linux-kernel - Re: [PATCH 3/3] media: mediatek: vcodedc: Fix Wcast-function-type-strict warnings

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Date: Thu, 1 Feb 2024 15:16:54 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, Mike Isely <isely@...ox.com>,
	Tiffany Lin <tiffany.lin@...iatek.com>,
	Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
	Yunfei Dong <yunfei.dong@...iatek.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH 3/3] media: mediatek: vcodedc: Fix
 Wcast-function-type-strict warnings

On Sun, Jan 28, 2024 at 02:12:22AM +0000, Ricardo Ribalda wrote:
> Building with LLVM=1 throws the following warning:
> drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_fw_vpu.c:38:32: warning: cast from 'mtk_vcodec_ipi_handler' (aka 'void (*)(void *, unsigned int, void *)') to 'ipi_handler_t' (aka 'void (*)(const void *, unsigned int, void *)') converts to incompatible function type [-Wcast-function-type-strict]
> 
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>

I am not positive because I don't have any hardware to test this driver
but I suspect this patch is just hiding the warning without actually
addressing the issue that it is pointing out. If handler is called
through vpu_ipi_register() indirectly (which it obviously is if it is
being passed down), there will be a CFI failure because the type of
mtk_vcodec_ipi_handler is not the same as ipi_handler_t, as the comment
mentions.

Has this seen testing on real hardware with kCFI?

> ---
>  drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_fw_vpu.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_fw_vpu.c b/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_fw_vpu.c
> index 9f6e4b59455d..781a0359afdc 100644
> --- a/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_fw_vpu.c
> +++ b/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_fw_vpu.c
> @@ -33,9 +33,11 @@ static int mtk_vcodec_vpu_set_ipi_register(struct mtk_vcodec_fw *fw, int id,
>  	 * The handler we receive takes a void * as its first argument. We
>  	 * cannot change this because it needs to be passed down to the rproc
>  	 * subsystem when SCP is used. VPU takes a const argument, which is
> -	 * more constrained, so the conversion below is safe.
> +	 * more constrained, so the conversion below is safe. We use the void
> +	 * casting, to convince clang with -Wcast-function-type-sctrict that
> +	 * this is safe.
>  	 */
> -	ipi_handler_t handler_const = (ipi_handler_t)handler;
> +	ipi_handler_t handler_const = (ipi_handler_t)((void *)handler);
>  
>  	return vpu_ipi_register(fw->pdev, id, handler_const, name, priv);
>  }
> 
> -- 
> 2.43.0.429.g432eaa2c6b-goog
>