linux-kernel - Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas

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Message-ID: <YCo7TqUnBdgJGkwN@dhcp22.suse.cz>
Date:   Mon, 15 Feb 2021 10:13:50 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     James Bottomley <jejb@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org, Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to
 create "secret" memory areas

On Sun 14-02-21 11:21:02, James Bottomley wrote:
> On Sun, 2021-02-14 at 10:58 +0100, David Hildenbrand wrote:
> [...]
> > > And here we come to the question "what are the differences that
> > > justify a new system call?" and the answer to this is very
> > > subjective. And as such we can continue bikeshedding forever.
> > 
> > I think this fits into the existing memfd_create() syscall just fine,
> > and I heard no compelling argument why it shouldn‘t. That‘s all I can
> > say.
> 
> OK, so let's review history.  In the first two incarnations of the
> patch, it was an extension of memfd_create().  The specific objection
> by Kirill Shutemov was that it doesn't share any code in common with
> memfd and so should be a separate system call:
> 
> https://lore.kernel.org/linux-api/20200713105812.dnwtdhsuyj3xbh4f@box/

Thanks for the pointer. But this argument hasn't been challenged at all.
It hasn't been brought up that the overlap would be considerable higher
by the hugetlb/sealing support. And so far nobody has claimed those
combinations as unviable.

> The other objection raised offlist is that if we do use memfd_create,
> then we have to add all the secret memory flags as an additional ioctl,
> whereas they can be specified on open if we do a separate system call. 
> The container people violently objected to the ioctl because it can't
> be properly analysed by seccomp and much preferred the syscall version.
> 
> Since we're dumping the uncached variant, the ioctl problem disappears
> but so does the possibility of ever adding it back if we take on the
> container peoples' objection.  This argues for a separate syscall
> because we can add additional features and extend the API with flags
> without causing anti-ioctl riots.

I am sorry but I do not understand this argument. What kind of flags are
we talking about and why would that be a problem with memfd_create
interface? Could you be more specific please?
-- 
Michal Hocko
SUSE Labs