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 11:19:54 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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>,
        James Bottomley <jejb@...ux.ibm.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 Fri, Feb 12, 2021 at 10:18:19AM +0100, David Hildenbrand wrote:
> On 12.02.21 00:09, Mike Rapoport wrote:
> > On Thu, Feb 11, 2021 at 01:07:10PM +0100, David Hildenbrand wrote:
> > > On 11.02.21 12:27, Mike Rapoport wrote:
> > > > On Thu, Feb 11, 2021 at 10:01:32AM +0100, David Hildenbrand wrote:
> > > 
> > > So let's talk about the main user-visible differences to other memfd files
> > > (especially, other purely virtual files like hugetlbfs). With secretmem:
> > > 
> > > - File content can only be read/written via memory mappings.
> > > - File content cannot be swapped out.
> > > 
> > > I think there are still valid ways to modify file content using syscalls:
> > > e.g., fallocate(PUNCH_HOLE). Things like truncate also seems to work just
> > > fine.
> > These work perfectly with any file, so maybe we should have added
> > memfd_create as a flag to open(2) back then and now the secretmem file
> > descriptors?
> 
> I think open() vs memfd_create() makes sense: for open, the path specifies
> main properties (tmpfs, hugetlbfs, filesystem). On memfd, there is no such
> path and the "type" has to be specified differently.
> 
> Also, open() might open existing files - memfd always creates new files.

Yes, but still open() returns a handle to a file and memfd_create() returns
a handle to a file. The differences may be well hidden by e.g. O_MEMORY and
than features unique to memfd files will have their set of O_SOMETHING
flags.

It's the same logic that says "we already have an interface that's close
enough and it's fine to add a bunch of new flags there".
 
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.

-- 
Sincerely yours,
Mike.