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Tue, 23 Nov 2021 03:32:40 +0000 Received: from SN6PR1501MB2064.namprd15.prod.outlook.com ([fe80::a91b:fba1:b79c:812c]) by SN6PR1501MB2064.namprd15.prod.outlook.com ([fe80::a91b:fba1:b79c:812c%5]) with mapi id 15.20.4713.026; Tue, 23 Nov 2021 03:32:39 +0000 Message-ID: <139df0c0-4379-b6ef-00fc-140adac17da5@fb.com> Date: Mon, 22 Nov 2021 19:32:36 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.3.2 Subject: Re: [PATCH dwarves 3/4] dwarf_loader: support btf_type_tag attribute Content-Language: en-US To: Andrii Nakryiko CC: Arnaldo Carvalho de Melo , , Alexei Starovoitov , Andrii Nakryiko , bpf , Daniel Borkmann , Kernel Team References: <20211117202214.3268824-1-yhs@fb.com> <20211117202229.3270304-1-yhs@fb.com> From: Yonghong Song In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed X-ClientProxiedBy: MWHPR22CA0068.namprd22.prod.outlook.com (2603:10b6:300:12a::30) To SN6PR1501MB2064.namprd15.prod.outlook.com (2603:10b6:805:d::27) Received: from [IPV6:2620:10d:c085:21e1::14f6] (2620:10d:c090:400::5:b4f1) by MWHPR22CA0068.namprd22.prod.outlook.com (2603:10b6:300:12a::30) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4713.19 via Frontend Transport; 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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; >> +} >> + > > [...] >