Re: [PATCH 2/4] mm: perform VMA allocation, freeing, duplication in mm

[Liam R. Howlett]

* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [250425 06:40]:
> On Thu, Apr 24, 2025 at 08:15:26PM -0700, Kees Cook wrote:
> >
> >
> > On April 24, 2025 2:15:27 PM PDT, Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote:
> > >+static void vm_area_init_from(const struct vm_area_struct *src,
> > >+			      struct vm_area_struct *dest)
> > >+{
> > >+	dest->vm_mm = src->vm_mm;
> > >+	dest->vm_ops = src->vm_ops;
> > >+	dest->vm_start = src->vm_start;
> > >+	dest->vm_end = src->vm_end;
> > >+	dest->anon_vma = src->anon_vma;
> > >+	dest->vm_pgoff = src->vm_pgoff;
> > >+	dest->vm_file = src->vm_file;
> > >+	dest->vm_private_data = src->vm_private_data;
> > >+	vm_flags_init(dest, src->vm_flags);
> > >+	memcpy(&dest->vm_page_prot, &src->vm_page_prot,
> > >+	       sizeof(dest->vm_page_prot));
> > >+	/*
> > >+	 * src->shared.rb may be modified concurrently when called from
> > >+	 * dup_mmap(), but the clone will reinitialize it.
> > >+	 */
> > >+	data_race(memcpy(&dest->shared, &src->shared, sizeof(dest->shared)));
> > >+	memcpy(&dest->vm_userfaultfd_ctx, &src->vm_userfaultfd_ctx,
> > >+	       sizeof(dest->vm_userfaultfd_ctx));
> > >+#ifdef CONFIG_ANON_VMA_NAME
> > >+	dest->anon_name = src->anon_name;
> > >+#endif
> > >+#ifdef CONFIG_SWAP
> > >+	memcpy(&dest->swap_readahead_info, &src->swap_readahead_info,
> > >+	       sizeof(dest->swap_readahead_info));
> > >+#endif
> > >+#ifdef CONFIG_NUMA
> > >+	dest->vm_policy = src->vm_policy;
> > >+#endif
> > >+}
> >
> > I know you're doing a big cut/paste here, but why in the world is this function written this way? Why not just:
> >
> > *dest = *src;
> >
> > And then do any one-off cleanups?
> 
> Yup I find it odd, and error prone to be honest. We'll end up with uninitialised
> state for some fields if we miss them here, seems unwise...
> 
> Presumably for performance?
> 
> This is, as you say, me simply propagating what exists, but I do wonder.

Two things come to mind:

1. How ctors are done.  (v3 of Suren's RCU safe patch series, willy made
a comment.. I think)

2. Some race that Vlastimil came up with the copy and the RCU safeness.
IIRC it had to do with the ordering of the setting of things?

Also, looking at it again...

How is it safe to do dest->anon_name = src->anon_name?  Isn't that ref
counted?

Pretty sure it's okay, but Suren would know for sure on all of this.

Suren, maybe you could send a patch with comments on this stuff?

Thanks,
Liam