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Message-ID: <CAP-5=fWQCZ6kC6rg5T1zOcshzEqhraXT5ix9kA=SjimiC4T-ZA@mail.gmail.com>
Date: Sun, 31 Aug 2025 22:22:01 -0700
From: Ian Rogers <irogers@...gle.com>
To: Yonghong Song <yonghong.song@...ux.dev>
Cc: Blake Jones <blakejones@...gle.com>, namhyung@...nel.org,
Arnaldo Carvalho de Melo <acme@...nel.org>, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
haoluo@...gle.com, jolsa@...nel.org, mykolal@...com, shuah@...nel.org,
ihor.solodrai@...ux.dev, bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org,
linux-perf-users <linux-perf-users@...r.kernel.org>, Howard Chu <howardchu95@...il.com>
Subject: Re: [PATCH v3 1/2] libbpf: add support for printing BTF character
arrays as strings
On Sun, Aug 31, 2025 at 9:17 PM Yonghong Song <yonghong.song@...ux.dev> wrote:
>
>
>
> On 8/29/25 10:19 PM, Ian Rogers wrote:
> > On Thu, Jun 5, 2025 at 2:00 PM <patchwork-bot+netdevbpf@...nel.org> wrote:
> >> Hello:
> >>
> >> This series was applied to bpf/bpf-next.git (master)
> >> by Andrii Nakryiko <andrii@...nel.org>:
> >>
> >> On Tue, 3 Jun 2025 13:37:00 -0700 you wrote:
> >>> The BTF dumper code currently displays arrays of characters as just that -
> >>> arrays, with each character formatted individually. Sometimes this is what
> >>> makes sense, but it's nice to be able to treat that array as a string.
> >>>
> >>> This change adds a special case to the btf_dump functionality to allow
> >>> 0-terminated arrays of single-byte integer values to be printed as
> >>> character strings. Characters for which isprint() returns false are
> >>> printed as hex-escaped values. This is enabled when the new ".emit_strings"
> >>> is set to 1 in the btf_dump_type_data_opts structure.
> >>>
> >>> [...]
> >> Here is the summary with links:
> >> - [v3,1/2] libbpf: add support for printing BTF character arrays as strings
> >> https://git.kernel.org/bpf/bpf-next/c/87c9c79a02b4
> >> - [v3,2/2] Tests for the ".emit_strings" functionality in the BTF dumper.
> >> https://git.kernel.org/bpf/bpf-next/c/a570f386f3d1
> >>
> >> You are awesome, thank you!
> > I believe this patch is responsible for segvs occurring in v6.17 in
> > various perf tests when the perf tests run in parallel. There's lots
>
> Could you share the command line to reproduce this failure?
> This will help debugging. Thanks!
Sure:
```
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
...
$ cd linux
$ mkdir /tmp/perf
$ make -C tools/perf O=/tmp/perf
make: Entering directory 'linux/tools/perf'
BUILD: Doing 'make -j28' parallel build
Warning: Kernel ABI header differences:
diff -u tools/arch/arm64/include/asm/cputype.h arch/arm64/include/asm/cputype.h
Auto-detecting system features:
... libdw: [ on ]
... glibc: [ on ]
... libelf: [ on ]
... libnuma: [ on ]
... numa_num_possible_cpus: [ on ]
... libperl: [ on ]
... libpython: [ on ]
... libcapstone: [ on ]
... llvm-perf: [ on ]
... zlib: [ on ]
... lzma: [ on ]
... get_cpuid: [ on ]
... bpf: [ on ]
... libaio: [ on ]
... libzstd: [ on ]
GEN /tmp/perf/common-cmds.h
...
LINK /tmp/perf/perf
GEN /tmp/perf/python/perf.cpython-313-x86_64-linux-gnu.so
make: Leaving directory 'linux/tools/perf'
$ git describe
v6.17-rc4
$ sudo /tmp/perf/perf test -v
1: vmlinux symtab matches kallsyms : Skip
2: Detect openat syscall event : Ok
...
79: build id cache operations : Ok
--- start ---
test child forked, pid 2546565
test daemon list
FAILED: wrong name
FAILED: wrong run
FAILED: wrong base
FAILED: wrong output
FAILED: wrong control
FAILED: wrong ack
test daemon reconfig
FAILED: Timeout waiting for daemon to ping
---- end(-1) ----
80: daemon operations : FAILED!
81: perf diff tests : Ok
82: DRM PMU : Ok
83: perf ftrace tests : Ok
--- start ---
test child forked, pid 2546573
Test perf header file
perf: Segmentation fault
linux/tools/perf/tests/shell/header.sh: line 51: 2546664 Segmentation
fault (core dumped
) perf record -o "${perfdata}" -- perf test -w noploop
Unexpected signal in test_file
---- end(-1) ----
84: perf header tests : FAILED!
85: perf list tests : Ok
...
