Re: 2.6.13: More on drivers/block/loop.c

[Anton Altaparmakov <aia21@cam.ac.uk>]

On Wed, 2005-09-14 at 13:51 +0100, Ian Collier wrote:
> Vanilla 2.6.13 doesn't crash.
> 
> However, unpack a fresh copy of 2.6.13, edit include/linux/loop.h to
> change LO_KEY_SIZE from 32 to 1844, and *boom*.  [Don't ask me why
> 1844... that's just what PPDD wants.]
> 
> It's crashing somewhere in loop_set_status_old, probably during the
> call to copy_from_user, but the crash messages aren't that helpful as
> they are different each time, often seem to happen during an interrupt,
> and usually contain pages of recursive calls to do_page_fault and
> error_code.
> 
> The loop_set_status_old function has two local variables, each of which
> is now 1812 bytes longer than it was, and I'm wondering if it's a stack
> overflow problem.  How much stack is a kernel function allowed to use,
> anyway?
> 
> Replacing these variables with kmalloc'd pointers seems to stop the crashes
> anyway, so I'll pass that tip on to the PPDD folks.

Not surprising.  The _entirety_ of the kernel, i.e. not just each
function, has either 4k or 8k of stack (depending on a .config option)
so having two local variables of 1812 bytes each is _guaranteed_ to blow
the stack.

Best regards,

        Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/