linux-kernel - Re: Oops in CDC ACM after an Openmoko phone (neo1973) changes its usb interface from ACM to Ethernet

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Message-ID: <81b0412b0908030217o1e687c2ewf0191687a78d6166@mail.gmail.com>
Date:	Mon, 3 Aug 2009 11:17:05 +0200
From:	Alex Riesen <raa.lkml@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Oliver Neukum <oliver@...kum.org>, linux-usb@...r.kernel.org,
	Oliver Neukum <oliver@...kum.name>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: Oops in CDC ACM after an Openmoko phone (neo1973) changes its usb 
	interface from ACM to Ethernet

On Sun, Aug 2, 2009 at 23:09, Alex Riesen<raa.lkml@...il.com> wrote:
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000003
> IP: [<ffffffffa005ce38>] acm_rx_tasklet+0x24e/0x3a3 [cdc_acm]

I tried to look a little around acm_rx_tasklet (cdc-acm.c, -rc5). I
think it is this code:

0000000000001bea <acm_rx_tasklet>:
...
    1e27:	4d 8b 44 24 18       	mov    0x18(%r12),%r8
    1e2c:	89 f8                	mov    %edi,%eax
    1e2e:	c1 f8 0f             	sar    $0xf,%eax
    1e31:	83 e0 0f             	and    $0xf,%eax
    1e34:	48 8b 14 c2          	mov    (%rdx,%rax,8),%rdx
    1e38:	0f b6 42 03          	movzbl 0x3(%rdx),%eax ; I think it crashed here

which is this:

	movq	24(%r12), %r8	# <variable>.base, temp.1149
	movl	%edi, %eax	# D.22806, tmp117
	sarl	$15, %eax	#, tmp117
	andl	$15, %eax	#, tmp119
	movq	(%rdx,%rax,8), %rdx	#, ep
	movzbl	3(%rdx), %eax	# <variable>.desc.bmAttributes,
<variable>.desc.bmAttributes

and correspondingly:

466		ep = (usb_pipein(acm->rx_endpoint) ? acm->dev->ep_in : acm->dev->ep_out)
467				[usb_pipeendpoint(acm->rx_endpoint)];
468		if (usb_endpoint_xfer_int(&ep->desc))

So it looks like ep is NULL (usb_endpoint_xfer_int accesses desc.bmAttributes,
which is 3 bytes off the beginning of struct usb_host_endpoint (desc being the
first field in usb_host_endpoint).

Any help with further debugging is very much appreciated (because
I don't know where to continue yet).
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