Re: [PATCH] mm: get_user_pages(write,force) refuse to COW in shared areas

[Oleg Nesterov <oleg@redhat.com>]

On 04/24, Hugh Dickins wrote:
>
> On Thu, 24 Apr 2014, Oleg Nesterov wrote:
>
> > So, what do you think about the patch below? It is probably fine in any case,
> > but is there any "strong" reason to follow the gup's behaviour and forbid the
> > anon page in VM_MAYSHARE && !VM_MAYWRITE vma?
>
> I don't think there is a "strong" reason to forbid it.
>
> The strongest reason is simply that it's much safer if uprobes follows
> the same conventions as mm, and get_user_pages() happens to have
> forbidden that all along.
>
> The philosophical reason to forbid it is that the user mmapped with
> MAP_SHARED, and it's merely a kernel-internal detail that we flip off
> VM_SHARED and treat these read-only shared mappings very much like
> private mappings.  The user asked for MAP_SHARED, and we prefer to
> respect that by not letting private COWs creep in.
>
> We could treat those mappings even more like private mappings, and
> allow the COWs; but better to be strict about it, so long as doing
> so doesn't give you regressions.

Great, thanks a lot! I was worried I missed something subtle.

And I forgot to mention, there is another reason why I would like to
change uprobes to follow the same convention. I still think it would
be better to kill __replace_page() and use gup(FOLL_WRITE | FORCE)
in uprobe_write_opcode().


> > --- x/kernel/events/uprobes.c
> > +++ x/kernel/events/uprobes.c
> > @@ -127,12 +127,13 @@ struct xol_area {
> >   */
> >  static bool valid_vma(struct vm_area_struct *vma, bool is_register)
> >  {
> > -	vm_flags_t flags = VM_HUGETLB | VM_MAYEXEC | VM_SHARED;
> > +	vm_flags_t flags = VM_HUGETLB | VM_MAYEXEC;
>
> I think a one-line patch changing VM_SHARED to VM_MAYSHARE would do it,
> wouldn't it?  And save you from having to export is_cow_mapping()
> from mm/memory.c.  (I used is_cow_mapping() because I had to make the
> test more complex anyway, just to exclude the case which had been
> oddly handled before.)

Indeed, thanks!

Oleg.

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