Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

[Hao Luo]

Thanks for reporting this and cc'ing me. I forgot to update the error
messages when renaming the flags. I will send a patch to fix the error
message.

The commit

commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
Author:     Hao Luo <haoluo@google.com>
AuthorDate: Wed Jul 8 13:44:10 2020 -0700

     btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.

encodes kernel global variables into BTF so that bpf programs can
directly access them. If there is no need to access kernel global
variables, it's perfectly fine to use '--btf_encode_force' to skip
encoding bad symbols into BTF, or '--skip_encoding_btf_vars' to skip
encoding all global vars all together. I will add these info into the
updated error message.

Also cc bpf folks for attention of this bug.

Hao

On Tue, Oct 20, 2020 at 5:20 AM Arnaldo Carvalho de Melo
<acme@kernel.org> wrote:
>
> Em Tue, Oct 20, 2020 at 11:01:39AM +0200, Jiri Slaby escreveu:
> > Hi,
> >
> > On 19. 10. 20, 1:18, Érico Rolim wrote:
> > > I'm trying to build kernel 5.9.1 for arm64, and my dotconfig has
> > > `CONFIG_DEBUG_INFO_BTF=y`, which requires pahole for building. However, pahole
> > > version 1.18 segfaults during the build, as can be seen below:
> > >
> > > PAHOLE: Error: Found symbol of zero size when encoding btf (sym:
> > > '__kvm_nvhe_arm64_ssbd_callback_required', cu:
> > > 'arch/arm64/kernel/cpu_errata.c').
> >
> > The symbol is an alias coming from arch/arm64/kernel/vmlinux.lds:
> > __kvm_nvhe_arm64_ssbd_callback_required = arm64_ssbd_callback_required;;
> >
> > > PAHOLE: Error: Use '-j' or '--force' to ignore such symbols and force
> > > emit the btf.
> > > scripts/link-vmlinux.sh: line 141: 43837 Segmentation fault
> > > LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > >    LD      .tmp_vmlinux.kallsyms1
> > >    KSYM    .tmp_vmlinux.kallsyms1.o
> > >    LD      .tmp_vmlinux.kallsyms2
> > >    KSYM    .tmp_vmlinux.kallsyms2.o
> > >    LD      vmlinux
> > >    BTFIDS  vmlinux
> > > FAILED: load BTF from vmlinux: Unknown error -2make: ***
> > > [Makefile:1162: vmlinux] Error 255
> > >
> > > It is possible to force the build to continue if
> > >
> > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > >
> > > in scripts/link-vmlinux.sh is changed to
> > >
> > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J --btf_encode_force ${1}
> > >
> > > The suggested `-j` or `--force` flags don't exist, since they were removed in
> > > [1]. I believe `--btf_encode_force` should be suggested instead.
> >
> > Agreed, '--btf_encode_force' makes pahole to proceed without crashes.
> >
> > > It should be noted that the same build, but with pahole version 1.17, works
> > > without issue, so I think this is either a regression in pahole or the script
> > > will need to be changed for newer versions of pahole.
> >
> > Yeah, I observe the very same. I reported it at:
> > https://bugzilla.suse.com/show_bug.cgi?id=1177921
>
> Would it be possible to try with
> https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?h=tmp.libbtf_encoder
> ?
>
> This switches to using libbpf for the BTF encoder and may have fixed
> this problem.
>
> - Arnaldo
>
> > The backtrace:
> > > (gdb) where
> > > #0  __memmove_sse2_unaligned_erms () at
> > ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:300
> > > #1  0x00007ffff7f78346 in memcpy (__len=<optimized out>, __src=<optimized
> > out>, __dest=<optimized out>, __dest=<optimized out>, __src=<optimized out>,
> > __len=<optimized out>) at /usr/include/bits/string_fortified.h:34
> > > #2  gobuffer__add (gb=0x555555569aa0, s=0x7fffffffb50c, len=12) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/gobuffer.c:87
