linux-kernel - Re: [PATCH] tracing: Have trace_marker writes be just half of TRACE_SEQ

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Message-ID: <c3051fd1-2aaa-485f-b23d-d98c3579e166@efficios.com>
Date: Mon, 4 Mar 2024 21:48:44 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Sachin Sant <sachinp@...ux.ibm.com>
Subject: Re: [PATCH] tracing: Have trace_marker writes be just half of
 TRACE_SEQ_SIZE

On 2024-03-04 21:37, Steven Rostedt wrote:
> On Mon, 4 Mar 2024 21:35:38 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>>> And it's not for debugging, it's for validation of assumptions
>>> made about an upper bound limit defined for a compile-time
>>> check, so as the code evolves issues are caught early.
>>
>> validating is debugging.
> 
> Did Linus put you up to this? To test me to see if I'm learning how to say "No" ;-)

No, he did not. I genuinely think that validating size limits like
this either at compile time or, when they can vary at runtime like
in this case, with a dynamic check, decreases the cognitive
load on the reviewers. We can then assume that whatever limit
was put in place is actually enforced and not just wishful
thinking.

If the "header" size upper bound is not validated at runtime, there
is not much point in adding the BUILD_BUG_ON() based on that value
in the first place, and you should then just add a runtime check that
you don't overflow the output buffer before writing the output to it.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com