Re: [PATCH v1 1/4] mm: Fix uprobe pte be overwritten when expanding vma

[Lorenzo Stoakes]

On Mon, Jun 02, 2025 at 02:26:21PM +0200, David Hildenbrand wrote:
> On 02.06.25 13:55, Lorenzo Stoakes wrote:
> > On Fri, May 30, 2025 at 08:51:14PM +0200, David Hildenbrand wrote:
> > > >    	if (vp->remove) {
> > > > @@ -1823,6 +1829,14 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> > > >    		faulted_in_anon_vma = false;
> > > >    	}
> > > > +	/*
> > > > +	 * If the VMA we are copying might contain a uprobe PTE, ensure
> > > > +	 * that we do not establish one upon merge. Otherwise, when mremap()
> > > > +	 * moves page tables, it will orphan the newly created PTE.
> > > > +	 */
> > > > +	if (vma->vm_file)
> > > > +		vmg.skip_vma_uprobe = true;
> > > > +
> > >
> > > Assuming we extend the VMA on the way (not merge), would we handle that
> > > properly?
> > >
> > > Or is that not possible on this code path or already broken either way?
> >
> > I'm not sure in what context you mean expand, vma_merge_new_range() calls
> > vma_expand() so we call an expand a merge here, and this flag will be
> > obeyed.
>
> Essentially, an mremap() that grows an existing mapping while moving it.
>
> Assume we have
>
> [ VMA 0 ] [ VMA X]
>
> And want to grow VMA 0 by 1 page.
>
> We cannot grow in-place, so we'll have to copy VMA 0 to another VMA, and
> while at it, expand it by 1 page.
>
> expand_vma()->move_vma()->copy_vma_and_data()->copy_vma()

OK so in that case you'd not have a merge at all, you'd have a new VMA and all
would be well and beautiful :) or I mean hopefully. Maybe?

>
>
> But maybe I'm getting lost in the code. (e.g., expand_vma() vs. vma_expand()
> ... confusing :) )

Yeah I think Liam or somebody else called me out for this :P I mean it's
accurate naming in mremap.c but that's kinda in the context of the mremap.

For VMA merging vma_expand() is used generally for a new VMA, since you're
always expanding into the gap, but because we all did terrible things in past
lives also called by relocate_vma_down() which is a kinda-hack for initial stack
relocation on initial process setup.

It maybe needs renaming... But expand kinda accurately describes what's going on
just semi-overloaded vs. mremap() now :>)

VMA merge code now at least readable enough that you can pick up on the various
oddnesses clearly :P

>
> --
> Cheers,
>
> David / dhildenb
>