linux-kernel - Re: [PATCH v9 2/3] mm: add a field to store names for private anonymous memory

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Date:   Fri, 1 Oct 2021 09:34:08 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
        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>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Muchun Song <songmuchun@...edance.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Cedeno <thomascedeno@...gle.com>, sashal@...nel.org,
        cxfcosmos@...il.com, 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 v9 2/3] mm: add a field to store names for private
 anonymous memory

On Fri, Oct 1, 2021 at 12:01 AM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> On 03/09/2021 01.18, Suren Baghdasaryan wrote:
> > From: Colin Cross <ccross@...gle.com>
> >
> >
> > changes in v9
> > - Changed max anon vma name length from 64 to 256 (as in the original patch)
> > because I found one case of the name length being 139 bytes. If anyone is
> > curious, here it is:
> > dalvik-/data/dalvik-cache/arm64/apex@....android.permission@...v-app@...glePermissionController@...glePermissionController.apk@...sses.art
>
> I'm not sure that's a very convincing argument. We don't add code
> arbitrarily just because some userspace code running on some custom
> kernel (ab)uses something in that kernel. Surely that user can come up
> with a name that doesn't contain GooglePermissionController twice.
>
> The argument for using strings and not just a 128 bit uuid was that it
> should (also) be human readable, and 250-byte strings are not that.
> Also, there's no natural law forcing this to be some power-of-two, and
> in fact the implementation means that it's actually somewhat harmful
> (give it a 256 char name, and we'll do a 260 byte alloc, which becomes a
> 512 byte alloc). So just make the limit 80, the kernel's definition of a
> sane line length.

Sounds reasonable. I'll set the limit to 80 and will look into the
userspace part if we can trim the names to abide by this limit.

> As for the allowed chars, it can be relaxed later if convincing arguments can be made.

For the disallowed chars, I would like to go with "\\`$[]" set because
of the example I presented in my last reply. Since we disallow $, the
parsers should be able to process parentheses with no issues I think.

>
>
> > +/* mmap_lock should be read-locked */
> > +static inline bool is_same_vma_anon_name(struct vm_area_struct *vma,
> > +                                      const char *name)
> > +{
> > +     const char *vma_name = vma_anon_name(vma);
> > +
> > +     if (likely(!vma_name))
> > +             return name == NULL;
> > +
> > +     return name && !strcmp(name, vma_name);
>
> It's probably preferable to spell this
>
>   /* either both NULL, or pointers to same refcounted string */
>   if (vma_name == name)
>       return true;
>
>   return name && vma_name && !strcmp(name, vma_name);
>
> so you have one less conditional in the common case.

Ack.

>
> Rasmus

Thanks for the review!
Suren.