"Couldn't open mtdev" crashing X

Thomas Bushnell, BSG tb at becket.net
Fri May 27 04:47:54 UTC 2011

I just noticed that the mtdev source I've been looking at is Debian
unstable, which is a tad newer than natty, and in particular, adds an
additional safety check in the front of mtdev_open. If indeed a null pointer
is passed in to mtdev_open's first arg, it would quickly segfault (though I
can't see why the fault would be in mtdev_open and not mtdev_init).

I wonder if this might not be the immediate cause of the crash. I don't know
how to quickly get the natty source for evdev (with the patches to call
mtdev from Chase Douglas); the Debian unstable version of evdev does not yet
include that patch. From the one I see (
http://lists.x.org/archives/xorg-devel/2011-January/018405.html) this
shouldn't happen:

The dev could only be bad if EvdevOpenDevice were to have pEvdev->device
set, but pEvdev->mtdev still unset. That can't happen afaict, though I would
prefer the patch initialized mtdev in a separate branch to make that

Still, I'm interested to know if the patch to check the args to mtdev_open
was prompted by actual problems or just general cleanup.

It is also odd to me that the crash is reported to be at mtdev_open+22, but
according to gdb, that's in the middle of an instruction, and if the PC got
pointed there by mistake, its a register-only instruction anyhow, so it
can't fault.
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