linux-kernel - Re: [PATCH v2 1/3] lib/vsprintf: Avoid redundant work with 0 size

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Message-ID: <Yfe7Q5cx+MoaOev/@smile.fi.intel.com>
Date:   Mon, 31 Jan 2022 12:34:43 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Waiman Long <longman@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v2 1/3] lib/vsprintf: Avoid redundant work with 0 size

On Mon, Jan 31, 2022 at 12:30:33PM +0200, Andy Shevchenko wrote:
> On Mon, Jan 31, 2022 at 12:25:09PM +0200, Andy Shevchenko wrote:
> > On Sun, Jan 30, 2022 at 12:49:37PM -0800, David Rientjes wrote:
> > > On Sat, 29 Jan 2022, Waiman Long wrote:
> > > 
> > > > For *scnprintf(), vsnprintf() is always called even if the input size is
> > > > 0. That is a waste of time, so just return 0 in this case.
> > 
> > Why do you think it's not legit?
> 
> I have to elaborate.
> 
> For *nprintf() the size=0 is quite useful to have.
> For *cnprintf() the size=0 makes less sense, but, if we read `man snprintf()`:
> 
>   The  functions  snprintf() and vsnprintf() do not write more than size bytes
>   (including the terminating null byte ('\0')). If the output was truncated due
>   to this limit, then the return value is the  number of  characters (excluding
>   the terminating null byte) which would have been written to the final string
>   if enough space had been available. Thus, a return value of size or more
>   means  that  the  output  was truncated.  (See also below under NOTES.)
> 
>   If an output error is encountered, a negative value is returned.
> 
> Note the last sentence there. You need to answer to it in the commit message
> why your change is okay and it will show that you thought through all possible
> scenarios.

Also it seems currently the kernel documentation is not aligned with the code

  "If @size is == 0 the function returns 0."

It should mention the (theoretical?) possibility of getting negative value,
if vsnprintf() returns negative value.

-- 
With Best Regards,
Andy Shevchenko