[PATCH dwarves v2 0/2] generate BTF_KIND_TAG types from DW_TAG_LLVM_annotation dwarf tags

[PATCH dwarves v2 0/2] generate BTF_KIND_TAG types from DW_TAG_LLVM_annotation dwarf tags

[Yonghong Song]

LLVM has implemented btf_tag attribute ([1]) which intended
to provide a "string" tag for struct/union or its member, var,
a func or its parameter. Such a "string" tag will be encoded
in dwarf. For non-BPF target like x86_64, pahole needs to
convert those dwarf btf_tag annotations to BTF so kernel
can utilize these "string" tags for bpf program verification, etc.
        
Patch 1 enhanced dwarf_loader to encode DW_TAG_LLVM_annotation
tags into internal data structure and Patch 2 will encode
such information to BTF with BTF_KIND_TAGs.

 [1] https://reviews.llvm.org/D106614

Changelog:
  v1 -> v2:
    - handle returning error cases for btf_encoder__add_tag().

Yonghong Song (2):
  dwarf_loader: parse dwarf tag DW_TAG_LLVM_annotation
  btf_encoder: generate BTF_KIND_TAG from llvm annotations

 btf_encoder.c  | 63 ++++++++++++++++++++++++++++++++++++-
 dwarf_loader.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++----
 dwarves.h      | 10 ++++++
 pahole.c       |  8 +++++
 4 files changed, 159 insertions(+), 7 deletions(-)

-- 
2.30.2

[PATCH dwarves v2 1/2] dwarf_loader: parse dwarf tag DW_TAG_LLVM_annotation

[Yonghong Song]

Parse dwarf tag DW_TAG_LLVM_annotation. Only record
annotations with btf_tag name which corresponds to
btf_tag attributes in C code. Such information will
be used later by btf_encoder for BTF conversion.

LLVM implementation only supports btf_tag annotations
on struct/union, func, func parameter and variable ([1]).
So we only check existence of corresponding DW tags
in these places. A flag "--skip_encoding_btf_tag"
is introduced if for whatever reason this feature
needs to be disabled.

 [1] https://reviews.llvm.org/D106614

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 dwarf_loader.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++----
 dwarves.h      | 10 ++++++
 pahole.c       |  8 +++++
 3 files changed, 97 insertions(+), 6 deletions(-)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index 0213e42..48e1bf0 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -52,6 +52,10 @@
 #define DW_OP_addrx 0xa1
 #endif
 
+#ifndef DW_TAG_LLVM_annotation
+#define DW_TAG_LLVM_annotation 0x6000
+#endif
+
 static pthread_mutex_t libdw__lock = PTHREAD_MUTEX_INITIALIZER;
 
 static uint32_t hashtags__bits = 12;
@@ -602,6 +606,7 @@ static void namespace__init(struct namespace *namespace, Dwarf_Die *die,
 {
 	tag__init(&namespace->tag, cu, die);
 	INIT_LIST_HEAD(&namespace->tags);
+	INIT_LIST_HEAD(&namespace->annots);
 	namespace->name  = attr_string(die, DW_AT_name, conf);
 	namespace->nr_tags = 0;
 	namespace->shared_tags = 0;
@@ -719,6 +724,7 @@ static struct variable *variable__new(Dwarf_Die *die, struct cu *cu, struct conf
 		/* non-defining declaration of an object */
 		var->declaration = dwarf_hasattr(die, DW_AT_declaration);
 		var->scope = VSCOPE_UNKNOWN;
+		INIT_LIST_HEAD(&var->annots);
 		var->ip.addr = 0;
 		if (!var->declaration && cu->has_addr_info)
 			var->scope = dwarf__location(die, &var->ip.addr, &var->location);
@@ -854,6 +860,51 @@ static int tag__recode_dwarf_bitfield(struct tag *tag, struct cu *cu, uint16_t b
 	return -ENOMEM;
 }
 
