[RFC] vivi, videobuf_to_vmalloc() and related breakage

[Al Viro]

	AFAICS, videobuf-vmalloc use of mem->vma and mem->vmalloc is
bogus.

You obtain the latter with vmalloc_user(); so far, so good.  Then you have
        retval=remap_vmalloc_range(vma, mem->vmalloc,0);
where vma is given to you by mmap(); again, fine - we get the memory
pointed to be mem->vmalloc() mapped at vma->vm_start.

Now we get the trouble: things like

static void vivi_fillbuff(struct vivi_dev *dev,struct vivi_buffer *buf)
{
	...
	void *vbuf=videobuf_to_vmalloc (&buf->vb);
	...
	copy_to_user(vbuf + ..., ..., ...)

get vbuf equal to ->vmalloc of buf->vp.priv and that is _not_ a userland
address.  Giving it to copy_to_user() is not going to do anything good.
On some targets it'll fail, on some - write to unrelated user memory.
What is going on there?  If that's an attempt to copy into that buffer
allocated by vmalloc_user(), why are we doing copy_to_user() at all?

But there's more; we have made a copy of vma (kmalloc+memcpy), stored it in
mem->vma and later we cheerfully do remap_vmalloc_range(mem->vma,....).
And kfree that mem->vma immediately afterwards.  What the hell?  It might
not break now, but that seems to be playing very fast and loose with the
warranties provided by VM.
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