linux-kernel - Re: [PATCH v2 3/4] pwm: sysfs: Replace sprintf() with sysfs

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Message-ID: <YzRmXAuJLnPLaTsp@orome>
Date:   Wed, 28 Sep 2022 17:21:00 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH v2 3/4] pwm: sysfs: Replace sprintf() with sysfs_emit()

On Wed, Sep 28, 2022 at 05:20:07PM +0200, Thierry Reding wrote:
> On Wed, Sep 28, 2022 at 04:58:17PM +0300, Andy Shevchenko wrote:
> > On Wed, Sep 28, 2022 at 04:40:35PM +0300, Andy Shevchenko wrote:
> > > On Wed, Sep 28, 2022 at 02:28:41PM +0200, Thierry Reding wrote:
> > > > On Fri, Aug 26, 2022 at 08:07:15PM +0300, Andy Shevchenko wrote:
> > > > > For sysfs outputs, it's safer to use a new helper, sysfs_emit(),
> > > > > instead of the raw sprintf() & co. This patch replaces such a
> > > > > sprintf() call straightforwardly with the new helper.
> > > 
> > > > How exactly is sysfs_emit() safer here? In all of these cases, the
> > > > values that sprintf() writes are the only values that are written into
> > > > the buffer and we know that none of them exceed PAGE_SIZE. So the
> > > > additional checks that sysfs_emit() performs are useless.
> > > 
> > > This is a recommended way to use sysfs_emit() mentioned in Documentation.
> > > Care to fix documentation?
> > 
> > For your convenience, Documentation/filesystems/sysfs.rst says:
> > 
> > - show() should only use sysfs_emit() or sysfs_emit_at() when formatting
> >   the value to be returned to user space.
> 
> Took some digging to find enough information to convince me. Again, the
> commit message says that sysfs_emit() is safer, but that's a bad reason
> in this case because these cases are fine. The sprintf() calls that this
> replaces aren't unbound and we're not appending to an existing seq_buf,
> so nothing to worry on that front.
> 
> I think the better argument for broadly applying this is to specifically
> distinguish the sysfs sprintf() calls from others so that they can be
> auditioned better and perhaps help with the documentation[0].
> 
> Do you mind if I apply this with a reworded documentation?

I meant "commit message", not documentation.

Thierry

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