linux-kernel - Re: [PATCH v3] media: mtk-vpu: avoid unaligned access to DTCM buffer.

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Date:   Thu, 27 Feb 2020 10:50:34 +0100
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Hsin-Yi Wang <hsinyi@...omium.org>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Minghsiu Tsai <minghsiu.tsai@...iatek.com>,
        Houlong Wei <houlong.wei@...iatek.com>,
        Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
        Tiffany Lin <tiffany.lin@...iatek.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-media@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] media: mtk-vpu: avoid unaligned access to DTCM buffer.

On 2/25/20 6:24 PM, Hsin-Yi Wang wrote:
> struct vpu_run *run in vpu_init_ipi_handler() is an ioremapped DTCM (Data
> Tightly Coupled Memory) buffer shared with AP.  It's not able to do
> unaligned access. Otherwise kernel would crash due to unable to handle
> kernel paging request.
> 
> struct vpu_run {
> 	u32 signaled;
> 	char fw_ver[VPU_FW_VER_LEN];
> 	unsigned int	dec_capability;
> 	unsigned int	enc_capability;
> 	wait_queue_head_t wq;
> };
> 
> fw_ver starts at 4 byte boundary. If system enables
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, strscpy() will do
> read_word_at_a_time(), which tries to read 8-byte: *(unsigned long *)addr
> 
> Copy the string by memcpy_fromio() for this buffer to avoid unaligned
> access.
> 
> Fixes: 85709cbf1524 ("media: replace strncpy() by strscpy()")
> Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> ---
> Change in v3:
> - fix sparse warnings.
> Change in v2:
> - fix sparse warnings.
> ---
>  drivers/media/platform/mtk-vpu/mtk_vpu.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/media/platform/mtk-vpu/mtk_vpu.c b/drivers/media/platform/mtk-vpu/mtk_vpu.c
> index a768707abb94..e3fd2d1814f3 100644
> --- a/drivers/media/platform/mtk-vpu/mtk_vpu.c
> +++ b/drivers/media/platform/mtk-vpu/mtk_vpu.c
> @@ -603,12 +603,14 @@ EXPORT_SYMBOL_GPL(vpu_load_firmware);
>  static void vpu_init_ipi_handler(void *data, unsigned int len, void *priv)
>  {
>  	struct mtk_vpu *vpu = (struct mtk_vpu *)priv;
> -	struct vpu_run *run = (struct vpu_run *)data;
> -
> -	vpu->run.signaled = run->signaled;
> -	strscpy(vpu->run.fw_ver, run->fw_ver, sizeof(vpu->run.fw_ver));
> -	vpu->run.dec_capability = run->dec_capability;
> -	vpu->run.enc_capability = run->enc_capability;
> +	struct vpu_run __iomem *run = (struct vpu_run __iomem __force *)data;

The use of __force is generally a bad sign. Shouldn't the 'void *data' be a
'void __iomem *data'? And vpu->recv_buf should be 'struct share_obj __iomem *recv_buf;'.
Probably send_buf as well.

In other words, the __iomem attribute should be wired up correctly throughout the
driver code, and not forcibly applied in one place. That is asking for trouble in
the future. Also, sparse only works well in detecting problems if such attributes
are applied at the right level.

Regards,

	Hans

> +
> +	vpu->run.signaled = readl(&run->signaled);
> +	memcpy_fromio(vpu->run.fw_ver, run->fw_ver, sizeof(vpu->run.fw_ver));
> +	/* Make sure the string is NUL-terminated */
> +	vpu->run.fw_ver[sizeof(vpu->run.fw_ver) - 1] = '\0';
> +	vpu->run.dec_capability = readl(&run->dec_capability);
> +	vpu->run.enc_capability = readl(&run->enc_capability);
>  	wake_up_interruptible(&vpu->run.wq);
>  }
>  
>