+static int add_llvm_annotation(Dwarf_Die *die, int component_idx, struct conf_load *conf,
+			       struct list_head *head)
+{
+	struct llvm_annotation *annot;
+	const char *name;
+
+	if (conf->skip_encoding_btf_tag)
+		return 0;
+
+	/* Only handle btf_tag annotation for now. */
+	name = attr_string(die, DW_AT_name, conf);
+	if (strcmp(name, "btf_tag") != 0)
+		return 0;
+
+	annot = zalloc(sizeof(*annot));
+	if (!annot)
+		return -ENOMEM;
+
+	annot->value = attr_string(die, DW_AT_const_value, conf);
+	annot->component_idx = component_idx;
+	list_add_tail(&annot->node, head);
+	return 0;
+}
+
+static int add_child_llvm_annotations(Dwarf_Die *die, int component_idx,
+				      struct conf_load *conf, struct list_head *head)
+{
+	Dwarf_Die child;
+	int ret;
+
+	if (!dwarf_haschildren(die) || dwarf_child(die, &child) != 0)
+		return 0;
+
+	die = &child;
+	do {
+		if (dwarf_tag(die) == DW_TAG_LLVM_annotation) {
+			ret = add_llvm_annotation(die, component_idx, conf, head);
+			if (ret)
+				return ret;
+		}
+	} while (dwarf_siblingof(die, die) == 0);
+
+	return 0;
+}
+
 int class_member__dwarf_recode_bitfield(struct class_member *member,
 					struct cu *cu)
 {
@@ -1092,6 +1143,7 @@ static struct function *function__new(Dwarf_Die *die, struct cu *cu, struct conf
 		func->accessibility   = attr_numeric(die, DW_AT_accessibility);
 		func->virtuality      = attr_numeric(die, DW_AT_virtuality);
 		INIT_LIST_HEAD(&func->vtable_node);
+		INIT_LIST_HEAD(&func->annots);
 		INIT_LIST_HEAD(&func->tool_node);
 		func->vtable_entry    = -1;
 		if (dwarf_hasattr(die, DW_AT_vtable_elem_location))
@@ -1304,16 +1356,21 @@ static struct tag *die__create_new_string_type(Dwarf_Die *die, struct cu *cu)
 static struct tag *die__create_new_parameter(Dwarf_Die *die,
 					     struct ftype *ftype,
 					     struct lexblock *lexblock,
-					     struct cu *cu, struct conf_load *conf)
+					     struct cu *cu, struct conf_load *conf,
+					     int param_idx)
 {
 	struct parameter *parm = parameter__new(die, cu, conf);
 
 	if (parm == NULL)
 		return NULL;
 
-	if (ftype != NULL)
+	if (ftype != NULL) {
 		ftype__add_parameter(ftype, parm);
-	else {
+		if (param_idx >= 0) {
+			if (add_child_llvm_annotations(die, param_idx, conf, &(tag__function(&ftype->tag)->annots)))
+				return NULL;
+		}
+	} else {
 		/*
 		 * DW_TAG_formal_parameters on a non DW_TAG_subprogram nor
 		 * DW_TAG_subroutine_type tag happens sometimes, likely due to
@@ -1346,7 +1403,10 @@ static struct tag *die__create_new_variable(Dwarf_Die *die, struct cu *cu, struc
 {
 	struct variable *var = variable__new(die, cu, conf);
 
-	return var ? &var->ip.tag : NULL;
+	if (var == NULL || add_child_llvm_annotations(die, -1, conf, &var->annots))
+		return NULL;
+
+	return &var->ip.tag;
 }
 
 static struct tag *die__create_new_subroutine_type(Dwarf_Die *die,
@@ -1371,7 +1431,7 @@ static struct tag *die__create_new_subroutine_type(Dwarf_Die *die,
 			tag__print_not_supported(dwarf_tag(die));
 			continue;
 		case DW_TAG_formal_parameter:
-			tag = die__create_new_parameter(die, ftype, NULL, cu, conf);
+			tag = die__create_new_parameter(die, ftype, NULL, cu, conf, -1);
 			break;
 		case DW_TAG_unspecified_parameters:
 			ftype->unspec_parms = 1;
@@ -1455,6 +1515,7 @@ static int die__process_class(Dwarf_Die *die, struct type *class,
 			      struct cu *cu, struct conf_load *conf)
 {
 	const bool is_union = tag__is_union(&class->namespace.tag);
+	int member_idx = 0;
 
 	do {
 		switch (dwarf_tag(die)) {
@@ -1497,8 +1558,15 @@ static int die__process_class(Dwarf_Die *die, struct type *class,
 
 			type__add_member(class, member);
 			cu__hash(cu, &member->tag);
+			if (add_child_llvm_annotations(die, member_idx, conf, &class->namespace.annots))
+				return -ENOMEM;
+			member_idx++;
 		}
 			continue;
+		case DW_TAG_LLVM_annotation:
+			if (add_llvm_annotation(die, -1, conf, &class->namespace.annots))
+				return -ENOMEM;
+			continue;
 		default: {
 			struct tag *tag = die__process_tag(die, cu, 0, conf);
 