> > > #3  0x00007ffff7f8671f in btf_elf__add_datasec_type
> > (btfe=btfe@entry=0x555555569a40,
> > section_name=section_name@entry=0x7ffff7fa43ad ".data..percpu",
> > var_secinfo_buf=var_secinfo_buf@entry=0x555555569ac0) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/libbtf.c:721
> > > #4  0x00007ffff7f8d766 in btf_elf__encode (flags=0 '\000',
> > btfe=0x555555569a40) at /usr/src/debug/dwarves-1.18-1.1.x86_64/libbtf.c:857
> > > #5  btf_elf__encode (btfe=0x555555569a40, flags=<optimized out>) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/libbtf.h:71
> > > #6  0x00007ffff7f7fc70 in btf_encoder__encode () at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/btf_encoder.c:213
> > > #7  0x00007ffff7f80d17 in cu__encode_btf (cu=0x55555638d9b0, verbose=0,
> > force=false, skip_encoding_vars=false) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/btf_encoder.c:255
> > > #8  0x000055555555ac4d in pahole_stealer (cu=0x55555638d9b0,
> > conf_load=<optimized out>) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/pahole.c:2366
> > > #9  0x00007ffff7f89dab in finalize_cu (cus=0x5555555622d0,
> > dcu=0x7fffffffd080, conf=0x5555555610e0 <conf_load>, cu=0x55555638d9b0) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarf_loader.c:2473
> > > #10 finalize_cu_immediately (conf=0x5555555610e0 <conf_load>,
> > dcu=0x7fffffffd080, cu=0x55555638d9b0, cus=0x5555555622d0) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarf_loader.c:2317
> > > #11 cus__load_module (cus=cus@entry=0x5555555622d0, conf=0x5555555610e0
> > <conf_load>, mod=mod@entry=0x555555564760, dw=0x555555565960,
> > elf=elf@entry=0x555555562360, filename=0x7fffffffe846 "ss") at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarf_loader.c:2473
> > > #12 0x00007ffff7f8a0f1 in cus__process_dwflmod (dwflmod=0x555555564760,
> > userdata=<optimized out>, name=<optimized out>, base=<optimized out>,
> > arg=0x7fffffffe1b0) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarf_loader.c:2518
> > > #13 0x00007ffff7d4f571 in dwfl_getmodules () from /usr/lib64/libdw.so.1
> > > #14 0x00007ffff7f823ed in cus__process_file (filename=0x7fffffffe846 "ss",
> > fd=3, conf=<optimized out>, cus=0x5555555622d0) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarf_loader.c:2571
> > > #15 dwarf__load_file (cus=0x5555555622d0, conf=<optimized out>,
> > filename=0x7fffffffe846 "ss") at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarf_loader.c:2588
> > > #16 0x00007ffff7f76771 in cus__load_file (cus=cus@entry=0x5555555622d0,
> > conf=conf@entry=0x5555555610e0 <conf_load>, filename=0x7fffffffe846 "ss") at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarves.c:1958
> > > #17 0x00007ffff7f798a8 in cus__load_files (cus=0x5555555622d0,
> > conf=0x5555555610e0 <conf_load>, filenames=0x7fffffffe518) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/dwarves.c:2316
> > > #18 0x00005555555576fc in main (argc=3, argv=0x7fffffffe508) at
> > /usr/src/debug/dwarves-1.18-1.1.x86_64/pahole.c:2687
> >
> >
> > I suspect:
> > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > Author:     Hao Luo <haoluo@google.com>
> > AuthorDate: Wed Jul 8 13:44:10 2020 -0700
> >
> >     btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
> >
> >
> > Which added this machinery (btf_elf__add_datasec_type in particular).
> >
> > > - [1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/pahole.c?h=v1.18&id=1abc001417b579b86a9b27ff88c9095d8f498a46
> > >
> > > Thanks,
> > > Érico
> > >
> >
> >
> > --
> > js
> > suse labs
>
> --
>
> - Arnaldo

Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

[Andrii Nakryiko]

