> - Recode phase is split in two sub-phases:
> - The existing `tag__recode_dwarf_type()` is executed first;
> - Newly added `update_btf_type_tag_refs()` is executed to update
> `->type` field of each tag if that type refers to an object with
> `btf:type_tag` annotation. The id of the type is replaced by id
> of the type tag.
> - When `btf:type_tag` annotations are present in compilation unit,
> all `btf_type_tag` annotations are ignored during BTF dump.
>
> See also:
> [1] Mailing list discussion regarding `btf:type_tag`
> https://lore.kernel.org/bpf/87r0w9jjoq.fsf@xxxxxxxxxx/
>
I know there's a v2 coming but one suggestion and one other issue below..
this is a great explanation, thanks for the details! one other thing that might help
here is specifying that the solution adopted is option 2 described in that discussion
(at least I think it is?).
> Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> ---
> btf_encoder.c | 13 +-
> btf_loader.c | 15 +-
> dwarf_loader.c | 763 +++++++++++++++++++++++++++++++++++++---------
> dwarves.c | 1 +
> dwarves.h | 68 +++--
> dwarves_fprintf.c | 13 +
> 6 files changed, 693 insertions(+), 180 deletions(-)
>
<snip>
> -static void ftype__recode_dwarf_types(struct tag *tag, struct cu *cu);
> +/** Add @tuple to @ctx->mappings array, extend it if necessary. */
> +static int push_btf_type_tag_mapping(struct btf_type_tag_mapping *tuple,
> + struct recode_context *ctx)
> +{
> + if (ctx->nr_allocated == ctx->nr_entries) {
> + uint32_t new_nr = ctx->nr_allocated * 2;
> + void *new_array = reallocarray(ctx->mappings, new_nr,
> + sizeof(ctx->mappings[0]));
older libcs won't have reallocarray; might be good to either replace with realloc
or define our own variant in dutil.h like was done with libbpf_reallocarray()?
