linux-hardening - Re: [PATCH] iio: sx9324: replace deprecated strncpy

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Date: Thu, 26 Oct 2023 16:54:41 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Lars-Peter Clausen <lars@...afoo.de>, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] iio: sx9324: replace deprecated strncpy

Hi Jonathan,

On Sat, Sep 23, 2023 at 10:47 AM Jonathan Cameron <jic23@...nel.org> wrote:
>
> On Thu, 21 Sep 2023 07:01:01 +0000
> Justin Stitt <justinstitt@...gle.com> wrote:
>
> > `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> >
> > We should prefer more robust and less ambiguous string interfaces.
> >
> > `prop` is defined as this string literal with size 30 (including null):
> > |       #define SX9324_PROXRAW_DEF "semtech,ph01-proxraw-strength"
> > |             char prop[] = SX9324_PROXRAW_DEF;
> >
> > Each of the strncpy->strscpy replacements involve string literals with a
> > size less than 30 which means there are no current problems with how
> > strncpy is used. However, let's move away from using strncpy entirely.
> >
> > A suitable replacement is `strscpy` [2] due to the fact that it
> > guarantees NUL-termination on the destination buffer without
> > unnecessarily NUL-padding.
> >
> > Moreover, let's opt for the more conventional `sizeof()` as opposed to
> > `ARRAY_SIZE` for these simple strings.
> >
> > 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@...r.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> > FWIW: It seems fragile to base future `prop` stores on the
> > size of it's default string.
>
> Agreed - can we just get rid of the copying?  Just set a const char *
> to point to appropriate string instead and use that?
>
> The dance is just about reasonable for the case where there is
> a string format being used to fill in the numbers, here I can't see any
> advantage at all. (for the other case, a kasprintf() or similar is probably
> more appropriate anyway) given this isn't a particular hot path.

I sent a [v2]! Can you see if this matches your vision here?

>
> Jonathan
> >
> > Note: build-tested
> > ---
> >  drivers/iio/proximity/sx9324.c | 12 ++++--------
> >  1 file changed, 4 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/iio/proximity/sx9324.c b/drivers/iio/proximity/sx9324.c
> > index 438f9c9aba6e..25ac2733bcef 100644
> > --- a/drivers/iio/proximity/sx9324.c
> > +++ b/drivers/iio/proximity/sx9324.c
> > @@ -937,11 +937,9 @@ sx9324_get_default_reg(struct device *dev, int idx,
> >       case SX9324_REG_AFE_CTRL4:
> >       case SX9324_REG_AFE_CTRL7:
> >               if (reg_def->reg == SX9324_REG_AFE_CTRL4)
> > -                     strncpy(prop, "semtech,ph01-resolution",
> > -                             ARRAY_SIZE(prop));
> > +                     strscpy(prop, "semtech,ph01-resolution", sizeof(prop));
> >               else
> > -                     strncpy(prop, "semtech,ph23-resolution",
> > -                             ARRAY_SIZE(prop));
> > +                     strscpy(prop, "semtech,ph23-resolution", sizeof(prop));
> >
> >               ret = device_property_read_u32(dev, prop, &raw);
> >               if (ret)
> > @@ -1012,11 +1010,9 @@ sx9324_get_default_reg(struct device *dev, int idx,
> >       case SX9324_REG_PROX_CTRL0:
> >       case SX9324_REG_PROX_CTRL1:
> >               if (reg_def->reg == SX9324_REG_PROX_CTRL0)
> > -                     strncpy(prop, "semtech,ph01-proxraw-strength",
> > -                             ARRAY_SIZE(prop));
> > +                     strscpy(prop, "semtech,ph01-proxraw-strength", sizeof(prop));
> >               else
> > -                     strncpy(prop, "semtech,ph23-proxraw-strength",
> > -                             ARRAY_SIZE(prop));
> > +                     strscpy(prop, "semtech,ph23-proxraw-strength", sizeof(prop));
> >               ret = device_property_read_u32(dev, prop, &raw);
> >               if (ret)
> >                       break;
> >
> > ---
> > base-commit: 2cf0f715623872823a72e451243bbf555d10d032
> > change-id: 20230921-strncpy-drivers-iio-proximity-sx9324-c-8c3437676039
> >
> > Best regards,
> > --
> > Justin Stitt <justinstitt@...gle.com>
> >
>

[v2]: https://lore.kernel.org/r/20231026-strncpy-drivers-iio-proximity-sx9324-c-v2-1-cee6e5db700c@google.com

Thanks
Justin