On 11/22/21 5:52 PM, Andrii Nakryiko wrote:
On Wed, Nov 17, 2021 at 12:25 PM Yonghong Song <yhs@xxxxxx> wrote:LLVM patches ([1] for clang, [2] and [3] for BPF backend) added support for btf_type_tag attributes. The following is an example: [$ ~] cat t.c #define __tag1 __attribute__((btf_type_tag("tag1"))) #define __tag2 __attribute__((btf_type_tag("tag2"))) int __tag1 * __tag1 __tag2 *g __attribute__((section(".data..percpu"))); [$ ~] clang -O2 -g -c t.c [$ ~] llvm-dwarfdump --debug-info t.o t.o: file format elf64-x86-64 ... 0x0000001e: DW_TAG_variable DW_AT_name ("g") DW_AT_type (0x00000033 "int **") DW_AT_external (true) DW_AT_decl_file ("/home/yhs/t.c") DW_AT_decl_line (3) DW_AT_location (DW_OP_addr 0x0) 0x00000033: DW_TAG_pointer_type DW_AT_type (0x0000004b "int *") 0x00000038: DW_TAG_LLVM_annotation DW_AT_name ("btf_type_tag") DW_AT_const_value ("tag1") 0x00000041: DW_TAG_LLVM_annotation DW_AT_name ("btf_type_tag") DW_AT_const_value ("tag2") 0x0000004a: NULL 0x0000004b: DW_TAG_pointer_type DW_AT_type (0x0000005a "int") 0x00000050: DW_TAG_LLVM_annotation DW_AT_name ("btf_type_tag") DW_AT_const_value ("tag1") 0x00000059: NULL 0x0000005a: DW_TAG_base_type DW_AT_name ("int") DW_AT_encoding (DW_ATE_signed) DW_AT_byte_size (0x04) 0x00000061: NULL From the above example, you can see that DW_TAG_pointer_type may contain one or more DW_TAG_LLVM_annotation btf_type_tag tags. If DW_TAG_LLVM_annotation tags are present inside DW_TAG_pointer_type, for BTF encoding, pahole will need to follow [3] to generate a type chain like var -> ptr -> tag2 -> tag1 -> ptr -> tag1 -> int This patch implemented dwarf_loader support. If a pointer type contains DW_TAG_LLVM_annotation tags, a new type btf_type_tag_ptr_type will be created which will store the pointer tag itself and all DW_TAG_LLVM_annotation tags. During recoding stage, the type chain will be formed properly based on the above example. An option "--skip_encoding_btf_type_tag" is added to disable this new functionality. [1] https://reviews.llvm.org/D111199 [2] https://reviews.llvm.org/D113222 [3] https://reviews.llvm.org/D113496 --- dwarf_loader.c | 116 +++++++++++++++++++++++++++++++++++++++++++++++-- dwarves.h | 33 +++++++++++++- pahole.c | 8 ++++ 3 files changed, 153 insertions(+), 4 deletions(-)[...]+ +static struct tag *die__create_new_pointer_tag(Dwarf_Die *die, struct cu *cu, + struct conf_load *conf) +{ + struct btf_type_tag_ptr_type *tag = NULL; + struct btf_type_tag_type *annot; + Dwarf_Die *cdie, child; + const char *name; + uint32_t id; + + /* If no child tags or skipping btf_type_tag encoding, just create a new tag + * and return + */ + if (!dwarf_haschildren(die) || dwarf_child(die, &child) != 0 || + conf->skip_encoding_btf_type_tag) + return tag__new(die, cu); + + /* Otherwise, check DW_TAG_LLVM_annotation child tags */ + cdie = &child; + do { + if (dwarf_tag(cdie) == DW_TAG_LLVM_annotation) {nit: inverting the condition and doing continue would reduce nestedness level
good point. Will send another revision.
+ /* Only check btf_type_tag annotations */ + name = attr_string(cdie, DW_AT_name, conf); + if (strcmp(name, "btf_type_tag") != 0) + continue; + + if (tag == NULL) { + /* Create a btf_type_tag_ptr type. */ + tag = die__create_new_btf_type_tag_ptr_type(die, cu); + if (!tag) + return NULL; + } + + /* Create a btf_type_tag type for this annotation. */ + annot = die__create_new_btf_type_tag_type(cdie, cu, conf); + if (annot == NULL) + return NULL; + + if (cu__table_add_tag(cu, &annot->tag, &id) < 0) + return NULL; + + struct dwarf_tag *dtag = annot->tag.priv; + dtag->small_id = id; + cu__hash(cu, &annot->tag); + + /* For a list of DW_TAG_LLVM_annotation like tag1 -> tag2 -> tag3, + * the tag->tags contains tag3 -> tag2 -> tag1. + */ + list_add(&annot->node, &tag->tags); + } + } while (dwarf_siblingof(cdie, cdie) == 0); + + return tag ? &tag->tag : tag__new(die, cu); +} + static struct tag *die__create_new_ptr_to_member_type(Dwarf_Die *die, struct cu *cu) { @@ -1903,12 +1985,13 @@ static struct tag *__die__process_tag(Dwarf_Die *die, struct cu *cu, case DW_TAG_const_type: case DW_TAG_imported_declaration: case DW_TAG_imported_module: - case DW_TAG_pointer_type: case DW_TAG_reference_type: case DW_TAG_restrict_type: case DW_TAG_unspecified_type: case DW_TAG_volatile_type: 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: tag = die__create_new_ptr_to_member_type(die, cu); break; case DW_TAG_enumeration_type: @@ -2192,6 +2275,26 @@ static void lexblock__recode_dwarf_types(struct lexblock *tag, struct cu *cu) } } +static void dwarf_cu__recode_btf_type_tag_ptr(struct btf_type_tag_ptr_type *tag, + uint32_t pointee_type) +{ + struct btf_type_tag_type *annot; + struct dwarf_tag *annot_dtag; + struct tag *prev_tag; + + /* If tag->tags contains tag3 -> tag2 -> tag1, the final type chain + * looks like: + * pointer -> tag3 -> tag2 -> tag1 -> pointee + */is the comment accurate or the final one should have looked like pointer -> tag1 -> tag2 -> tag3 -> pointee? Basically, trying to understand if the final BTF represents the source-level order of tags or not?
The comment is accurate. Given source like int tag1 tag2 tag3 *p; the final type chain is p -> tag3 -> tag2 -> tag1 -> int basically it means - '*' applies to "int tag1 tag2 tag3" - tag3 applies to "int tag1 tag2" - tag2 applies to "int tag1" - tag1 applies to "int" This also makes final source code (format c) easier as we can do emit for "tag3 -> tag2 -> tag1 -> int" emit '*' For 'tag3 -> tag2 -> tag1 -> int": emit for "tag2 -> tag1 -> int" emit tag3 Eventually we can get the source code like int tag1 tag2 tag3 *p and this matches the user/kernel code.
+ prev_tag = &tag->tag; + list_for_each_entry(annot, &tag->tags, node) { + annot_dtag = annot->tag.priv; + prev_tag->type = annot_dtag->small_id; + prev_tag = &annot->tag; + } + prev_tag->type = pointee_type; +} +[...]