On Tue, Oct 20, 2020 at 10:05 AM Hao Luo <haoluo@google.com> wrote:
>
> Thanks for reporting this and cc'ing me. I forgot to update the error
> messages when renaming the flags. I will send a patch to fix the error
> message.
>
> The commit
>
> commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> Author:     Hao Luo <haoluo@google.com>
> AuthorDate: Wed Jul 8 13:44:10 2020 -0700
>
>      btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
>
> encodes kernel global variables into BTF so that bpf programs can
> directly access them. If there is no need to access kernel global
> variables, it's perfectly fine to use '--btf_encode_force' to skip
> encoding bad symbols into BTF, or '--skip_encoding_btf_vars' to skip
> encoding all global vars all together. I will add these info into the
> updated error message.
>
> Also cc bpf folks for attention of this bug.

I've already fixed the message as part of
2e719cca6672 ("btf_encoder: revamp how per-CPU variables are encoded")

It's currently still in the tmp.libbtf_encoder branch in pahole repo.

>
> Hao
>
> On Tue, Oct 20, 2020 at 5:20 AM Arnaldo Carvalho de Melo
> <acme@kernel.org> wrote:
> >
> > Em Tue, Oct 20, 2020 at 11:01:39AM +0200, Jiri Slaby escreveu:
> > > Hi,
> > >
> > > On 19. 10. 20, 1:18, Érico Rolim wrote:
> > > > I'm trying to build kernel 5.9.1 for arm64, and my dotconfig has
> > > > `CONFIG_DEBUG_INFO_BTF=y`, which requires pahole for building. However, pahole
> > > > version 1.18 segfaults during the build, as can be seen below:
> > > >
> > > > PAHOLE: Error: Found symbol of zero size when encoding btf (sym:
> > > > '__kvm_nvhe_arm64_ssbd_callback_required', cu:
> > > > 'arch/arm64/kernel/cpu_errata.c').
> > >
> > > The symbol is an alias coming from arch/arm64/kernel/vmlinux.lds:
> > > __kvm_nvhe_arm64_ssbd_callback_required = arm64_ssbd_callback_required;;
> > >
> > > > PAHOLE: Error: Use '-j' or '--force' to ignore such symbols and force
> > > > emit the btf.
> > > > scripts/link-vmlinux.sh: line 141: 43837 Segmentation fault
> > > > LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > >    LD      .tmp_vmlinux.kallsyms1
> > > >    KSYM    .tmp_vmlinux.kallsyms1.o
> > > >    LD      .tmp_vmlinux.kallsyms2
> > > >    KSYM    .tmp_vmlinux.kallsyms2.o
> > > >    LD      vmlinux
> > > >    BTFIDS  vmlinux
> > > > FAILED: load BTF from vmlinux: Unknown error -2make: ***
> > > > [Makefile:1162: vmlinux] Error 255
> > > >
> > > > It is possible to force the build to continue if
> > > >
> > > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > >
> > > > in scripts/link-vmlinux.sh is changed to
> > > >
> > > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J --btf_encode_force ${1}
> > > >
> > > > The suggested `-j` or `--force` flags don't exist, since they were removed in
> > > > [1]. I believe `--btf_encode_force` should be suggested instead.
> > >
> > > Agreed, '--btf_encode_force' makes pahole to proceed without crashes.
> > >
> > > > It should be noted that the same build, but with pahole version 1.17, works
> > > > without issue, so I think this is either a regression in pahole or the script
> > > > will need to be changed for newer versions of pahole.
> > >
> > > Yeah, I observe the very same. I reported it at:
> > > https://bugzilla.suse.com/show_bug.cgi?id=1177921
> >
> > Would it be possible to try with
> > https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?h=tmp.libbtf_encoder
> > ?
> >
> > This switches to using libbpf for the BTF encoder and may have fixed
> > this problem.
> >
> > - Arnaldo
> >

[...]

