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

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Date:   Sun, 14 Feb 2021 11:21:02 -0800
From:   James Bottomley <jejb@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...nel.org>
Cc:     Michal Hocko <mhocko@...e.com>, 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, 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/

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.

James