@@ -1699,6 +1767,7 @@ static struct tag *die__create_new_inline_expansion(Dwarf_Die *die,
 static int die__process_function(Dwarf_Die *die, struct ftype *ftype,
 				 struct lexblock *lexblock, struct cu *cu, struct conf_load *conf)
 {
+	int param_idx = 0;
 	Dwarf_Die child;
 	struct tag *tag;
 
@@ -1742,7 +1811,7 @@ static int die__process_function(Dwarf_Die *die, struct ftype *ftype,
 			tag__print_not_supported(dwarf_tag(die));
 			continue;
 		case DW_TAG_formal_parameter:
-			tag = die__create_new_parameter(die, ftype, lexblock, cu, conf);
+			tag = die__create_new_parameter(die, ftype, lexblock, cu, conf, param_idx++);
 			break;
 		case DW_TAG_variable:
 			tag = die__create_new_variable(die, cu, conf);
@@ -1771,6 +1840,10 @@ static int die__process_function(Dwarf_Die *die, struct ftype *ftype,
 			if (die__create_new_lexblock(die, cu, lexblock, conf) != 0)
 				goto out_enomem;
 			continue;
+		case DW_TAG_LLVM_annotation:
+			if (add_llvm_annotation(die, -1, conf, &(tag__function(&ftype->tag)->annots)))
+				goto out_enomem;
+			continue;
 		default:
 			tag = die__process_tag(die, cu, 0, conf);
 
diff --git a/dwarves.h b/dwarves.h
index 0b69312..30d33fa 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -58,6 +58,7 @@ struct conf_load {
 	bool			ignore_inline_expansions;
 	bool			ignore_labels;
 	bool			ptr_table_stats;
+	bool			skip_encoding_btf_tag;
 	uint8_t			hashtable_bits;
 	uint8_t			max_hashtable_bits;
 	uint16_t		kabi_prefix_len;
@@ -594,6 +595,12 @@ static inline struct ptr_to_member_type *
 	return (struct ptr_to_member_type *)tag;
 }
 
+struct llvm_annotation {
+	const char		*value;
+	int16_t			component_idx;
+	struct list_head	node;
+};
+
 /** struct namespace - base class for enums, structs, unions, typedefs, etc
  *
  * @tags - class_member, enumerators, etc
@@ -605,6 +612,7 @@ struct namespace {
 	uint16_t	 nr_tags;
 	uint8_t		 shared_tags;
 	struct list_head tags;
+	struct list_head annots;
 };
 
 static inline struct namespace *tag__namespace(const struct tag *tag)
@@ -686,6 +694,7 @@ struct variable {
 	enum vscope	 scope;
 	struct location	 location;
 	struct hlist_node tool_hnode;
+	struct list_head annots;
 	struct variable  *spec;
 };
 
@@ -818,6 +827,7 @@ struct function {
 	uint8_t		 btf:1;
 	int32_t		 vtable_entry;
 	struct list_head vtable_node;
+	struct list_head annots;
 	/* fields used by tools */
 	union {
 		struct list_head  tool_node;
diff --git a/pahole.c b/pahole.c
index 7f1771b..80271b5 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1124,6 +1124,7 @@ ARGP_PROGRAM_VERSION_HOOK_DEF = dwarves_print_version;
 #define ARGP_sort_output	   328
 #define ARGP_hashbits		   329
 #define ARGP_devel_stats	   330
+#define ARGP_skip_encoding_btf_tag 331
 
 static const struct argp_option pahole__options[] = {
 	{
@@ -1494,6 +1495,11 @@ static const struct argp_option pahole__options[] = {
 		.key  = ARGP_devel_stats,
 		.doc  = "Print internal data structures stats",
 	},
+	{
+		.name = "skip_encoding_btf_tag",
+		.key  = ARGP_skip_encoding_btf_tag,
+		.doc  = "Do not encode TAGs in BTF."
+	},
 	{
 		.name = NULL,
 	}
@@ -1642,6 +1648,8 @@ static error_t pahole__options_parser(int key, char *arg,
 		conf_load.hashtable_bits = atoi(arg);	break;
 	case ARGP_devel_stats:
 		conf_load.ptr_table_stats = true;	break;
+	case ARGP_skip_encoding_btf_tag:
+		conf_load.skip_encoding_btf_tag = true;	break;
 	default:
 		return ARGP_ERR_UNKNOWN;
 	}
-- 
2.30.2

