Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 3 Sep 2021 13:49:01 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Colin Cross <ccross@google.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Randy Dunlap <rdunlap@infradead.org>,
        Kalesh Singh <kaleshsingh@google.com>,
        Peter Xu <peterx@redhat.com>, rppt@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        vincenzo.frascino@arm.com,
        Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= 
        <chinwen.chang@mediatek.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Jann Horn <jannh@google.com>, apopple@nvidia.com,
        John Hubbard <jhubbard@nvidia.com>,
        Yu Zhao <yuzhao@google.com>, Will Deacon <will@kernel.org>,
        fenghua.yu@intel.com, thunder.leizhen@huawei.com,
        Hugh Dickins <hughd@google.com>, feng.tang@intel.com,
        Jason Gunthorpe <jgg@ziepe.ca>, Roman Gushchin <guro@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>, krisman@collabora.com,
        chris.hyser@oracle.com, Peter Collingbourne <pcc@google.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Jens Axboe <axboe@kernel.dk>, legion@kernel.org,
        Rolf Eike Beer <eb@emlix.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Thomas Cedeno <thomascedeno@google.com>, sashal@kernel.org,
        cxfcosmos@gmail.com, Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-mm <linux-mm@kvack.org>,
        kernel-team <kernel-team@android.com>
Subject: Re: [PATCH v8 2/3] mm: add a field to store names for private
 anonymous memory
Message-ID: <YTILrVHLMBky9YjP@dhcp22.suse.cz>
References: <20210827191858.2037087-1-surenb@google.com>
 <20210827191858.2037087-3-surenb@google.com>
 <YS81abHD8KZMrX8D@dhcp22.suse.cz>
 <CAJuCfpHWCtqCcuZdyfc4-virtynOMv2f_iU=OJUB_6b2Xz+k9g@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAJuCfpHWCtqCcuZdyfc4-virtynOMv2f_iU=OJUB_6b2Xz+k9g@mail.gmail.com>
Precedence: bulk

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@android.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