linux-kernel - Re: [PATCH v2 05/15] tracefs: add __get_str_strip_nl

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Date:   Thu, 30 Nov 2023 19:25:54 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Łukasz Bartosik <lb@...ihalf.com>
Cc:     Jason Baron <jbaron@...mai.com>, Jim Cromie <jim.cromie@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <groeck@...gle.com>,
        Yaniv Tzoreff <yanivt@...gle.com>,
        Benson Leung <bleung@...gle.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        upstream@...ihalf.com, pmladek@...e.com,
        sergey.senozhatsky@...il.com, john.ogness@...utronix.de,
        Simon Ser <contact@...rsion.fr>
Subject: Re: [PATCH v2 05/15] tracefs: add __get_str_strip_nl - RFC

On Fri,  1 Dec 2023 00:40:38 +0100
Łukasz Bartosik <lb@...ihalf.com> wrote:

> From: Jim Cromie <jim.cromie@...il.com>
> 
> This variant of __get_str() removes the trailing newline. It is for
> use by printk/debug-ish events which already have a trailing newline.
> It is here to support:
> 
> https://lore.kernel.org/lkml/
> 20200825153338.17061-1-vincent.whitchurch@...s.com/

Line wrap breakage.

> which taught dyndbg to send pr_debug() msgs to tracefs, via -x/T flag.
> 
> It "reused" the include/trace/events/printk.h console event,
> which does the following:
> 
>        TP_fast_assign(
>                /*
>                 * Each trace entry is printed in a new line.
>                 * If the msg finishes with '\n', cut it off
>                 * to avoid blank lines in the trace.
>                 */
>                if (len > 0 && (msg[len-1] == '\n'))
>                        len -= 1;
> 
>                memcpy(__get_str(s), msg, len);
>                __get_str(s)[len] = 0;
> 	),
> 
> That trim work could be avoided, *if* all pr_debug() callers are
> known to have no '\n' to strip.  While that's not true for *all*
> callsites, it is 99+% true for DRM.debug callsites, and can be made
> true for some subsets of prdbg/dyndbg callsites.
> 
> WANTED: macros to validate that a literal format-str has or doesn't
> have a trailing newline, or to provide or trim trailing newline(s?).
> Should be usable in TP_printk* defns, for use in new event defns.
> 
> Cc: <rostedt@...dmis.org>
> Cc: Vincent Whitchurch <vincent.whitchurch@...s.com>
> Cc: <daniel@...ll.ch>
> Cc: <pmladek@...e.com>
> Cc: <sergey.senozhatsky@...il.com>
> Cc: <john.ogness@...utronix.de>
> Cc: Simon Ser <contact@...rsion.fr>
> Cc: Sean Paul <seanpaul@...omium.org>
> Signed-off-by: Jim Cromie <jim.cromie@...il.com>
> Signed-off-by: Łukasz Bartosik <lb@...ihalf.com>
> ---
>  include/trace/stages/stage3_trace_output.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/include/trace/stages/stage3_trace_output.h b/include/trace/stages/stage3_trace_output.h
> index c1fb1355d309..92a79bd5c0cd 100644
> --- a/include/trace/stages/stage3_trace_output.h
> +++ b/include/trace/stages/stage3_trace_output.h
> @@ -19,6 +19,15 @@
>  #undef __get_str
>  #define __get_str(field) ((char *)__get_dynamic_array(field))
>  
> +#undef __get_str_strip_nl
> +#define __get_str_strip_nl(field)					\
> +	({								\
> +		char *s = __get_str(field);				\
> +		size_t len = strlen(s);					\
> +		if (len && s[len-1] == '\n')				\
> +			s[len-1] = '\0'; s;				\
> +	})

I'd be worried about modifying the string itself as you are doing above.
That's modifying the source which may have unintended consequences.

That said, there's a trace_seq that is available for stage 3 called "p".
You can use that:

#define __get_str_strip_nl(field)					\
	({								\
		char *s = trace_seq_buffer_ptr(p);			\
		size_t len;						\
		trace_seq_printf(p, "%s", __get_str(field));		\
		trace_seq_putc(p, '\0');				\
		len = strlen(s);					\
		if (len && s[len-1] == '\n')				\
			s[len-1] = '\0';				\
		s;							\
	})

-- Steve

> +
>  #undef __get_rel_dynamic_array
>  #define __get_rel_dynamic_array(field)					\
>  		((void *)__entry + 					\