[PATCH dwarves v2 2/2] btf_encoder: generate BTF_KIND_TAG from llvm annotations

[Yonghong Song]

The following is an example with latest upstream clang:
  $ cat t.c
  #define __tag1 __attribute__((btf_tag("tag1")))
  #define __tag2 __attribute__((btf_tag("tag2")))

  struct t {
          int a:1 __tag1;
          int b __tag2;
  } __tag1 __tag2;

  int g __tag1 __attribute__((section(".data..percpu")));

  int __tag1 foo(struct t *a1, int a2 __tag2) {
    return a1->b + a2 + g;
  }

  $ clang -O2 -g -c t.c
  $ pahole -JV t.o
  Found per-CPU symbol 'g' at address 0x0
  Found 1 per-CPU variables!
  Found 1 functions!
  File t.o:
  [1] INT int size=4 nr_bits=32 encoding=SIGNED
  [2] PTR (anon) type_id=3
  [3] STRUCT t size=8
        a type_id=1 bitfield_size=1 bits_offset=0
        b type_id=1 bitfield_size=0 bits_offset=32
  [4] TAG tag1 type_id=3 component_idx=0
  [5] TAG tag2 type_id=3 component_idx=1
  [6] TAG tag1 type_id=3 component_idx=-1
  [7] TAG tag2 type_id=3 component_idx=-1
  [8] FUNC_PROTO (anon) return=1 args=(2 a1, 1 a2)
  [9] FUNC foo type_id=8
  [10] TAG tag2 type_id=9 component_idx=1
  [11] TAG tag1 type_id=9 component_idx=-1
  search cu 't.c' for percpu global variables.
  Variable 'g' from CU 't.c' at address 0x0 encoded
  [12] VAR g type=1 linkage=1
  [13] TAG tag1 type_id=12 component_idx=-1
  [14] DATASEC .data..percpu size=4 vlen=1
        type=12 offset=0 size=4
  $ ...

With additional option --skip_encoding_btf_tag, pahole doesn't
generate BTF_KIND_TAGs any more.
  $ pahole -JV --skip_encoding_btf_tag t.o
  Found per-CPU symbol 'g' at address 0x0
  Found 1 per-CPU variables!
  Found 1 functions!
  File t.o:
  [1] INT int size=4 nr_bits=32 encoding=SIGNED
  [2] PTR (anon) type_id=3
  [3] STRUCT t size=8
        a type_id=1 bitfield_size=1 bits_offset=0
        b type_id=1 bitfield_size=0 bits_offset=32
  [4] FUNC_PROTO (anon) return=1 args=(2 a1, 1 a2)
  [5] FUNC foo type_id=4
  search cu 't.c' for percpu global variables.
  Variable 'g' from CU 't.c' at address 0x0 encoded
  [6] VAR g type=1 linkage=1
  [7] DATASEC .data..percpu size=4 vlen=1
        type=6 offset=0 size=4
  $ ...

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 btf_encoder.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 62 insertions(+), 1 deletion(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 1b4e83d..c341f95 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -141,6 +141,7 @@ static const char * const btf_kind_str[NR_BTF_KINDS] = {
 	[BTF_KIND_VAR]          = "VAR",
 	[BTF_KIND_DATASEC]      = "DATASEC",
 	[BTF_KIND_FLOAT]        = "FLOAT",
+	[BTF_KIND_TAG]          = "TAG",
 };
 
 static const char *btf__printable_name(const struct btf *btf, uint32_t offset)
@@ -644,6 +645,26 @@ static int32_t btf_encoder__add_datasec(struct btf_encoder *encoder, const char
 	return id;
 }
 
+static int32_t btf_encoder__add_tag(struct btf_encoder *encoder, const char *value, uint32_t type,
+				    int component_idx)
+{
+	struct btf *btf = encoder->btf;
+	const struct btf_type *t;
+	int32_t id;
+
+	id = btf__add_tag(btf, value, type, component_idx);
+	if (id > 0) {
+		t = btf__type_by_id(btf, id);
+		btf_encoder__log_type(encoder, t, false, true, "type_id=%u component_idx=%d",
+				      t->type, component_idx);
+	} else {
+		btf__log_err(btf, BTF_KIND_TAG, value, true, "component_idx=%d Error emitting BTF type",
+			     component_idx);
+	}
+
+	return id;
+}
+
 /*
  * This corresponds to the same macro defined in
  * include/linux/kallsyms.h
@@ -1158,6 +1179,7 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, struct
 		struct variable *var = tag__variable(pos);
 		uint32_t size, type, linkage;
 		const char *name, *dwarf_name;
+		struct llvm_annotation *annot;
 		const struct tag *tag;
 		uint64_t addr;
 		int id;
@@ -1244,6 +1266,15 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, struct
 			goto out;
 		}
 
