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Subject: Re: [PATCH 2/2] virtio-console: remove unnecessary kmemdup()
To:     Xianting Tian <xianting.tian@linux.alibaba.com>,
        gregkh@linuxfoundation.org, amit@kernel.org, arnd@arndb.de
Cc:     linuxppc-dev@lists.ozlabs.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, osandov@fb.com
References: <20210801051655.79048-1-xianting.tian@linux.alibaba.com>
From:   Jiri Slaby <jirislaby@kernel.org>
Message-ID: <b5554967-a683-96ae-deb9-2d4980d33b41@kernel.org>
Date:   Mon, 2 Aug 2021 09:25:13 +0200
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Hi,

why is this 2/2? I seem (Lore neither) to find 1/2.

On 01. 08. 21, 7:16, Xianting Tian wrote:
> hvc framework will never pass stack memory to the put_chars() function,

Am I blind or missing something?

hvc_console_print(...)
{
   char c[N_OUTBUF]
...
   cons_ops[index]->put_chars(vtermnos[index], c, i);

The same here:

hvc_poll_put_char(..., char ch)
{
...
    n = hp->ops->put_chars(hp->vtermno, &ch, 1);

AFAICS both of them *pass* a pointer to stack variable.

> So the calling of kmemdup() is unnecessary, remove it.
> 
> Fixes: c4baad5029 ("virtio-console: avoid DMA from stack")

This patch doesn't "Fix" -- it reverts the commit. You should've CCed 
the author too.

> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
> ---
>   drivers/char/virtio_console.c | 12 ++----------
>   1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index 7eaf303a7..4ed3ffb1d 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -1117,8 +1117,6 @@ static int put_chars(u32 vtermno, const char *buf, int count)
>   {
>   	struct port *port;
>   	struct scatterlist sg[1];
> -	void *data;
> -	int ret;
>   
>   	if (unlikely(early_put_chars))
>   		return early_put_chars(vtermno, buf, count);
> @@ -1127,14 +1125,8 @@ static int put_chars(u32 vtermno, const char *buf, int count)
>   	if (!port)
>   		return -EPIPE;
>   
> -	data = kmemdup(buf, count, GFP_ATOMIC);
> -	if (!data)
> -		return -ENOMEM;
> -
> -	sg_init_one(sg, data, count);
> -	ret = __send_to_port(port, sg, 1, count, data, false);
> -	kfree(data);
> -	return ret;
> +	sg_init_one(sg, buf, count);
> +	return __send_to_port(port, sg, 1, count, (void *)buf, false);
>   }
>   
>   /*
> 


-- 
js
suse labs