92: perf sched tests : Ok
--- start ---
test child forked, pid 2546593
DB test
perf: Segmentation fault
/tmp/x/linux/tools/perf/tests/shell/script.sh: line 35: 2546822
Segmentation fault (core dumped
) perf record $cmd_flags -o "${perfdatafile}" true
--- Cleaning up ---
---- end(-1) ----
93: perf script tests : FAILED!
...
$ /tmp/perf/perf version --build-options
perf version 6.17.rc4.gb320789d6883
aio: [ on ] # HAVE_AIO_SUPPORT
bpf: [ on ] # HAVE_LIBBPF_SUPPORT
bpf_skeletons: [ on ] # HAVE_BPF_SKEL
debuginfod: [ on ] # HAVE_DEBUGINFOD_SUPPORT
dwarf: [ on ] # HAVE_LIBDW_SUPPORT
dwarf_getlocations: [ on ] # HAVE_LIBDW_SUPPORT
dwarf-unwind: [ on ] # HAVE_DWARF_UNWIND_SUPPORT
auxtrace: [ on ] # HAVE_AUXTRACE_SUPPORT
libbfd: [ OFF ] # HAVE_LIBBFD_SUPPORT ( tip:
Deprecated, license incompatibility, u
se BUILD_NONDISTRO=1 and install binutils-dev[el] )
libbpf-strings: [ on ] # HAVE_LIBBPF_STRINGS_SUPPORT
libcapstone: [ on ] # HAVE_LIBCAPSTONE_SUPPORT
libdw-dwarf-unwind: [ on ] # HAVE_LIBDW_SUPPORT
libelf: [ on ] # HAVE_LIBELF_SUPPORT
libnuma: [ on ] # HAVE_LIBNUMA_SUPPORT
libopencsd: [ OFF ] # HAVE_CSTRACE_SUPPORT
libperl: [ on ] # HAVE_LIBPERL_SUPPORT
libpfm4: [ on ] # HAVE_LIBPFM
libpython: [ on ] # HAVE_LIBPYTHON_SUPPORT
libslang: [ on ] # HAVE_SLANG_SUPPORT
libtraceevent: [ on ] # HAVE_LIBTRACEEVENT
libunwind: [ OFF ] # HAVE_LIBUNWIND_SUPPORT ( tip:
Deprecated, use LIBUNWIND=1 and ins
tall libunwind-dev[el] to build with it )
lzma: [ on ] # HAVE_LZMA_SUPPORT
numa_num_possible_cpus: [ on ] # HAVE_LIBNUMA_SUPPORT
zlib: [ on ] # HAVE_ZLIB_SUPPORT
zstd: [ on ] # HAVE_ZSTD_SUPPORT
```
Thanks,
Ian
> > of BPF things happening in parallel in the test but the failures are
> > happening in a shell and I did get to attach a debugger. I've not seen
> > this problem earlier as the patches weren't in the perf-tools-next
> > tree. Through bisection I was able to blame the patch and I came up
> > with this minimal fix:
> > ```
> > diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> > index ccfd905f03df..71e198b30c5f 100644
> > --- a/tools/lib/bpf/btf.h
> > +++ b/tools/lib/bpf/btf.h
> > @@ -326,10 +326,10 @@ struct btf_dump_type_data_opts {
> > bool compact; /* no newlines/indentation */
> > bool skip_names; /* skip member/type names */
> > bool emit_zeroes; /* show 0-valued fields */
> > - bool emit_strings; /* print char arrays as strings */
> > + //bool emit_strings; /* print char arrays as strings */
> > size_t :0;
> > };
> > -#define btf_dump_type_data_opts__last_field emit_strings
> > +#define btf_dump_type_data_opts__last_field emit_zeroes
> >
> > LIBBPF_API int
> > btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
> > diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> > index f09f25eccf3c..c7b5a376642f 100644
> > --- a/tools/lib/bpf/btf_dump.c
> > +++ b/tools/lib/bpf/btf_dump.c
> > @@ -2599,7 +2599,7 @@ int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
> > d->typed_dump->compact = OPTS_GET(opts, compact, false);
> > d->typed_dump->skip_names = OPTS_GET(opts, skip_names, false);
> > d->typed_dump->emit_zeroes = OPTS_GET(opts, emit_zeroes, false);
> > - d->typed_dump->emit_strings = OPTS_GET(opts, emit_strings, false);
> > + d->typed_dump->emit_strings = true; // OPTS_GET(opts,
> > emit_strings, false);
> >
> > ret = btf_dump_dump_type_data(d, NULL, t, id, data, 0, 0);
> >
> >
> > ```
> > So I think the problem relates to modifying struct
> > btf_dump_type_data_opts. Given I'm statically linking libbpf into perf
> > I'm not sure on the exact route of the segv, no doubt this report will
> > be enough for someone else to figure it out.
> >
> > Given this is a regression what should the fix be?
> >
> > Thanks,
> > Ian
>
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