Re: [RFC PATCH] Remove flush_icache_user_range()

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Apr 09, 2014 at 12:09:26PM -0400, David Miller wrote:
> From: Russell King - ARM Linux <linux@arm.linux.org.uk>
> 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@redhat.com>
> 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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