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Date: Fri, 10 Jan 2020 09:22:31 -0800
From: Stanislav Fomichev <sdf@...ichev.me>
To: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, daniel@...earbox.net,
ast@...com, andriin@...com, morbo@...gle.com
Subject: Re: pahole and LTO
On 01/10, Arnaldo Carvalho de Melo wrote:
> Em Fri, Jan 10, 2020 at 08:44:10AM -0800, Stanislav Fomichev escreveu:
> > tl;dr - building the kernel with clang and lto breaks BTF generation because
> > pahole doesn't seem to understand cross-cu references.
>
> tl;dr response:
>
> Yeah, so it may be the time to fix that, elfutils has interfaces for it,
> and the tools that come with it handle cross-cu references, so we need
> to study that and make pahole understand it.
Sure, we can definitely help with the implementation unless someone
is already actively working on it. Just wanted to make sure that's
a known problem.
>From my (limited) looking at pahole sources, it seems that building
and index on the first pass and doing a second pass to resolve
cross-cu references is relatively easy to implement. Am I missing
anything? (not a dwarf expert in any sense).
And where do the patches for pahole go? I don't see any pahole patches
in bpf/netdev mailing lists.
> - Arnaldo
>
> > 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
>
> --
>
> - Arnaldo
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