linux-kernel - Re: [PATCH 1/2] tracing/user_events: Fix non-spaced field matching

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Message-Id: <20240423085056.5deb4c21122c5b66e17b21c0@kernel.org>
Date: Tue, 23 Apr 2024 08:50:56 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Beau Belgrave <beaub@...ux.microsoft.com>
Cc: rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 dcook@...ux.microsoft.com
Subject: Re: [PATCH 1/2] tracing/user_events: Fix non-spaced field matching

On Mon, 22 Apr 2024 14:55:25 -0700
Beau Belgrave <beaub@...ux.microsoft.com> wrote:

> On Sat, Apr 20, 2024 at 09:50:52PM +0900, Masami Hiramatsu wrote:
> > On Fri, 19 Apr 2024 14:13:34 -0700
> > Beau Belgrave <beaub@...ux.microsoft.com> wrote:
> > 
> > > On Fri, Apr 19, 2024 at 11:33:05AM +0900, Masami Hiramatsu wrote:
> > > > On Tue, 16 Apr 2024 22:41:01 +0000
> > > > Beau Belgrave <beaub@...ux.microsoft.com> wrote:
> 
> *SNIP*
> 
> > > > nit: This loop can be simpler, because we are sure fixed has enough length;
> > > > 
> > > > /* insert a space after ';' if there is no space. */
> > > > while(*args) {
> > > > 	*pos = *args++;
> > > > 	if (*pos++ == ';' && !isspace(*args))
> > > > 		*pos++ = ' ';
> > > > }
> > > > 
> > > 
> > > I was worried that if count_semis_no_space() ever had different logic
> > > (maybe after this commit) that it could cause an overflow if the count
> > > was wrong, etc.
> > > 
> > > I don't have an issue making it shorter, but I was trying to be more on
> > > the safe side, since this isn't a fast path (event register).
> > 
> > OK, anyway current code looks correct. But note that I don't think
> > "pos++; len--;" is safer, since it is not atomic. This pattern
> > easily loose "len--;" in my experience. So please carefully use it ;)
> > 
> 
> I'll stick with your loop. Perhaps others will chime in on the v2 and
> state a stronger opinion.
> 
> You scared me with the atomic comment, I went back and looked at all the
> paths for this. In the user_events IOCTL the buffer is copied from user
> to kernel, so it cannot change (and no other threads access it). I also
> checked trace_parse_run_command() which is the same. So at least in this
> context the non-atomic part is OK.

Oh, sorry if I scared you. I've seen bugs get introduced into loops like
this many times (while updating the code), so I try to keep it simple.
I'm sure that your code has no bugs.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>