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

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Message-ID: <CAK8ByeK8dGcbxfXghw6=LrhSWLmO0a4XuB8C0nsUc812aoU0Pw@mail.gmail.com>
Date:   Fri, 1 Dec 2023 10:00:51 +0100
From:   Łukasz Bartosik <lb@...ihalf.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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

pt., 1 gru 2023 o 01:25 Steven Rostedt <rostedt@...dmis.org> napisał(a):
>
> 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.
>

I will fix it.

> > 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
>

Thank you Steve. I will update the __get_str_strip_nl macro per your
recommendation.

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