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 13:49:01 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.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>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jann Horn <jannh@...gle.com>, apopple@...dia.com,
        John Hubbard <jhubbard@...dia.com>,
        Yu Zhao <yuzhao@...gle.com>, Will Deacon <will@...nel.org>,
        fenghua.yu@...el.com, thunder.leizhen@...wei.com,
        Hugh Dickins <hughd@...gle.com>, feng.tang@...el.com,
        Jason Gunthorpe <jgg@...pe.ca>, Roman Gushchin <guro@...com>,
        Thomas Gleixner <tglx@...utronix.de>, krisman@...labora.com,
        chris.hyser@...cle.com, Peter Collingbourne <pcc@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jens Axboe <axboe@...nel.dk>, legion@...nel.org,
        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,
        cxfcosmos@...il.com, Rasmus Villemoes <linux@...musvillemoes.dk>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        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 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.
-- 
Michal Hocko
SUSE Labs