Re: [PATCH v8 2/3] mm: add a field to store names for private anonymous memory

[Suren Baghdasaryan]

On Fri, Sep 3, 2021 at 4:49 AM 'Michal Hocko' via kernel-team
<kernel-team@xxxxxxxxxxx> wrote:
>
> On Wed 01-09-21 08:42:29, Suren Baghdasaryan wrote:
> > On Wed, Sep 1, 2021 at 1:10 AM 'Michal Hocko' via kernel-team
> > <kernel-team@xxxxxxxxxxx> wrote:
> > >
> > > On Fri 27-08-21 12:18:57, Suren Baghdasaryan wrote:
> > > [...]
> > > > +static void replace_vma_anon_name(struct vm_area_struct *vma, const char *name)
> > > > +{
> > > > +     if (!name) {
> > > > +             free_vma_anon_name(vma);
> > > > +             return;
> > > > +     }
> > > > +
> > > > +     if (vma->anon_name) {
> > > > +             /* Should never happen, to dup use dup_vma_anon_name() */
> > > > +             WARN_ON(vma->anon_name == name);
> > >
> > > What is the point of this warning?
> >
> > I wanted to make sure replace_vma_anon_name() is not used from inside
> > vm_area_dup() or some similar place (does not exist today but maybe in
> > the future) where "new" vma is a copy of "orig" vma and
> > new->anon_name==orig->anon_name. If someone by mistake calls
> > replace_vma_anon_name(new, orig->anon_name) and
> > new->anon_name==orig->anon_name then they will keep pointing to the
> > same name pointer, which breaks an assumption that ->anon_name
> > pointers are not shared among vmas even if the string is the same.
> > That would eventually lead to use-after-free error. After the next
> > patch implementing refcounting, the similar situation would lead to
> > both new and orig vma pointing to the same anon_vma_name structure
> > without raising the refcount, which would also lead to use-after-free
> > error. That's why the above comment asks to use dup_vma_anon_name() if
> > this warning ever happens.
> > I can remove the warning but I thought the problem is subtle enough to
> > put some safeguards.
>
> This to me sounds very much like a debugging code that shouldn't make it
> to the final patch to be merged. I do see your point of an early
> diagnostic but we are talking about an internal MM code and that is not
> really designed to be robust against its own failures so I do not see
> why this should be any special.

Fair enough. I posted v9 yesterday but will respin another version in
a couple days. Will remove the warning then.
Thanks,
Suren.

> --
> Michal Hocko
> SUSE Labs
>
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