linux-kernel - Re: [PATCH] tracing: events_hist: avoid using excessive stack space

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20210514101610.4392adbc@gandalf.local.home>
Date:   Fri, 14 May 2021 10:16:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Steven Rostedt (VMware)" <rostedt@...mis.org>,
        Tom Zanussi <zanussi@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Qiujun Huang <hqjagain@...il.com>, Tom Rix <trix@...hat.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] tracing: events_hist: avoid using excessive stack space

On Fri, 14 May 2021 16:04:25 +0200
Arnd Bergmann <arnd@...nel.org> wrote:

> From: Arnd Bergmann <arnd@...db.de>
> 
> In some configurations, clang produces a warning about an overly large
> amount of stack space used in hist_trigger_print_key():
> 
> kernel/trace/trace_events_hist.c:4594:13: error: stack frame size of 1248 bytes in function 'hist_trigger_print_key' [-Werror,-Wframe-larger-than=]
> static void hist_trigger_print_key(struct seq_file *m,
> 
> Moving the 'str' variable into a more local scope in the two places
> where it gets used actually reduces the the used stack space here
> and gets it below the warning limit, because the compiler can now
> assume that it is safe to use the same stack slot that it has for
> the stack of any inline function.

Thanks Arnd for the nice explanation of the rationale for this change.

But I still find it too subtle to my liking that we need to move the
declaration like this (and duplicate it twice) for internal behavior of the
compiler (where it can't figure out itself by the use cases if it can
optimize the stack).

> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  kernel/trace/trace_events_hist.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index c1abd63f1d6c..e3fe84f017a8 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -4597,7 +4597,6 @@ static void hist_trigger_print_key(struct seq_file *m,
>  				   struct tracing_map_elt *elt)
>  {
>  	struct hist_field *key_field;
> -	char str[KSYM_SYMBOL_LEN];

Instead, I think we should just make str static, as this should only be
called under the event_mutex. To be sure, we can also add:

	/* To protect the static str variable */
	lockdep_assert_held(&event_mutex);

-- Steve

>  	bool multiline = false;
>  	const char *field_name;
>  	unsigned int i;
> @@ -4617,11 +4616,13 @@ static void hist_trigger_print_key(struct seq_file *m,
>  			uval = *(u64 *)(key + key_field->offset);
>  			seq_printf(m, "%s: %llx", field_name, uval);
>  		} else if (key_field->flags & HIST_FIELD_FL_SYM) {
> +			char str[KSYM_SYMBOL_LEN];
>  			uval = *(u64 *)(key + key_field->offset);
>  			sprint_symbol_no_offset(str, uval);
>  			seq_printf(m, "%s: [%llx] %-45s", field_name,
>  				   uval, str);
>  		} else if (key_field->flags & HIST_FIELD_FL_SYM_OFFSET) {
> +			char str[KSYM_SYMBOL_LEN];
>  			uval = *(u64 *)(key + key_field->offset);
>  			sprint_symbol(str, uval);
>  			seq_printf(m, "%s: [%llx] %-55s", field_name,