+		list_for_each_entry(annot, &var->annots, node) {
+			int tag_type_id = btf_encoder__add_tag(encoder, annot->value, id, annot->component_idx);
+			if (tag_type_id < 0) {
+				fprintf(stderr, "error: failed to encode tag '%s' to variable '%s' with component_idx %d\n",
+					annot->value, name, annot->component_idx);
+				goto out;
+			}
+		}
+
 		/*
 		 * add a BTF_VAR_SECINFO in encoder->percpu_secinfo, which will be added into
 		 * encoder->types later when we add BTF_VAR_DATASEC.
@@ -1359,6 +1390,8 @@ void btf_encoder__delete(struct btf_encoder *encoder)
 int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu)
 {
 	uint32_t type_id_off = btf__get_nr_types(encoder->btf);
+	struct llvm_annotation *annot;
+	int btf_type_id, tag_type_id;
 	uint32_t core_id;
 	struct function *fn;
 	struct tag *pos;
@@ -1378,7 +1411,7 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu)
 	}
 
 	cu__for_each_type(cu, core_id, pos) {
-		int32_t btf_type_id = btf_encoder__encode_tag(encoder, pos, type_id_off);
+		btf_type_id = btf_encoder__encode_tag(encoder, pos, type_id_off);
 
 		if (btf_type_id < 0 ||
 		    tag__check_id_drift(pos, core_id, btf_type_id, type_id_off)) {
@@ -1396,6 +1429,25 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu)
 		encoder->has_index_type = true;
 	}
 
+	cu__for_each_type(cu, core_id, pos) {
+		struct namespace *ns;
+
+		if (pos->tag != DW_TAG_structure_type && pos->tag != DW_TAG_union_type)
+			continue;
+
+		btf_type_id = type_id_off + core_id;
+		ns = tag__namespace(pos);
+		list_for_each_entry(annot, &ns->annots, node) {
+			tag_type_id = btf_encoder__add_tag(encoder, annot->value, btf_type_id, annot->component_idx);
+			if (tag_type_id < 0) {
+				fprintf(stderr, "error: failed to encode tag '%s' to %s '%s' with component_idx %d\n",
+					annot->value, pos->tag == DW_TAG_structure_type ? "struct" : "union",
+					namespace__name(ns), annot->component_idx);
+				goto out;
+			}
+		}
+	}
+
 	cu__for_each_function(cu, core_id, fn) {
 		int btf_fnproto_id, btf_fn_id;
 		const char *name;
@@ -1436,6 +1488,15 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu)
 			printf("error: failed to encode function '%s'\n", function__name(fn));
 			goto out;
 		}
+
+		list_for_each_entry(annot, &fn->annots, node) {
+			tag_type_id = btf_encoder__add_tag(encoder, annot->value, btf_fn_id, annot->component_idx);
+			if (tag_type_id < 0) {
+				fprintf(stderr, "error: failed to encode tag '%s' to func %s with component_idx %d\n",
+					annot->value, name, annot->component_idx);
+				goto out;
+			}
+		}
 	}
 
 	if (!encoder->skip_encoding_vars)
-- 
2.30.2

Re: [PATCH dwarves v2 0/2] generate BTF_KIND_TAG types from DW_TAG_LLVM_annotation dwarf tags

[Arnaldo Carvalho de Melo]

Em Tue, Sep 21, 2021 at 07:13:21PM -0700, Yonghong Song escreveu:
> LLVM has implemented btf_tag attribute ([1]) which intended
> to provide a "string" tag for struct/union or its member, var,
> a func or its parameter. Such a "string" tag will be encoded
> in dwarf. For non-BPF target like x86_64, pahole needs to
> convert those dwarf btf_tag annotations to BTF so kernel
> can utilize these "string" tags for bpf program verification, etc.
>         
> Patch 1 enhanced dwarf_loader to encode DW_TAG_LLVM_annotation
> tags into internal data structure and Patch 2 will encode
> such information to BTF with BTF_KIND_TAGs.
> 
>  [1] https://reviews.llvm.org/D106614

