pahole and LTO

[Stanislav Fomichev <sdf@fomichev.me>]

tl;dr - building the kernel with clang and lto breaks BTF generation because
pahole doesn't seem to understand cross-cu references.

Can be reproduced with the following:
    $ cat a.c
    struct s;

    void f1() {}

    __attribute__((always_inline)) void f2(struct s *p)
    {
            if (p)
                    f1();
    }
    $ cat b.c
    struct s {
            int x;
    };

    void f2(struct s *p);

    int main()
    {
            struct s s = { 10 };
            f2(&s);
    }
    $ clang -fuse-ld=lld -flto {a,b}.c -g

    $ pahole a.out
    tag__recode_dwarf_type: couldn't find 0x3f type for 0x99 (inlined_subroutine)!
    lexblock__recode_dwarf_types: couldn't find 0x3f type for 0x99 (inlined_subroutine)!
    struct s {
            int                        x;                    /*     0     4 */

            /* size: 4, cachelines: 1, members: 1 */
            /* last cacheline: 4 bytes */
    };

From what I can tell, pahole internally loops over each cu and resolves only
local references, while the dwarf spec (table 2.3) states the following
about 'reference':
"Refers to one of the debugging information entries that describe the program.
There are four types of reference. The first is an offset relative to the
beginning of the compilation unit in which the reference occurs and must
refer to an entry within that same compilation unit. The second type of
reference is the offset of a debugging information entry in any compilation
unit, including one different from the unit containing the reference. The
third type of reference is an indirect reference to a type definition using
an 8-byte signature for that type. The fourth type of reference is a reference
from within the .debug_info section of the executable or shared object file to
a debugging information entry in the .debug_info section of a supplementary
object file."

In particular: "The second type of reference is the offset of a debugging
information entry in any compilation unit, including one different from the
unit containing the reference."


So the question is: is it a (known) issue? Is it something that's ommitted
on purpose? Or it's not implemented because lto is not (yet) widely used?


Here is the dwarf:

$ readelf --debug-dump=info a.out
Contents of the .debug_info section:

  Compilation Unit @ offset 0x0:
   Length:        0x44 (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  8
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <c>   DW_AT_producer    : (indirect string, offset: 0x11): clang version 10.0.0 (https://github.com/llvm/llvm-project.git 5fe4679cc9cfb4941b766db07bf3cd928075d204)
    <10>   DW_AT_language    : 12	(ANSI C99)
    <12>   DW_AT_name        : (indirect string, offset: 0x0): a.c
    <16>   DW_AT_stmt_list   : 0x0
    <1a>   DW_AT_comp_dir    : (indirect string, offset: 0x7a): /usr/local/google/home/sdf/tmp/lto
    <1e>   DW_AT_low_pc      : 0x201730
    <26>   DW_AT_high_pc     : 0x6
 <1><2a>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2b>   DW_AT_low_pc      : 0x201730
    <33>   DW_AT_high_pc     : 0x6
    <37>   DW_AT_frame_base  : 1 byte block: 56 	(DW_OP_reg6 (rbp))
    <39>   DW_AT_name        : (indirect string, offset: 0xa4): f1
    <3d>   DW_AT_decl_file   : 1
    <3e>   DW_AT_decl_line   : 3
    <3f>   DW_AT_external    : 1
 <1><3f>: Abbrev Number: 3 (DW_TAG_subprogram)
    <40>   DW_AT_name        : (indirect string, offset: 0x4): f2
    <44>   DW_AT_decl_file   : 1
    <45>   DW_AT_decl_line   : 5
    <46>   DW_AT_prototyped  : 1
    <46>   DW_AT_external    : 1
    <46>   DW_AT_inline      : 1	(inlined)
 <1><47>: Abbrev Number: 0
  Compilation Unit @ offset 0x48:
   Length:        0x7f (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  8
 <0><53>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <54>   DW_AT_producer    : (indirect string, offset: 0x11): clang version 10.0.0 (https://github.com/llvm/llvm-project.git 5fe4679cc9cfb4941b766db07bf3cd928075d204)
    <58>   DW_AT_language    : 12	(ANSI C99)
    <5a>   DW_AT_name        : (indirect string, offset: 0x7): b.c
    <5e>   DW_AT_stmt_list   : 0x3a
    <62>   DW_AT_comp_dir    : (indirect string, offset: 0x7a): /usr/local/google/home/sdf/tmp/lto
    <66>   DW_AT_low_pc      : 0x201740
    <6e>   DW_AT_high_pc     : 0x1f
 <1><72>: Abbrev Number: 4 (DW_TAG_subprogram)
    <73>   DW_AT_low_pc      : 0x201740
    <7b>   DW_AT_high_pc     : 0x1f
    <7f>   DW_AT_frame_base  : 1 byte block: 56 	(DW_OP_reg6 (rbp))
    <81>   DW_AT_name        : (indirect string, offset: 0x9d): main
    <85>   DW_AT_decl_file   : 1
    <86>   DW_AT_decl_line   : 7
    <87>   DW_AT_type        : <0xae>
    <8b>   DW_AT_external    : 1
 <2><8b>: Abbrev Number: 5 (DW_TAG_variable)
    <8c>   DW_AT_location    : 2 byte block: 91 78 	(DW_OP_fbreg: -8)
    <8f>   DW_AT_name        : (indirect string, offset: 0xb): s
    <93>   DW_AT_decl_file   : 1
    <94>   DW_AT_decl_line   : 9
    <95>   DW_AT_type        : <0xb5>
 <2><99>: Abbrev Number: 6 (DW_TAG_inlined_subroutine)
    <9a>   DW_AT_abstract_origin: <0x3f>
    <9e>   DW_AT_low_pc      : 0x201752
    <a6>   DW_AT_high_pc     : 0x5
    <aa>   DW_AT_call_file   : 1
    <ab>   DW_AT_call_line   : 10
    <ac>   DW_AT_call_column : 
 <2><ad>: Abbrev Number: 0
 <1><ae>: Abbrev Number: 7 (DW_TAG_base_type)
    <af>   DW_AT_name        : (indirect string, offset: 0xd): int
    <b3>   DW_AT_encoding    : 5	(signed)
    <b4>   DW_AT_byte_size   : 4
 <1><b5>: Abbrev Number: 8 (DW_TAG_structure_type)
    <b6>   DW_AT_name        : (indirect string, offset: 0xb): s
    <ba>   DW_AT_byte_size   : 4
    <bb>   DW_AT_decl_file   : 1
    <bc>   DW_AT_decl_line   : 1
 <2><bd>: Abbrev Number: 9 (DW_TAG_member)
    <be>   DW_AT_name        : (indirect string, offset: 0xa2): x
    <c2>   DW_AT_type        : <0xae>
    <c6>   DW_AT_decl_file   : 1
    <c7>   DW_AT_decl_line   : 2
    <c8>   DW_AT_data_member_location: 0
 <2><c9>: Abbrev Number: 0
 <1><ca>: Abbrev Number: 0