> > >
> > >
> > > I suspect:
> > > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > > Author:     Hao Luo <haoluo@google.com>
> > > AuthorDate: Wed Jul 8 13:44:10 2020 -0700
> > >
> > >     btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
> > >
> > >
> > > Which added this machinery (btf_elf__add_datasec_type in particular).
> > >
> > > > - [1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/pahole.c?h=v1.18&id=1abc001417b579b86a9b27ff88c9095d8f498a46
> > > >
> > > > Thanks,
> > > > Érico
> > > >
> > >
> > >
> > > --
> > > js
> > > suse labs
> >
> > --
> >
> > - Arnaldo

Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

[Hao Luo]

On Tue, Oct 20, 2020 at 10:10 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Oct 20, 2020 at 10:05 AM Hao Luo <haoluo@google.com> wrote:
> >
> > Thanks for reporting this and cc'ing me. I forgot to update the error
> > messages when renaming the flags. I will send a patch to fix the error
> > message.
> >
> > The commit
> >
> > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > Author:     Hao Luo <haoluo@google.com>
> > AuthorDate: Wed Jul 8 13:44:10 2020 -0700
> >
> >      btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
> >
> > encodes kernel global variables into BTF so that bpf programs can
> > directly access them. If there is no need to access kernel global
> > variables, it's perfectly fine to use '--btf_encode_force' to skip
> > encoding bad symbols into BTF, or '--skip_encoding_btf_vars' to skip
> > encoding all global vars all together. I will add these info into the
> > updated error message.
> >
> > Also cc bpf folks for attention of this bug.
>
> I've already fixed the message as part of
> 2e719cca6672 ("btf_encoder: revamp how per-CPU variables are encoded")
>

Ah, that's awesome! Thanks for fixing this, Andrii. I haven't got time
to take a look at your patches last week, I will try to look at it
ASAP.

> It's currently still in the tmp.libbtf_encoder branch in pahole repo.
>
> >
> > Hao
> >
> > On Tue, Oct 20, 2020 at 5:20 AM Arnaldo Carvalho de Melo
> > <acme@kernel.org> wrote:
> > >
> > > Em Tue, Oct 20, 2020 at 11:01:39AM +0200, Jiri Slaby escreveu:
> > > > Hi,
> > > >
> > > > On 19. 10. 20, 1:18, Érico Rolim wrote:
> > > > > I'm trying to build kernel 5.9.1 for arm64, and my dotconfig has
> > > > > `CONFIG_DEBUG_INFO_BTF=y`, which requires pahole for building. However, pahole
> > > > > version 1.18 segfaults during the build, as can be seen below:
> > > > >
> > > > > PAHOLE: Error: Found symbol of zero size when encoding btf (sym:
> > > > > '__kvm_nvhe_arm64_ssbd_callback_required', cu:
> > > > > 'arch/arm64/kernel/cpu_errata.c').
> > > >
> > > > The symbol is an alias coming from arch/arm64/kernel/vmlinux.lds:
> > > > __kvm_nvhe_arm64_ssbd_callback_required = arm64_ssbd_callback_required;;
> > > >
> > > > > PAHOLE: Error: Use '-j' or '--force' to ignore such symbols and force
> > > > > emit the btf.
> > > > > scripts/link-vmlinux.sh: line 141: 43837 Segmentation fault
> > > > > LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > > >    LD      .tmp_vmlinux.kallsyms1
> > > > >    KSYM    .tmp_vmlinux.kallsyms1.o
> > > > >    LD      .tmp_vmlinux.kallsyms2
> > > > >    KSYM    .tmp_vmlinux.kallsyms2.o
> > > > >    LD      vmlinux
> > > > >    BTFIDS  vmlinux
> > > > > FAILED: load BTF from vmlinux: Unknown error -2make: ***
> > > > > [Makefile:1162: vmlinux] Error 255
> > > > >
> > > > > It is possible to force the build to continue if
> > > > >
> > > > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > > >
> > > > > in scripts/link-vmlinux.sh is changed to
> > > > >
> > > > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J --btf_encode_force ${1}
> > > > >
> > > > > The suggested `-j` or `--force` flags don't exist, since they were removed in
> > > > > [1]. I believe `--btf_encode_force` should be suggested instead.
> > > >
> > > > Agreed, '--btf_encode_force' makes pahole to proceed without crashes.
> > > >
> > > > > It should be noted that the same build, but with pahole version 1.17, works
> > > > > without issue, so I think this is either a regression in pahole or the script
> > > > > will need to be changed for newer versions of pahole.
> > > >
> > > > Yeah, I observe the very same. I reported it at:
> > > > https://bugzilla.suse.com/show_bug.cgi?id=1177921
> > >
> > > Would it be possible to try with
> > > https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?h=tmp.libbtf_encoder
> > > ?
> > >
> > > This switches to using libbpf for the BTF encoder and may have fixed
> > > this problem.
> > >
> > > - Arnaldo
> > >
>
> [...]
>
> > > >
> > > >
> > > > I suspect:
> > > > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > > > Author:     Hao Luo <haoluo@google.com>
> > > > AuthorDate: Wed Jul 8 13:44:10 2020 -0700
> > > >
> > > >     btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
> > > >
> > > >
> > > > Which added this machinery (btf_elf__add_datasec_type in particular).
> > > >
> > > > > - [1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/pahole.c?h=v1.18&id=1abc001417b579b86a9b27ff88c9095d8f498a46
> > > > >
> > > > > Thanks,
> > > > > Érico
> > > > >
> > > >
> > > >
> > > > --
> > > > js
> > > > suse labs
> > >
> > > --
> > >
> > > - Arnaldo

Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

[Arnaldo Carvalho de Melo]

Em Tue, Oct 20, 2020 at 10:10:19AM -0700, Andrii Nakryiko escreveu:
> On Tue, Oct 20, 2020 at 10:05 AM Hao Luo <haoluo@google.com> wrote:
> > Thanks for reporting this and cc'ing me. I forgot to update the error
> > messages when renaming the flags. I will send a patch to fix the error
> > message.

> > The commit

> > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > Author:     Hao Luo <haoluo@google.com>
> > AuthorDate: Wed Jul 8 13:44:10 2020 -0700

> >      btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.

> > encodes kernel global variables into BTF so that bpf programs can
> > directly access them. If there is no need to access kernel global
> > variables, it's perfectly fine to use '--btf_encode_force' to skip
> > encoding bad symbols into BTF, or '--skip_encoding_btf_vars' to skip
> > encoding all global vars all together. I will add these info into the
> > updated error message.

> > Also cc bpf folks for attention of this bug.

> I've already fixed the message as part of
> 2e719cca6672 ("btf_encoder: revamp how per-CPU variables are encoded")

> It's currently still in the tmp.libbtf_encoder branch in pahole repo.

I'm now running:

  $ grep BTF=y ../build/s390x-v5.9.0+/.config
  CONFIG_DEBUG_INFO_BTF=y
  $ make -j24 CROSS_COMPILE=s390x-linux-gnu- ARCH=s390 O=../build/s390x-v5.9.0+/

To do the last test I wanted before moving it to master.

- Arnaldo

Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

[Arnaldo Carvalho de Melo]

