linux-kernel - Re: [PATCH] i915/intel_tv_get_modes: fix strncpy truncation warning

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Date:   Wed, 11 Jul 2018 15:48:40 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Dominique Martinet <asmadeus@...ewreck.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     David Airlie <airlied@...ux.ie>, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dominique Martinet <asmadeus@...ewreck.org>
Subject: Re: [PATCH] i915/intel_tv_get_modes: fix strncpy truncation warning

Quoting Dominique Martinet (2018-07-11 08:46:15)
> This is effectively no-op as the next line writes a nul at the final
> byte of the buffer, so copying one letter less does not change the
> behaviour.
> 
> Signed-off-by: Dominique Martinet <asmadeus@...ewreck.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
> 
> 
> 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);

strlcpy
-Chris