linux-hardening - Re: [PATCH 03/10] scsi: NCR5380: Replace snprintf() with the safer scnprintf() variant

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Message-ID: <20240220082816.GI10170@google.com>
Date: Tue, 20 Feb 2024 08:28:16 +0000
From: Lee Jones <lee@...nel.org>
To: James Bottomley <jejb@...ux.ibm.com>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org, Finn Thain <fthain@...ux-m68k.org>,
	Michael Schmitz <schmitzmic@...il.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	drew@...orado.edu, Tnx to <Thomas_Roesch@...maus.de>,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH 03/10] scsi: NCR5380: Replace snprintf() with the safer
 scnprintf() variant

On Mon, 19 Feb 2024, James Bottomley wrote:

> On Mon, 2024-02-19 at 15:23 +0000, Lee Jones wrote:
> > On Sat, 10 Feb 2024, James Bottomley wrote:
> > 
> > > On Thu, 2024-02-08 at 10:29 +0000, Lee Jones wrote:
> > > > On Thu, 08 Feb 2024, Geert Uytterhoeven wrote:
> > > > 
> > > > > Hi Lee,
> > > > > 
> > > > > Thanks for your patch!
> > > > > 
> > > > > On Thu, Feb 8, 2024 at 9:48 AM Lee Jones <lee@...nel.org>
> > > > > wrote:
> > > > > > There is a general misunderstanding amongst engineers that
> > > > > > {v}snprintf() returns the length of the data *actually*
> > > > > > encoded into the destination array.  However, as per the C99
> > > > > > standard {v}snprintf() really returns the length of the data
> > > > > > that *would have been* written if there were enough space for
> > > > > > it.  This misunderstanding has led to buffer-overruns in the
> > > > > > past.  It's generally considered safer to use the
> > > > > > {v}scnprintf() variants in their place (or even sprintf() in
> > > > > > simple cases).  So let's do that.
> > > > > 
> > > > > Confused... The return value is not used at all?
> > > > 
> > > > Future proofing.  The idea of the effort is to rid the use
> > > > entirely.
> > > > 
> > > >  - Usage is inside a sysfs handler passing PAGE_SIZE as the size
> > > >    - s/snprintf/sysfs_emit/
> > > >  - Usage is inside a sysfs handler passing a bespoke value as the
> > > > size
> > > >    - s/snprintf/scnprintf/
> > > >  - Return value used, but does *not* care about overflow
> > > >    - s/snprintf/scnprintf/
> > > >  - Return value used, caller *does* care about overflow
> > > >    - s/snprintf/seq_buf/
> > > >  - Return value not used
> > > >    - s/snprintf/scnprintf/
> > > > 
> > > > This is the final case.
> > > 
> > > To re-ask Geert's question: the last case can't ever lead to a bug
> > > orproblem, what value does churning the kernel to change it
> > > provide? As Finn said, if we want to deprecate it as a future
> > > pattern, put it in checkpatch.
> > 
> > Adding this to checkpatch is a good idea.
> > 
> > What if we also take Kees's suggestion and hit all of these found in
> > SCSI in one patch to keep the churn down to a minimum?
> 
> That doesn't fix the churn problem because you're still changing the
> source.  For ancient drivers, we keep the changes to a minimum to avoid
> introducing inadvertent bugs which aren't discovered until months
> later.  If there's no actual bug in the driver, there's no reason to
> change the code.

Okay, no problem.  Would you like me to drop these from the set and
resubmit or are you happy to cherry-pick the remainder?

-- 
Lee Jones [李琼斯]