Em Tue, Oct 20, 2020 at 03:14:59PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Oct 20, 2020 at 10:10:19AM -0700, Andrii Nakryiko escreveu:
> > On Tue, Oct 20, 2020 at 10:05 AM Hao Luo <haoluo@google.com> wrote:
> > > Thanks for reporting this and cc'ing me. I forgot to update the error
> > > messages when renaming the flags. I will send a patch to fix the error
> > > message.
> 
> > > The commit
> 
> > > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > > Author:     Hao Luo <haoluo@google.com>
> > > AuthorDate: Wed Jul 8 13:44:10 2020 -0700
> 
> > >      btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
> 
> > > encodes kernel global variables into BTF so that bpf programs can
> > > directly access them. If there is no need to access kernel global
> > > variables, it's perfectly fine to use '--btf_encode_force' to skip
> > > encoding bad symbols into BTF, or '--skip_encoding_btf_vars' to skip
> > > encoding all global vars all together. I will add these info into the
> > > updated error message.
> 
> > > Also cc bpf folks for attention of this bug.
> 
> > I've already fixed the message as part of
> > 2e719cca6672 ("btf_encoder: revamp how per-CPU variables are encoded")
> 
> > It's currently still in the tmp.libbtf_encoder branch in pahole repo.
> 
> I'm now running:
> 
>   $ grep BTF=y ../build/s390x-v5.9.0+/.config
>   CONFIG_DEBUG_INFO_BTF=y
>   $ make -j24 CROSS_COMPILE=s390x-linux-gnu- ARCH=s390 O=../build/s390x-v5.9.0+/

  $ ls -la /home/acme/git/build/s390x-v5.9.0+/.tmp_vmlinux.btf
  -rwxrwxr-x. 1 acme acme 304592928 Oct 20 15:26 /home/acme/git/build/s390x-v5.9.0+/.tmp_vmlinux.btf
  $ file /home/acme/git/build/s390x-v5.9.0+/.tmp_vmlinux.btf
  /home/acme/git/build/s390x-v5.9.0+/.tmp_vmlinux.btf: ELF 64-bit MSB executable, IBM S/390, version 1 (SYSV), statically linked, BuildID[sha1]=ed39402fdbd7108c1055baaa61cfc6b0e431901d, with debug_info, not stripped
  $ pahole -F btf -C list_head /home/acme/git/build/s390x-v5.9.0+/.tmp_vmlinux.btf
  struct list_head {
  	struct list_head *         next;                 /*     0     8 */
  	struct list_head *         prev;                 /*     8     8 */
  
  	/* size: 16, cachelines: 1, members: 2 */
  	/* last cacheline: 16 bytes */
  };
  $
  $ readelf -wi /home/acme/git/build/s390x-v5.9.0+/vmlinux | grep -m2 DW_AT_producer
      <28>   DW_AT_producer    : (indirect string, offset: 0x51): GNU AS 2.34
      <3b>   DW_AT_producer    : (indirect string, offset: 0xeb46): GNU C89 9.2.1 20190827 (Red Hat Cross 9.2.1-3) -m64 -mwarn-dynamicstack -mbackchain -msoft-float -march=z196 -mtune=z196 -mpacked-stack -mindirect-branch=thunk -mfunction-return=thunk -mindirect-branch-table -mrecord-mcount -mnop-mcount -mfentry -mzarch -g -O2 -std=gnu90 -p -fno-strict-aliasing -fno-common -fshort-wchar -fPIE -fno-asynchronous-unwind-tables -fno-delete-null-pointer-checks -fno-reorder-blocks -fno-ipa-cp-clone -fno-partial-inlining -fno-stack-protector -fno-var-tracking-assignments -fno-inline-functions-called-once -falign-functions=32 -fno-strict-overflow -fstack-check=no -fconserve-stack -fno-function-sections -fno-data-sections -fsanitize=kernel-address -fasan-shadow-offset=0x18000000000000 -fsanitize=bounds -fsanitize=shift -fsanitize=integer-divide-by-zero -fsanitize=unreachable -fsanitize=signed-integer-overflow -fsanitize=object-size -fsanitize=bool -fsanitize=enum -fsanitize-undefined-trap-on-error -fsanitize-coverage=trace-pc -fsanitize-coverage=trace-cmp --param allow-store-data-races=0 --param asan-globals=1 --param asan-instrumentation-with-call-threshold=0 --param asan-stack=1 --param asan-instrument-allocas=1
  $
  $ file /home/acme/git/build/s390x-v5.9.0+/vmlinux
  /home/acme/git/build/s390x-v5.9.0+/vmlinux: ELF 64-bit MSB executable, IBM S/390, version 1 (SYSV), statically linked, BuildID[sha1]=fbb252d8dccc11d8e66d6f248d06bcdca4e7db7a, with debug_info, not stripped
  $

