Re: [Intel-gfx] [PATCH] i915/intel_tv_get_modes: fix strncpy truncation warning

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Dominique Martinet <asmadeus@codewreck.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@intel.com>,
	David Airlie <airlied@linux.ie>,
	intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH] i915/intel_tv_get_modes: fix strncpy truncation warning
Date: Thu, 12 Jul 2018 15:44:01 +0300	[thread overview]
Message-ID: <20180712124401.GZ5565@intel.com> (raw)
In-Reply-To: <1531295175-24052-1-git-send-email-asmadeus@codewreck.org>

On Wed, Jul 11, 2018 at 09:46:15AM +0200, Dominique Martinet wrote:
> This is effectively no-op as the next line writes a nul at the final

What is "This". Please write self contained commit messages.

> byte of the buffer, so copying one letter less does not change the
> behaviour.
> 
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> ---
> 
> gcc 8 gives the following warning, which I am not sure why is treated
> as error for this file, thus making me fix it:
> drivers/gpu/drm/i915/intel_tv.c: In function ‘intel_tv_get_modes’:
> drivers/gpu/drm/i915/intel_tv.c:1358:3: error: ‘strncpy’ specified bound 32 equals destination size [-Werror=stringop-truncation]
>    strncpy(mode_ptr->name, input->name, DRM_DISPLAY_MODE_LEN);
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors

That warning should be in the actual commit message.

> 
> 
> Also, as a side note, while checking if this was already sent I stumbled
> uppon this: https://lists.freedesktop.org/archives/intel-gfx/2018-July/170638.html
> ([Intel-gfx] [PATCH i-g-t 6/7] Fix truncate string in the strncpy)
> which replaces strncpy(dest, src, sizeof(dest)) by strncpy(dest, src,
> strlen(dest))... This isn't for linux but this looks like a pretty bad
> idea to me... (I'm not on the list to reply there, sorry)
> 
>  drivers/gpu/drm/i915/intel_tv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_tv.c b/drivers/gpu/drm/i915/intel_tv.c
> index b55b5c157e38..f5614d07b10d 100644
> --- a/drivers/gpu/drm/i915/intel_tv.c
> +++ b/drivers/gpu/drm/i915/intel_tv.c
> @@ -1355,7 +1355,7 @@ intel_tv_get_modes(struct drm_connector *connector)
>  		mode_ptr = drm_mode_create(connector->dev);
>  		if (!mode_ptr)
>  			continue;
> -		strncpy(mode_ptr->name, input->name, DRM_DISPLAY_MODE_LEN);
> +		strncpy(mode_ptr->name, input->name, DRM_DISPLAY_MODE_LEN - 1);
>  		mode_ptr->name[DRM_DISPLAY_MODE_LEN - 1] = '\0';

This same pattern is used all over drm. Can you go and fix them all up?
One might even consider writing a cocci patch for it ;)

>  
>  		mode_ptr->hdisplay = hactive_s;
> -- 
> 2.17.1
> 
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-- 
Ville Syrjälä
Intel