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

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Message-ID: <CAP-5=fWJQcmUOP7MuCA2ihKnDAHUCOBLkQFEkQES-1ZZTrgf8Q@mail.gmail.com>
Date: Fri, 29 Aug 2025 22:19:17 -0700
From: Ian Rogers <irogers@...gle.com>
To: 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, 
	yonghong.song@...ux.dev, 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 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
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