linux-kernel - Re: [PATCH 3/4] perf list: Don't write to const memory

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Message-ID: <20260121112536.27fd5d11@pumpkin>
Date: Wed, 21 Jan 2026 11:25:36 +0000
From: David Laight <david.laight.linux@...il.com>
To: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, James Clark <james.clark@...aro.org>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian
 Hunter <adrian.hunter@...el.com>, Kan Liang <kan.liang@...ux.intel.com>,
 Clark Williams <williams@...hat.com>, linux-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org, Arnaldo Carvalho de Melo
 <acme@...hat.com>
Subject: Re: [PATCH 3/4] perf list: Don't write to const memory

On Tue, 20 Jan 2026 19:08:59 -0300
Arnaldo Carvalho de Melo <acme@...nel.org> wrote:

> From: Arnaldo Carvalho de Melo <acme@...hat.com>
> 
> Something now detected on fedora 44, where strchr() returns const if it
> is passed a const pointer:
> 
>   util/print-events.c: In function 'print_sdt_events':
>   util/print-events.c:89:29: error: initialization discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
>      89 |                 char *bid = strchr(sdt_name->s, '@');
>         |                             ^~~~~~
> 
> Fix it by using strnchr() if strchr finds the separator instead of
> temporarily scrubbing it with '\0'.
> 
> Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> ---
>  tools/perf/util/print-events.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/perf/util/print-events.c b/tools/perf/util/print-events.c
> index 8f3ed83853a9e468..898cf426509790cd 100644
> --- a/tools/perf/util/print-events.c
> +++ b/tools/perf/util/print-events.c
> @@ -97,14 +97,11 @@ void print_sdt_events(const struct print_callbacks *print_cb, void *print_state)
>  		} else {
>  			next_sdt_name = strlist__next(sdt_name);
>  			if (next_sdt_name) {
> -				char *bid2 = strchr(next_sdt_name->s, '@');
> -
> -				if (bid2)
> -					*bid2 = '\0';
> -				if (strcmp(sdt_name->s, next_sdt_name->s) == 0)
> -					show_detail = true;
> -				if (bid2)
> -					*bid2 = '@';
> +				const char *bid2 = strchr(next_sdt_name->s, '@');
> +
> +				show_detail = bid2 ?
> +						strncmp(sdt_name->s, next_sdt_name->s, bid2 - next_sdt_name->s) == 0 :
> +						strcmp(sdt_name->s, next_sdt_name->s) == 0;

You could use:
	show_detail = strncmp(sdt_name->s, next_sdt_name->s, strcspn(sdt_name->s, "@"));
strcspn() will be slower, but it is succinct.
I'm sure there is a function like strchr() that returns a pointer to the '\0'
when the character isn't found - but I can't remember what it is called :-(

	David

>  			}
>  		}
>  		last_sdt_name = sdt_name->s;