Re: [PATCH] EDAC/mc_sysfs: refactor deprecated strncpy

[Justin Stitt]

On Wed, Sep 13, 2023 at 8:13 AM Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>
> > `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> >
> > We should prefer more robust and less ambiguous string interfaces.
> >
> > A suitable replacement is `strscpy_pad` [2] due to the fact that it guarantees
> > NUL-termination on the destination buffer whilst maintaining the
> > NUL-padding behavior that `strncpy` provides. This may not be strictly
> > necessary but as I couldn't understand what this code does I wanted to
> > ensure that the functionality is the same.
> >
> > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> > Link: https://github.com/KSPP/linux/issues/90
> > Cc: linux-hardening@xxxxxxxxxxxxxxx
> > Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> > ---
> > Note: build-tested only.
> > ---
> >  drivers/edac/edac_mc_sysfs.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/drivers/edac/edac_mc_sysfs.c b/drivers/edac/edac_mc_sysfs.c
> > index 15f63452a9be..b303309a63cf 100644
> > --- a/drivers/edac/edac_mc_sysfs.c
> > +++ b/drivers/edac/edac_mc_sysfs.c
> > @@ -229,8 +229,7 @@ static ssize_t channel_dimm_label_store(struct device *dev,
> >         if (copy_count == 0 || copy_count >= sizeof(rank->dimm->label))
> >                 return -EINVAL;
> >
> > -       strncpy(rank->dimm->label, data, copy_count);
> > -       rank->dimm->label[copy_count] = '\0';
> > +       strscpy_pad(rank->dimm->label, data, copy_count);
>
> That doc page says the problem with strncpy() is that it doesn't guarantee to
> NUL terminate the target string. But this code is aware of that limitation and
> zaps a '\0' at the end to be sure.
>
> So this code doesn't suffer from the potential problems.

Right, the original code did not have an existing bug due to the
reason you mentioned. However, I'm pretty keen on eliminating uses of
this interface treewide as there is always a more robust and less
ambiguous option.


>
> If it is going to be fixed, then some further analysis of the original code
> would be wise. Just replacing with strscpy_pad() means the code probably
> still suffers from the "needless performance penalty" also mentioned in
> the deprecation document.
Got it, sending a v2 that prefers `strscpy` to `strscpy_pad` resolving
the performance issue.

>
> -Tony
>

Thanks for the timely review!
Justin