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

James Bottomley
2024-02-10T12:57:00Z

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@kernel.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 or
problem, 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.

James