linux-kernel - Re: [PATCH 2/2] Eliminate double kfree

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Date:	Thu, 29 May 2008 17:57:50 +0200 (CEST)
From:	Julia Lawall <julia@...u.dk>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 2/2] Eliminate double kfree

On Thu, 29 May 2008, Dmitry Torokhov wrote:

> Hi Julia,
> 
> On Thu, May 29, 2008 at 03:05:07PM +0200, Julia Lawall wrote:
> > From: Julia Lawall <julia@...u.dk>
> > 
> > The variable report is only non-NULL and non-freed in a small region of
> > code, so it should only be freed in error handling code that comes from
> > that region.
> > 
> 
> Thank you for your patch. There isn't a double-kfree though, as far
> as I can see.

There was a double free because the error handling at the end of the 
function (for input_register_device) would fall through to the 
kfree(report) under err_free_devs, even though report is already freed at 
that point.

Your solution does indeed look more elegant, and seems to preserve the 
semantics of the original code.

julia

> Because we need to free the report in both success and
> failure cases the error handling is a bit unwieldy I agree. I don't
> want to apply your patch though because I don't like when we bypass
> parts of error handling path that weren't bypassed if we aborted
> earlier, if you follow me.
> 
> What do you think about the patch below?
> 
> -- 
> Dmitry
> 
> Input: gtco - clean up error handling in gtco_probe
> 
> Thanks to Julia Lawall for noticing ugliness.
> 
> Signed-off-by: Dmitry Torokhov <dtor@...l.ru>
> ---
>  drivers/input/tablet/gtco.c |   17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
> 
> Index: linux/drivers/input/tablet/gtco.c
> ===================================================================
> --- linux.orig/drivers/input/tablet/gtco.c
> +++ linux/drivers/input/tablet/gtco.c
> @@ -830,7 +830,7 @@ static int gtco_probe(struct usb_interfa
>  	struct gtco             *gtco;
>  	struct input_dev        *input_dev;
>  	struct hid_descriptor   *hid_desc;
> -	char                    *report = NULL;
> +	char                    *report;
>  	int                     result = 0, retry;
>  	int			error;
>  	struct usb_endpoint_descriptor *endpoint;
> @@ -916,12 +916,16 @@ static int gtco_probe(struct usb_interfa
>  					 le16_to_cpu(hid_desc->wDescriptorLength),
>  					 5000); /* 5 secs */
>  
> -		if (result == le16_to_cpu(hid_desc->wDescriptorLength))
> +		dbg("usb_control_msg result: %d", result);
> +		if (result == le16_to_cpu(hid_desc->wDescriptorLength)) {
> +			parse_hid_report_descriptor(gtco, report, result);
>  			break;
> +		}
>  	}
>  
> +	kfree(report);
> +
>  	/* If we didn't get the report, fail */
> -	dbg("usb_control_msg result: :%d", result);
>  	if (result != le16_to_cpu(hid_desc->wDescriptorLength)) {
>  		err("Failed to get HID Report Descriptor of size: %d",
>  		    hid_desc->wDescriptorLength);
> @@ -929,12 +933,6 @@ static int gtco_probe(struct usb_interfa
>  		goto err_free_urb;
>  	}
>  
> -	/* Now we parse the report */
> -	parse_hid_report_descriptor(gtco, report, result);
> -
> -	/* Now we delete it */
> -	kfree(report);
> -
>  	/* Create a device file node */
>  	usb_make_path(gtco->usbdev, gtco->usbpath, sizeof(gtco->usbpath));
>  	strlcat(gtco->usbpath, "/input0", sizeof(gtco->usbpath));
> @@ -988,7 +986,6 @@ static int gtco_probe(struct usb_interfa
>  	usb_buffer_free(gtco->usbdev, REPORT_MAX_SIZE,
>  			gtco->buffer, gtco->buf_dma);
>   err_free_devs:
> -	kfree(report);
>  	input_free_device(input_dev);
>  	kfree(gtco);
>  	return error;
> --
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