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

From: Michal Hocko <mhocko@suse.com>
To: James Bottomley <jejb@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>,
	Mike Rapoport <rppt@kernel.org>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christopher Lameter <cl@linux.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Matthew Wilcox <willy@infradead.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Roman Gushchin <guro@fb.com>, Shakeel Butt <shakeelb@google.com>,
	Shuah Khan <shuah@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Tycho Ander sen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
	linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
	x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>,
	Palmer Dabbelt <palmerdabbelt@google.com>
Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Mon, 15 Feb 2021 10:13:50 +0100	[thread overview]
Message-ID: <YCo7TqUnBdgJGkwN@dhcp22.suse.cz> (raw)
In-Reply-To: <244f86cba227fa49ca30cd595c4e5538fe2f7c2b.camel@linux.ibm.com>

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
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