On Wed, Apr 09, 2014 at 12:09:26PM -0400, David Miller wrote:
> From: Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>
> Date: Wed, 9 Apr 2014 11:01:50 +0100
>
> > What has been proposed for uprobes is a uprobe specific function -
> > flush_uprobe_xol_access() which is used after uprobes writes via the
> > kmap_atomic() mapping of the page:
> >
> > + xol_page_kaddr = kmap_atomic(area->page);
> > +
> > + /* Initialize the slot */
> > + memcpy(xol_page_kaddr + (xol_vaddr & ~PAGE_MASK),
> > + &uprobe->arch.ixol,
> > + sizeof(uprobe->arch.ixol));
> > +
> > + arch_uprobe_flush_xol_access(area->page, xol_vaddr,
> > + xol_page_kaddr + (xol_vaddr & ~PAGE_MASK),
> > + sizeof(uprobe->arch.ixol));
> > +
> > + kunmap_atomic(xol_page_kaddr);
> >
> > Comments?
>
> ptrace() accesses (via __access_remote_vm()) already use an existing
> helper function for these sorts of situations, in the form of
> copy_{to,from}_user_page(). I would suggest that uprobes uses that
> as well.
Yes, I agree with you.
I made that suggestion too (because it's really the same problem)
but uprobes people are concerned that it's too much of an overhead:
From: Oleg Nesterov <oleg@xxxxxxxxxx>
> On 04/08, Russell King - ARM Linux wrote:
> > Well, isn't this code doing the same thing as ptrace? It seems to want
> > to modify a page in userspace of another process to change instructions
> > that are going to be executed. That's what ptrace does, and ptrace
> > already copes with all the issues there.
>
> Yes, but it does get_user_pages(&vma) and thus it knows vma.
>
> > Given that we've already solved that problem, wouldn't it be a good idea
> > if the tracing code would stop trying to reinvent broken solutions to
> > problems we have already solved?
>
> But uprobes can't do this. Of course, I am not saying this is impossible,
> but it would be nice to avoid mmap_sem/find_vma/etc.
>
> Almost nobody (iirc only sparc?) actually uses this "vma" arguments. And
> at least the supported architectures do not (at least this is what I think
> after the quick grep).
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