linux-kernel - Re: [PATCH] USB: serial: mos7720: improve handling of a kmalloc failure in read_mos

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Message-ID: <X/1ke4/PaRQRGJTg@hovoldconsulting.com>
Date:   Tue, 12 Jan 2021 09:57:31 +0100
From:   Johan Hovold <johan@...nel.org>
To:     trix@...hat.com
Cc:     johan@...nel.org, gregkh@...uxfoundation.org,
        natechancellor@...il.com, ndesaulniers@...gle.com,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] USB: serial: mos7720: improve handling of a kmalloc
 failure in read_mos_reg()

On Mon, Jan 11, 2021 at 02:09:04PM -0800, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> clang static analysis reports this problem
> 
> mos7720.c:352:2: warning: Undefined or garbage value returned to caller
>         return d;
>         ^~~~~~~~
> 
> In the parport_mos7715_read_data()'s call to read_mos_reg(), 'd' is
> only set after the alloc block.
> 
> 	buf = kmalloc(1, GFP_KERNEL);
> 	if (!buf)
> 		return -ENOMEM;
> 
> Although the problem is reported in parport_most7715_read_data(),
> none of the callee's of read_mos_reg() check the return status.
> 
> So move the clearing of data to before the malloc.
> 
> Fixes: 0d130367abf5 ("USB: serial: mos7720: fix control-message error handling")
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  drivers/usb/serial/mos7720.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/serial/mos7720.c b/drivers/usb/serial/mos7720.c
> index 41ee2984a0df..23e8162c768b 100644
> --- a/drivers/usb/serial/mos7720.c
> +++ b/drivers/usb/serial/mos7720.c
> @@ -214,6 +214,7 @@ static int read_mos_reg(struct usb_serial *serial, unsigned int serial_portnum,
>  	u8 *buf;
>  	int status;
>  
> +	*data = 0;
>  	buf = kmalloc(1, GFP_KERNEL);
>  	if (!buf)
>  		return -ENOMEM;

I added a clearing of the buffer to this error path instead to avoid the
redundant assignment for every call due to something which will
basically never happen.

> @@ -227,7 +228,6 @@ static int read_mos_reg(struct usb_serial *serial, unsigned int serial_portnum,
>  			"mos7720: usb_control_msg() failed: %d\n", status);
>  		if (status >= 0)
>  			status = -EIO;
> -		*data = 0;
>  	}
>  
>  	kfree(buf);

Johan