linux-kernel - Re: [PATCH v2] iio: st_lsm6dsx: Replace scnprintf with sysfs

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Message-ID: <20250706115003.5752261c@jic23-huawei>
Date: Sun, 6 Jul 2025 11:50:03 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: akshay bansod <akbansd@...il.com>
Cc: Andy Shevchenko <andriy.shevchenko@...el.com>, Lorenzo Bianconi
 <lorenzo@...nel.org>, David Lechner <dlechner@...libre.com>, Nuno
 Sá <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>,
 linux-kernel-mentees@...ts.linuxfoundation.org, skhan@...uxfoundation.org,
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: st_lsm6dsx: Replace scnprintf with sysfs_emit

On Thu, 03 Jul 2025 22:28:13 +0530
akshay bansod <akbansd@...il.com> wrote:

> On Thursday, 3 July 2025 10:12 pm +0530 Andy Shevchenko wrote:
> > On Thu, Jul 03, 2025 at 11:08:59AM +0530, Akshay Bansod wrote:  
> > > Update the sysfs interface for sampling frequency and scale attributes.
> > > Replace `scnprintf()` with `sysfs_emit_at()` which is PAGE_SIZE-aware
> > > and recommended for use in sysfs.  
> > 
> > 'must' is stronger than 'recommendation'.
> > Of has the documentation been changed lately?
> > 
> > ...
> >   
> > > st_lsm6dsx_sysfs_sampling_frequency_avail(struct device *dev,  
> >   
> > >  	odr_table = &sensor->hw->settings->odr_table[sensor->id];
> > >  	for (i = 0; i < odr_table->odr_len; i++)
> > > -		len += scnprintf(buf + len, PAGE_SIZE - len, "%d.%03d ",
> > > -				 odr_table->odr_avl[i].milli_hz / 1000,
> > > -				 odr_table->odr_avl[i].milli_hz % 1000);
> > > +		len += sysfs_emit_at(buf, len, "%d.%03d ",
> > > +				     odr_table->odr_avl[i].milli_hz / 1000,
> > > +				     odr_table->odr_avl[i].milli_hz % 1000);
> > >  	buf[len - 1] = '\n';  
> > 
> > My gosh, this is error prone. I'm wondering when some CIs will start to
> > complain on this line. But this was already before your change...
> >   
> I'm planning to drop It entirely or should I replace it with another `sysfs_emit_at()` ?
> I've seen other device driver returning space terminated buffers. Maybe I'm overlooking
> something.

It is rather ugly currently but not a bug as such as we know we don't actually run
out of space in the page (it would just overwrite last byte in that case so odd
output, but not a bug) and that we always print something so just as you suggest
sysfs_emit_at(buf, len - 1, "\n"); is safe.  It also checks under and overflow
so that safe + hopefully won't trip up static analysis tools.

> 
> > >  	return len;  
> > 
> > ...
> >   
> > >  	fs_table = &hw->settings->fs_table[sensor->id];
> > >  	for (i = 0; i < fs_table->fs_len; i++)
> > > -		len += scnprintf(buf + len, PAGE_SIZE - len, "0.%09u ",
> > > -				 fs_table->fs_avl[i].gain);
> > > +		len += sysfs_emit_at(buf, len, "0.%09u ",
> > > +				     fs_table->fs_avl[i].gain);
> > >  	buf[len - 1] = '\n';  
> > 
> > Ditto.
> > 
> >   
> 
> regards,
> Akshay Bansod
> 
> 
> 
>