Re: [PATCH] omap_vout: find_vma() needs ->mmap_sem held

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Dec 15, 2012 at 08:12:37PM +0000, Al Viro wrote:
> 	Walking rbtree while it's modified is a Bad Idea(tm); besides,
> the result of find_vma() can be freed just as it's getting returned
> to caller.  Fortunately, it's easy to fix - just take ->mmap_sem a bit
> earlier (and don't bother with find_vma() at all if virtp >= PAGE_OFFSET -
> in that case we don't even look at its result).

	While we are at it, what prevents VIDIOC_PREPARE_BUF calling
v4l_prepare_buf() -> (e.g) vb2_ioctl_prepare_buf() -> vb2_prepare_buf() ->
__buf_prepare() -> __qbuf_userptr() -> vb2_vmalloc_get_userptr() -> find_vma(),
AFAICS without having taken ->mmap_sem anywhere in process?  The code flow
is bloody convoluted and depends on a bunch of things done by initialization,
so I certainly might've missed something...