Re: [PATCH dwarves] dwarf_loader: Fix for BTF id drift caused by adding unspecified types

[Arnaldo Carvalho de Melo <acme@kernel.org>]

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@kernel.org/
> > [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@google.com/
> > 
> > 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@kernel.org>
> > Reported-by: Matt Bobrowski <mattbobrowski@google.com>
> > Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> 
> lgtm, tested on top of the pahole next branch with bpf selftests
> 
> Tested-by: Jiri Olsa <jolsa@kernel.org>

Looks good to me as well, and way more elegant, thanks!

Applied.

- Arnaldo