linux-kernel - Re: [PATCH v3 1/2] libbpf: add support for printing BTF character arrays as strings

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Message-ID: <466d45ae-ce97-4256-9444-9f25f3328c51@linux.dev>
Date: Sun, 31 Aug 2025 21:17:34 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Ian Rogers <irogers@...gle.com>, Blake Jones <blakejones@...gle.com>,
 namhyung@...nel.org, Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: 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 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!

> 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