linux-kernel - Re: [PATCH 1/4] tracing/histogram: Don't use strlen to find length of stacktrace variables

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Message-ID: <20230213105633.006a22fc@rorschach.local.home>
Date:   Mon, 13 Feb 2023 10:56:33 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Tom Zanussi <zanussi@...nel.org>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        zwisler@...gle.com, chinglinyu@...gle.com
Subject: Re: [PATCH 1/4] tracing/histogram: Don't use strlen to find length
 of stacktrace variables

On Tue, 14 Feb 2023 00:24:18 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> > -				e = stack_trace_save((void *)str,
> > +				e = stack_trace_save((void *)stack_start,
> >  						     HIST_STACKTRACE_DEPTH,
> >  						     HIST_STACKTRACE_SKIP);  
> 
> BTW, the size of "str" is enough to store HIST_STACKTRACE_DEPTH?
> In string case, 
> 
> size = min(val->size, STR_VAR_LEN_MAX);
> 
> will limit the max size.

Well, we have:

#define HIST_STACKTRACE_DEPTH  16

And 16 * 8 = 128

#define STR_VAR_LEN_MAX              MAX_FILTER_STR_VAL
#define MAX_FILTER_STR_VAL 256U

So 128 < 256, with plenty of room. I wouldn't do this runtime, but we
should add here:

	BUILD_BUG_ON((HIST_STACKTRACE_DEPTH + 1) * sizeof(long) >= STR_VAR_LEN_MAX);

-- Steve