Arnaldo,
On Wed, Aug 26, 2020 at 6:12 AM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> Em Mon, Aug 24, 2020 at 05:45:23PM -0700, Hao Luo escreveu:
> > It is found on gcc 8.2 that global percpu variables generate the
> > following dwarf entry in the cu where the variable is defined[1].
> >
> > Take the global variable "bpf_prog_active" defined in
> > kernel/bpf/syscall.c as an example. The debug info for syscall.c
> > has two dwarf entries for "bpf_prog_active".
> >
[...]
>
> Interesting, here I get, with binutils' readelf:
>
> [root@quaco perf]# readelf -wi ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep bpf_prog_active
> <f6a1> DW_AT_name : (indirect string, offset: 0xb70d): bpf_prog_active
> [root@quaco perf]#
>
> Just one, as:
>
> [root@quaco perf]# readelf -wi ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep bpf_prog_active -B1 -A8
> <1><f6a0>: Abbrev Number: 103 (DW_TAG_variable)
> <f6a1> DW_AT_name : (indirect string, offset: 0xb70d): bpf_prog_active
> <f6a5> DW_AT_decl_file : 11
> <f6a6> DW_AT_decl_line : 1008
> <f6a8> DW_AT_decl_column : 1
> <f6a9> DW_AT_type : <0xcf>
> <f6ad> DW_AT_external : 1
> <f6ad> DW_AT_declaration : 1
> <1><f6ad>: Abbrev Number: 103 (DW_TAG_variable)
> <f6ae> DW_AT_name : (indirect string, offset: 0x3a5d): bpf_stats_enabled_mutex
> [root@quaco perf]#
>
> I get what you have when I use elfutils' readelf:
>
> [root@quaco perf]# eu-readelf -winfo ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep bpf_prog_active
> name (strp) "bpf_prog_active"
> [ 0] addr .data..percpu+0 <bpf_prog_active>
> [root@quaco perf]#
>
> [root@quaco perf]# eu-readelf -winfo ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep -B1 -A8 \"bpf_prog_active\"
> [ f6a0] variable abbrev: 103
> name (strp) "bpf_prog_active"
> decl_file (data1) bpf.h (11)
> decl_line (data2) 1008
> decl_column (data1) 1
> type (ref4) [ cf]
> external (flag_present) yes
> declaration (flag_present) yes
> [ f6ad] variable abbrev: 103
> name (strp) "bpf_stats_enabled_mutex"
> [root@quaco perf]#
>
> And:
>
> [root@quaco perf]# eu-readelf -winfo ../build/v5.8-rc5+/kernel/bpf/syscall.o | grep -B5 \<bpf_prog_active\>
> [ 1bdf5] variable abbrev: 212
> specification (ref4) [ f6a0]
> decl_file (data1) syscall.c (1)
> decl_line (data1) 43
> location (exprloc)
> [ 0] addr .data..percpu+0 <bpf_prog_active>
> [root@quaco perf]#
>
In binutils readelf, there is a extra entry
<1><1b24c>: Abbrev Number: 195 (DW_TAG_variable)
<1b24e> DW_AT_specification: <0xf335>
<1b252> DW_AT_decl_file : 1
<1b253> DW_AT_decl_line : 43
<1b254> DW_AT_location : 9 byte block: 3 0 0 0 0 0 0 0 0
(DW_OP_addr: 0)
which points to
<1><f335>: Abbrev Number: 95 (DW_TAG_variable)
<f336> DW_AT_name : (indirect string, offset: 0xb37a):
bpf_prog_active
It just doesn't have the string 'bpf_prog_active', annotating entry.
So eu-readelf and binutils readelf have the same results.
> > Note that second DW_TAG_variable entry contains specification that
> > points to the first entry.
>
> So you are not considering the first when encoding since it is just a
> DW_AT_declaration, considers the second, as it should be, and then needs
> to go see its DW_AT_specification, right?
>
> Sounds correct, applying, will test further and then push out,
>
Yes, exactly. The var tags to be considered are those that either have
DW_AT_specification or not have DW_AT_declaration. This makes sure
btf_encoder works correctly on both old and new gcc.
> Thanks,
>
> - Arnaldo
Suggested by Yonghong, I tested this change on a larger set of
compilers this time and works correctly. See below.
Could you also add 'Reported-by: Yonghong Song <yhs@xxxxxx>'? I should
have done that when sending out this patch. The credit goes to
Yonghong.
Thank you,
Hao
clang 10:
[67] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[20168] VAR 'bpf_prog_active' type_id=67, linkage=global-alloc
clang 9:
[64] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[19789] VAR 'bpf_prog_active' type_id=64, linkage=global-alloc
gcc 10.2
[18] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[20319] VAR 'bpf_prog_active' type_id=18, linkage=global-alloc
gcc 9.3:
[21] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[21085] VAR 'bpf_prog_active' type_id=21, linkage=global-alloc
gcc 8
[21] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[21084] VAR 'bpf_prog_active' type_id=21, linkage=global-alloc
gcc 6.2
[22] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[21083] VAR 'bpf_prog_active' type_id=22, linkage=global-alloc
gcc 4.9
[17] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[20410] VAR 'bpf_prog_active' type_id=17, linkage=global-alloc
