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

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Date:   Fri, 3 Sep 2021 08:47:01 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Peter Xu <peterx@...hat.com>, rppt@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        vincenzo.frascino@....com,
        Chinwen Chang (張錦文) 
        <chinwen.chang@...iatek.com>,
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        Rolf Eike Beer <eb@...ix.com>,
        Muchun Song <songmuchun@...edance.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Cedeno <thomascedeno@...gle.com>, sashal@...nel.org,
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        linux-mm <linux-mm@...ck.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH v8 2/3] mm: add a field to store names for private
 anonymous memory

On Fri, Sep 3, 2021 at 4:49 AM 'Michal Hocko' via kernel-team
<kernel-team@...roid.com> 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@...roid.com> 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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