From b55a35326c9ea1cd9287a7ce87193443bb523c9b Mon Sep 17 00:00:00 2001

From: Arnaldo Carvalho de Melo <acme@redhat.com>

Date: Mon, 10 Oct 2022 11:20:07 -0300

Subject: [PATCH 6/7] btf_encoder: Encode DW_TAG_unspecified_type returning

 routines as void

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Since we don´t have how to encode this info in BTF, and from what we

saw, at least in this case:

Built binutils from git://sourceware.org/git/binutils-gdb.git, then used

gcc's -B option to point to the directory with the new as, that is built

as as-new, so make a symlink, ending up with:

  15e20ce2324a:~/git/linux # readelf -wi ./arch/x86/entry/entry.o

  Contents of the .debug_info section:

    Compilation Unit @ offset 0:

     Length:        0x35 (32-bit)

     Version:       5

     Unit Type:     DW_UT_compile (1)

     Abbrev Offset: 0

     Pointer Size:  8

   <0><c>: Abbrev Number: 1 (DW_TAG_compile_unit)

      <d>   DW_AT_stmt_list   : 0

      <11>   DW_AT_low_pc      : 0

      <19>   DW_AT_high_pc     : 19

      <1a>   DW_AT_name        : (indirect string, offset: 0): arch/x86/entry/entry.S

      <1e>   DW_AT_comp_dir    : (indirect string, offset: 0x17): /root/git/linux

      <22>   DW_AT_producer    : (indirect string, offset: 0x27): GNU AS 2.39.50

      <26>   DW_AT_language    : 32769  (MIPS assembler)

   <1><28>: Abbrev Number: 2 (DW_TAG_subprogram)

      <29>   DW_AT_name        : (indirect string, offset: 0x36): entry_ibpb

      <2d>   DW_AT_external    : 1

      <2d>   DW_AT_type        : <0x37>

      <2e>   DW_AT_low_pc      : 0

      <36>   DW_AT_high_pc     : 19

   <1><37>: Abbrev Number: 3 (DW_TAG_unspecified_type)

   <1><38>: Abbrev Number: 0

So we have that asm label encoded by GNU AS 2.39.50 as a

DW_TAG_subprogram that has as its DW_AT_type the DW_TAG_unspecified_type

0x37 that we convert to 0 (void):

  15e20ce2324a:~/git/linux # pahole -J ./arch/x86/entry/entry.o

  15e20ce2324a:~/git/linux # pahole -JV ./arch/x86/entry/entry.o

  btf_encoder__new: 'entry.o' doesn't have '.data..percpu' section

  Found 0 per-CPU variables!

  Found 1 functions!

  File entry.o:

  [1] FUNC_PROTO (anon) return=0 args=(void)

  [2] FUNC entry_ibpb type_id=1

  15e20ce2324a:~/git/linux # pfunct -F btf ./arch/x86/entry/entry.o

  entry_ibpb

  15e20ce2324a:~/git/linux # pfunct --proto -F btf ./arch/x86/entry/entry.o

  void  entry_ibpb(void);

  15e20ce2324a:~/git/linux #

  15e20ce2324a:~/git/linux # tools/bpf/bpftool/bpftool btf dump file ./arch/x86/entry/entry.o format raw

  [1] FUNC_PROTO '(anon)' ret_type_id=0 vlen=0

  [2] FUNC 'entry_ibpb' type_id=1 linkage=static

  15e20ce2324a:~/git/linux #

I think this is what can be done to avoid having to skip ASM DWARF when

gets widely used, i.e. binutils gets updated.

Acked-by: Yonghong Song <yhs@fb.com>

Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,

Cc: Martin Liška <mliska@suse.cz>

Link: https://lore.kernel.org/all/Y0R7uu3s%2FimnvPzM@kernel.org/

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

---

 btf_encoder.c | 33 ++++++++++++++++++++++++++++++---

 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c

index e74862cfc0a3a802..eee55a0ade5e74ee 100644

--- a/btf_encoder.c

+++ b/btf_encoder.c

@@ -570,6 +570,19 @@ static int32_t btf_encoder__add_func_param(struct btf_encoder *encoder, const ch

 	}

 }

+static int32_t btf_encoder__tag_type(struct btf_encoder *encoder, uint32_t type_id_off, uint32_t tag_type)

+{

+	if (tag_type == 0)

+		return 0;

+

+	if (encoder->cu->unspecified_type.tag && tag_type == encoder->cu->unspecified_type.type) {

+		// No provision for encoding this, turn it into void.

+		return 0;

+	}

+

+	return type_id_off + tag_type;

+}

+

 static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct ftype *ftype, uint32_t type_id_off)

 {

 	struct btf *btf = encoder->btf;

@@ -580,7 +593,7 @@ static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct f

 	/* add btf_type for func_proto */

 	nr_params = ftype->nr_parms + (ftype->unspec_parms ? 1 : 0);

-	type_id = ftype->tag.type == 0 ? 0 : type_id_off + ftype->tag.type;

+	type_id = btf_encoder__tag_type(encoder, type_id_off, ftype->tag.type);

 	id = btf__add_func_proto(btf, type_id);

 	if (id > 0) {

@@ -943,6 +956,15 @@ static int btf_encoder__encode_tag(struct btf_encoder *encoder, struct tag *tag,

 		return btf_encoder__add_enum_type(encoder, tag, conf_load);

 	case DW_TAG_subroutine_type:

 		return btf_encoder__add_func_proto(encoder, tag__ftype(tag), type_id_off);

+        case DW_TAG_unspecified_type:

+		/* Just don't encode this for now, converting anything with this type to void (0) instead.

+		 *

+		 * If we end up needing to encode this, one possible hack is to do as follows, as "const void".

+		 *

+		 * Returning zero means we skipped encoding a DWARF type.

+		 */

+               // btf_encoder__add_ref_type(encoder, BTF_KIND_CONST, 0, NULL, false);

+               return 0;

 	default:

 		fprintf(stderr, "Unsupported DW_TAG_%s(0x%x): type: 0x%x\n",

 			dwarf_tag_name(tag->tag), tag->tag, ref_type_id);

@@ -1464,7 +1486,7 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu, struct co

 {

 	uint32_t type_id_off = btf__type_cnt(encoder->btf) - 1;

 	struct llvm_annotation *annot;

-	int btf_type_id, tag_type_id;

+	int btf_type_id, tag_type_id, skipped_types = 0;

 	uint32_t core_id;

 	struct function *fn;

 	struct tag *pos;

@@ -1487,8 +1509,13 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu, struct co

 	cu__for_each_type(cu, core_id, pos) {

 		btf_type_id = btf_encoder__encode_tag(encoder, pos, type_id_off, conf_load);

+		if (btf_type_id == 0) {

+			++skipped_types;

+			continue;

+		}

+

 		if (btf_type_id < 0 ||

-		    tag__check_id_drift(pos, core_id, btf_type_id, type_id_off)) {

+		    tag__check_id_drift(pos, core_id, btf_type_id + skipped_types, type_id_off)) {

 			err = -1;

 			goto out;

 		}

-- 

2.39.1