Em Wed, Mar 01, 2023 at 10:19:56PM +0100, Jiri Olsa escreveu: > On Tue, Feb 28, 2023 at 10:23:57PM +0200, Eduard Zingerman wrote: > > Recent changes to handle unspecified types (see [1]) cause BTF ID drift. > > > > Specifically, the intent of commits [2], [3] and [4] is to render > > references to unspecified types as void type. > > However, as a consequence: > > - in `die__process_unit()` call to `cu__add_tag()` allocates `small_id` > > for unspecified type tags and adds these tags to `cu->types_table`; > > - `btf_encoder__encode_tag()` skips generation of BTF entries for > > `DW_TAG_unspecified_type` tags. > > > > Such logic causes ID drift if unspecified type is not the last type > > processed for compilation unit. `small_id` of each type following > > unspecified type in the `cu->types_table` would have its BTF id off by -1. > > Thus, rendering references established on recode phase invalid. > > > > This commit reverts `unspecified_type` id/tag tracking. > > Instead, the following is done: > > - `small_id` for unspecified type tags is set to 0, thus reference to > > unspecified type tag would render BTF id of a `void` on recode phase; > > - unspecified type tags are not added to `cu->types_table`. > > > > This change also happens to fix issue reported in [5], the gist of > > that issue is that the field `encoder->unspecified_type` is set but > > not reset by function `btf_encoder__encode_cu()`. Thus, the following > > sequence of events might occur when BTF encoding is requested: > > - CU with unspecified type is processed: > > - unspecified type id is 42 > > - encoder->unspecified_type is set to 42 > > - CU without unspecified type is processed next using the same > > `encoder` object: > > - some `struct foo` has id 42 in this CU > > - the references to `struct foo` are set 0 by function > > `btf_encoder__tag_type()`. > > > > [1] https://lore.kernel.org/all/Y0R7uu3s%2FimnvPzM@xxxxxxxxxx/ > > [2] bcc648a10cbc ("btf_encoder: Encode DW_TAG_unspecified_type returning routines as void") > > [3] cffe5e1f75e1 ("core: Record if a CU has a DW_TAG_unspecified_type") > > [4] 75e0fe28bb02 ("core: Add DW_TAG_unspecified_type to tag__is_tag_type() set") > > [5] https://lore.kernel.org/bpf/Y%2FP1yxAuV6Wj3A0K@xxxxxxxxxx/ > > > > Fixes: bcc648a10cbc ("btf_encoder: Encode DW_TAG_unspecified_type returning routines as void") > > Fixes: 52b25808e44a ("btf_encoder: Store type_id_off, unspecified type in encoder") > > Tested-by: KP Singh <kpsingh@xxxxxxxxxx> > > Reported-by: Matt Bobrowski <mattbobrowski@xxxxxxxxxx> > > Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx> > > lgtm, tested on top of the pahole next branch with bpf selftests > > Tested-by: Jiri Olsa <jolsa@xxxxxxxxxx> Looks good to me as well, and way more elegant, thanks! Applied. - Arnaldo