Applied both locally, now building HEAD llvm/clang to test everything,

- Arnaldo
 
> Changelog:
>   v1 -> v2:
>     - handle returning error cases for btf_encoder__add_tag().
> 
> Yonghong Song (2):
>   dwarf_loader: parse dwarf tag DW_TAG_LLVM_annotation
>   btf_encoder: generate BTF_KIND_TAG from llvm annotations
> 
>  btf_encoder.c  | 63 ++++++++++++++++++++++++++++++++++++-
>  dwarf_loader.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++----
>  dwarves.h      | 10 ++++++
>  pahole.c       |  8 +++++
>  4 files changed, 159 insertions(+), 7 deletions(-)
> 
> -- 
> 2.30.2

-- 

- Arnaldo

Re: [PATCH dwarves v2 0/2] generate BTF_KIND_TAG types from DW_TAG_LLVM_annotation dwarf tags

[Arnaldo Carvalho de Melo]

Em Mon, Sep 27, 2021 at 05:40:09PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Sep 21, 2021 at 07:13:21PM -0700, Yonghong Song escreveu:
> > LLVM has implemented btf_tag attribute ([1]) which intended
> > to provide a "string" tag for struct/union or its member, var,
> > a func or its parameter. Such a "string" tag will be encoded
> > in dwarf. For non-BPF target like x86_64, pahole needs to
> > convert those dwarf btf_tag annotations to BTF so kernel
> > can utilize these "string" tags for bpf program verification, etc.
> >         
> > Patch 1 enhanced dwarf_loader to encode DW_TAG_LLVM_annotation
> > tags into internal data structure and Patch 2 will encode
> > such information to BTF with BTF_KIND_TAGs.
> > 
> >  [1] https://reviews.llvm.org/D106614
> 
> Applied both locally, now building HEAD llvm/clang to test everything,

⬢[acme@toolbox pahole]$ pahole -JV t.o
Found per-CPU symbol 'g' at address 0x0
Found 1 per-CPU variables!
Found 1 functions!
File t.o:
[1] INT int size=4 nr_bits=32 encoding=SIGNED
[2] PTR (anon) type_id=3
[3] STRUCT t size=8
	a type_id=1 bitfield_size=1 bits_offset=0
	b type_id=1 bitfield_size=0 bits_offset=32
[4] TAG tag1 type_id=3 component_idx=0
[5] TAG tag2 type_id=3 component_idx=1
[6] TAG tag1 type_id=3 component_idx=-1
[7] TAG tag2 type_id=3 component_idx=-1
[8] FUNC_PROTO (anon) return=1 args=(2 a1, 1 a2)
[9] FUNC foo type_id=8
[10] TAG tag2 type_id=9 component_idx=1
[11] TAG tag1 type_id=9 component_idx=-1
search cu 't.c' for percpu global variables.
Variable 'g' from CU 't.c' at address 0x0 encoded
[12] VAR g type=1 linkage=1
[13] TAG tag1 type_id=12 component_idx=-1
[14] DATASEC .data..percpu size=4 vlen=1
	type=12 offset=0 size=4
⬢[acme@toolbox pahole]$ pahole -JV --skip_encoding_btf_tag t.o
Found per-CPU symbol 'g' at address 0x0
Found 1 per-CPU variables!
Found 1 functions!
File t.o:
[1] INT int size=4 nr_bits=32 encoding=SIGNED
[2] PTR (anon) type_id=3
[3] STRUCT t size=8
	a type_id=1 bitfield_size=1 bits_offset=0
	b type_id=1 bitfield_size=0 bits_offset=32
[4] FUNC_PROTO (anon) return=1 args=(2 a1, 1 a2)
[5] FUNC foo type_id=4
search cu 't.c' for percpu global variables.
Variable 'g' from CU 't.c' at address 0x0 encoded
[6] VAR g type=1 linkage=1
[7] DATASEC .data..percpu size=4 vlen=1
	type=6 offset=0 size=4
⬢[acme@toolbox pahole]$ clang -v
clang version 14.0.0 (https://github.com/llvm/llvm-project f7e82e4fa849376ea9226220847a098dc92d74a0)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/11
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/11
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
⬢[acme@toolbox pahole]$

And the usual:

⬢[acme@toolbox pahole]$ pahole -J vmlinux
⬢[acme@toolbox pahole]$ btfdiff vmlinux
⬢[acme@toolbox pahole]$

So applied, pushing out to the tmp.master branch so that libbpf's CI can
have a go at it, then after the next run I'll push to master, thanks!

- Arnaldo