[Qemu-devel] Guest OS hangs on usb_add

[TJ]

> ---------- Forwarded message ----------
> From: Timothy Jones <address@hidden>
> Date: Wed, Jun 23, 2010 at 9:07 PM
> Subject: Guest OS hangs on usb_add
> To: address@hidden
> 
> 
> With some digging around I found out that the qemu hangs in
> usb_host_claim_interfaces, which is caused by screwed up usb
> descriptor. The device reports the following:
> 
> (gdb) p dev->descr_len
> $21 = 50
> (gdb) p /x dev->address@hidden
> $23 = {0x18, 0x1, 0x0, 0x1, 0xff, 0xff, 0xff, 0x8, 0x47, 0x46, 0x0,
> 0x30, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1, 0x9, 0x2, 0x20,
>  0x0, 0x1, 0x1, 0x0, 0x80, 0x19, 0x9, 0x4, 0x0, 0x0, 0x2, 0xff, 0xff,
> 0xff, 0x0, 0x7, 0x5, 0x81, 0x2, 0x40, 0x0, 0x0,
>  0x7, 0x5, 0x3, 0x2, 0x10, 0x0, 0x0}
> 
> The first 0x18 (Device Descriptor bLength) is supposed to be decimal
> 18, not hex! According to USB spec, if the device reports size greater
> than expected, the host is supposed ignore the extra bytes. So qemu
> behaves correctly here. However, with this length, the following
> Configuration Descriptor length falls on a 0x0 and so the qemu spins
> in an endless loop. (This is prolly something that should be detected
> and reported as error by qemu.)
> 
> My question is: This 0x18 -- is this something that comes from the
> device itself (ie, firmware bug)? Or does it come from the USB
> subsystem?
> 
> I don't mind writing a small patch to make descriptor parsing a bit
> more intelligent, but I am very unfamiliar with the code, so I might
> botch things up. Or is the above data sufficient for one of the devs
> to take a look at the code and improve it?
> 
> Thank you.
> 
> -TJ
> 

Here is small patch that fixed my problem.

In looking at the USB spec, it seems pretty clear cut about the whole
device/config/interface/endpoint descriptor hierarchy, so the
usb_host_claim_interfaces can be optimized instead of parsing through each
descriptor to skip through config descriptors using wTotalLength field. And
again, some checks can be done for descriptor types and/or sizes.

Just my 2 cents.

-TJ

hex-dev-descr-len.patch
Description: Text document