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Subject: Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64
To:     =?UTF-8?Q?=c3=89rico_Rolim?= <erico.erc@gmail.com>,
        dwarves@vger.kernel.org, linux-kernel@vger.kernel.org,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Hao Luo <haoluo@google.com>
References: <CAFDeuWM7D-Upi84-JovKa3g8Y_4fjv65jND3--e9u-tER3WmVA@mail.gmail.com>
From:   Jiri Slaby <jirislaby@kernel.org>
Message-ID: <82b757bb-1f49-ab02-2f4b-89577d56fec9@kernel.org>
Date:   Tue, 20 Oct 2020 11:01:39 +0200
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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

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