But I noticed that 'btfdiff' is showing differences from output
generated from DWARF and the one generated from BTF, the first issue
is:

[acme@five pahole]$ btfdiff /home/acme/git/build/v5.9.0+/vmlinux
<SNIP>
@@ -115549,7 +120436,7 @@ struct irq_router_handler {
 
 	/* XXX 6 bytes hole, try to pack */
 
-	int                        (*probe)(struct irq_router * , struct pci_dev * , u16 ); /*     8     8 */
+	int                        (*probe)(struct irq_router *, struct pci_dev *, u16); /*     8     8 */
 
 	/* size: 16, cachelines: 1, members: 2 */
 	/* sum members: 10, holes: 1, sum holes: 6 */
[acme@five pahole]$

The BTF output (the one starting with '+' in the diff output) is better, just
different than it was before, I'll fix the DWARF one to avoid that needless
space for arg lists without names.

The other problem I noticed is a bit more worrying:

@@ -52,13 +52,29 @@ struct file_system_type {
        /* last cacheline: 8 bytes */
 };
 struct qspinlock {
-       union                      ;                     /*     0     4 */
+       union {
+               atomic_t           val;                  /*     0     4 */
+               struct {
+                       u8         locked;               /*     0     1 */
+                       u8         pending;              /*     1     1 */
+               };                                       /*     0     2 */
+               struct {
+                       u16        locked_pending;       /*     0     2 */
+                       u16        tail;                 /*     2     2 */
+               };                                       /*     0     4 */
+       };                                               /*     0     4 */
 
        /* size: 4, cachelines: 1, members: 1 */
        /* last cacheline: 4 bytes */
 };
 struct qrwlock {
-       union                      ;                     /*     0     4 */
+       union {
+               atomic_t           cnts;                 /*     0     4 */
+               struct {
+                       u8         wlocked;              /*     0     1 */
+                       u8         __lstate[3];          /*     1     3 */
+               };                                       /*     0     4 */
+       };                                               /*     0     4 */
        arch_spinlock_t            wait_lock;            /*     4     4 */
 
        /* size: 8, cachelines: 1, members: 2 */

But again, its the DWARF code that is wrong :-)

So, for what is being tested here, which is BTF generation, things looks Ok:

i.e. using BTF:

[acme@five perf]$ pahole qspinlock
struct qspinlock {
	union {
		atomic_t           val;                  /*     0     4 */
		struct {
			u8         locked;               /*     0     1 */
			u8         pending;              /*     1     1 */
		};                                       /*     0     2 */
		struct {
			u16        locked_pending;       /*     0     2 */
			u16        tail;                 /*     2     2 */
		};                                       /*     0     4 */
	};                                               /*     0     4 */

	/* size: 4, cachelines: 1, members: 1 */
	/* last cacheline: 4 bytes */
};
[acme@five perf]$

While using DWARF:

[acme@five perf]$ pahole -F dwarf -C qspinlock
struct qspinlock {
	union                      ;                     /*     0     4 */

	/* size: 4, cachelines: 1, members: 1 */
	/* last cacheline: 4 bytes */
};
[acme@five perf]$

typedef struct qspinlock {
        union {
                atomic_t val;

                /*
                 * By using the whole 2nd least significant byte for the
                 * pending bit, we can allow better optimization of the lock
                 * acquisition for the pending bit holder.
                 */
#ifdef __LITTLE_ENDIAN
                struct {
                        u8      locked;
                        u8      pending;
                };
                struct {
                        u16     locked_pending;
                        u16     tail;
                };
#else
                struct {
                        u16     tail;
                        u16     locked_pending;
                };
                struct {
                        u8      reserved[2];
                        u8      pending;
                        u8      locked;
                };
#endif
        };
} arch_spinlock_t;

This is just a heads up, will investigate further...

- Arnaldo

 
> To do the last test I wanted before moving it to master.