Hi, Greg, Alan, All,
On 05/18/2013 06:17 PM, Greg KH wrote:
On Sat, May 18, 2013 at 12:14:08PM -0400, Alan Stern wrote:
On Sat, 18 May 2013, Hans de Goede wrote:
But the sysfs descriptors file will just packs the
rawdescriptors one behind the other, using
usb_device->config[x].desc.wTotalLength, where as
userspace only sees the length advertised by
the rawdescriptors, which may be different, and when
it is userspace will this have no idea where the next
descriptor starts.
I believe the proper way to fix this is to make the
sysfs code deal with this the same way the usbfs code
does (filling the holes with 0 to avoid leaking kmem),
if people agree I can write a patch for this.
That's okay with me.
Sounds good to me as well.
Thanks for the input. I've been thinking about this some-more,
and I see a possible other solution which I think we need to
consider.
Although the config.wTotalLength value in /sys/.../descriptors
can not be trusted, the kernel does sanitize things to such a
degree that the standard descriptor header bLength can be
trusted to always be valid inside /sys/.../descriptors, so
alternatively to using config.wTotalLength userspace could
simple parse things one descriptor at a time, until it
encounters another config descriptor or EOF, and reliable figure
out where the next config descriptor starts that way.
I'm sort of tending towards using that solution instead
Advantages:
1) If libusb is modified to do thing this way it will work with
older kernels, without needing to switch to using usbfs (which
in some rough benchmarks I've run turns out to be 7 times as slow
when it comes to open file, read desc, close file).
2) We're not breaking the kernel ABI, which technically my
other proposal does. I know devices with multiple configs are
rare, and one could argue the old behavior is a bug, but we
may have something out there depending on it.
Disadvantages:
1) It means that there will be some inconsistencies to how this
is handled between usbfs and sysfs (note there already is such
an inconsistency with regards to the endian-ness of the device
descriptor).
As said I tend towards using the alternative proposal I've
just suggested, input very much welcome!
Either way I noticed that descriptors is absent from
Documentation/ABI/testing/sysfs-bus-usb
Once we've a decision on which way to go, I'll document the
behavior there.
Regards,
Hans
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