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> jirka > --- > btf_encoder.c | 8 -------- > dwarf_loader.c | 25 +++++++++++++++++++------ > dwarves.h | 8 -------- > 3 files changed, 19 insertions(+), 22 deletions(-) > > diff --git a/btf_encoder.c b/btf_encoder.c > index da776f4..07a9dc5 100644 > --- a/btf_encoder.c > +++ b/btf_encoder.c > @@ -69,7 +69,6 @@ struct btf_encoder { > const char *filename; > struct elf_symtab *symtab; > uint32_t type_id_off; > - uint32_t unspecified_type; > int saved_func_cnt; > bool has_index_type, > need_index_type, > @@ -635,11 +634,6 @@ static int32_t btf_encoder__tag_type(struct btf_encoder *encoder, uint32_t tag_t > if (tag_type == 0) > return 0; > > - if (encoder->unspecified_type && tag_type == encoder->unspecified_type) { > - // No provision for encoding this, turn it into void. > - return 0; > - } > - > return encoder->type_id_off + tag_type; > } > > @@ -1746,8 +1740,6 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu, struct co > > encoder->cu = cu; > encoder->type_id_off = btf__type_cnt(encoder->btf) - 1; > - if (encoder->cu->unspecified_type.tag) > - encoder->unspecified_type = encoder->cu->unspecified_type.type; > > if (!encoder->has_index_type) { > /* cu__find_base_type_by_name() takes "type_id_t *id" */ > diff --git a/dwarf_loader.c b/dwarf_loader.c > index 014e130..c37bd7b 100644 > --- a/dwarf_loader.c > +++ b/dwarf_loader.c > @@ -2155,8 +2155,7 @@ static struct tag *__die__process_tag(Dwarf_Die *die, struct cu *cu, > case DW_TAG_atomic_type: > tag = die__create_new_tag(die, cu); break; > case DW_TAG_unspecified_type: > - cu->unspecified_type.tag = > - tag = die__create_new_tag(die, cu); break; > + tag = die__create_new_tag(die, cu); break; > case DW_TAG_pointer_type: > tag = die__create_new_pointer_tag(die, cu, conf); break; > case DW_TAG_ptr_to_member_type: > @@ -2219,13 +2218,27 @@ static int die__process_unit(Dwarf_Die *die, struct cu *cu, struct conf_load *co > continue; > } > > - uint32_t id; > - cu__add_tag(cu, tag, &id); > + uint32_t id = 0; > + /* There is no BTF representation for unspecified types. > + * Currently we want such types to be represented as `void` > + * (and thus skip BTF encoding). > + * > + * As BTF encoding is skipped, such types must not be added to type table, > + * otherwise an ID for a type would be allocated and we would be forced > + * to put something in BTF at this ID. > + * Thus avoid `cu__add_tag()` call for such types. > + * > + * On the other hand, there might be references to this type from other > + * tags, so `dwarf_cu__find_tag_by_ref()` must return something. > + * Thus call `cu__hash()` for such types. > + * > + * Note, that small_id of zero would be assigned to unspecified type entry. > + */ > + if (tag->tag != DW_TAG_unspecified_type) > + cu__add_tag(cu, tag, &id); > cu__hash(cu, tag); > struct dwarf_tag *dtag = tag->priv; > dtag->small_id = id; > - if (tag->tag == DW_TAG_unspecified_type) > - cu->unspecified_type.type = id; > } while (dwarf_siblingof(die, die) == 0); > > return 0; > diff --git a/dwarves.h b/dwarves.h > index 5074cf8..e92b2fd 100644 > --- a/dwarves.h > +++ b/dwarves.h > @@ -236,10 +236,6 @@ struct debug_fmt_ops { > > #define ARCH_MAX_REGISTER_PARAMS 8 > > -/* > - * unspecified_type: If this CU has a DW_TAG_unspecified_type, as BTF doesn't have a representation for this > - * and thus we need to check functions returning this to convert it to void. > - */ > struct cu { > struct list_head node; > struct list_head tags; > @@ -248,10 +244,6 @@ struct cu { > struct ptr_table functions_table; > struct ptr_table tags_table; > struct rb_root functions; > - struct { > - struct tag *tag; > - uint32_t type; > - } unspecified_type; > char *name; > char *filename; > void *priv; > -- > 2.39.1 >
