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From: Hao Luo <haoluo@google.com>
To: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>,
	dwarves@vger.kernel.org, Yonghong Song <yhs@fb.com>
Subject: Re: [PATCH v1] btf_encoder: Handle DW_TAG_variable that has DW_AT_specification
Date: Tue, 29 Sep 2020 23:52:32 -0700
Message-ID: <CA+khW7imZ+1to15Y+6Suw5_RRQfOQ32X_mkcFACDedjHrNYFaQ@mail.gmail.com> (raw)
In-Reply-To: <DEC4CC81-88CE-4476-A631-2BBB6E922F5C@gmail.com>

Arnaldo,

Is this patch ready to be merged into Pahole's master branch? Alexei
is testing the kernel patches that need this patch. Please let me know
if there is anything I can do to help merging.

Thank you,
Hao

On Wed, Aug 26, 2020 at 6:56 PM Arnaldo Carvalho de Melo
<arnaldo.melo@gmail.com> wrote:
>
>
>
> On August 26, 2020 3:35:17 PM GMT-03:00, Hao Luo <haoluo@google.com> wrote:
> >Arnaldo,
> >
> >On Wed, Aug 26, 2020 at 6:12 AM Arnaldo Carvalho de Melo
> ><acme@kernel.org> 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
>
> Not here, tomorrow I'll triple check.
>
> >
> > <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@fb.com>'? I should
> >have done that when sending out this patch. The credit goes to
> >Yonghong.
>
> Sure, and I'll add your results with different computers, for the record.
>
> Thanks,
>
> - Arnaldo
> >
> >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
>
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.

       reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200825004523.1353133-1-haoluo@google.com>
     [not found] ` <20200826131143.GF1059382@kernel.org>
     [not found]   ` <CA+khW7jf7Z=sMC1u5eyn6XOZDTFJaNjV-D0ogvQSyUGSKjC3LQ@mail.gmail.com>
     [not found]     ` <DEC4CC81-88CE-4476-A631-2BBB6E922F5C@gmail.com>
2020-09-30  6:52       ` Hao Luo [this message]
2020-10-01 15:47         ` Alexei Starovoitov
2020-10-01 18:24           ` Arnaldo Carvalho de Melo
2020-10-01 18:40             ` Hao Luo
2020-10-01 20:27               ` Arnaldo Carvalho de Melo
2020-10-01 20:57                 ` Hao Luo
2020-10-01 21:07                   ` Alexei Starovoitov
2020-10-01 21:31                     ` Hao Luo

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