linux-hardening - Re: [PATCH] drm/i915: refactor deprecated strncpy

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Date:   Thu, 14 Sep 2023 21:38:12 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] drm/i915: refactor deprecated strncpy

On Thu, Sep 14, 2023 at 09:14:07PM +0000, Justin Stitt 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` [2] due to the fact that it guarantees
> NUL-termination on the destination buffer without unnecessarily NUL-padding.
> 
> `ctx` is zero allocated and as such strncpy's NUL-padding behavior was
> strictly a performance hit which should now be resolved. It should be
> noted, however, that performance is not critical in these selftests,
> especially by these margins.
> 
> 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>
> ---
>  drivers/gpu/drm/i915/gem/selftests/mock_context.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_context.c b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> index 8ac6726ec16b..025b9c773b93 100644
> --- a/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> +++ b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> @@ -36,7 +36,7 @@ mock_context(struct drm_i915_private *i915,
>  	if (name) {
>  		struct i915_ppgtt *ppgtt;
>  
> -		strncpy(ctx->name, name, sizeof(ctx->name) - 1);
> +		strscpy(ctx->name, name, sizeof(ctx->name) - 1);

I'd expect the "- 1" to go away...

-Kees

>  
>  		ppgtt = mock_ppgtt(i915, name);
>  		if (!ppgtt)
> 
> ---
> base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
> change-id: 20230914-strncpy-drivers-gpu-drm-i915-gem-selftests-mock_context-c-980c8ecc9142
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook