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Date:   Fri, 12 Feb 2021 01:09:10 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Michal Hocko <mhocko@suse.com>, Mike Rapoport <rppt@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Catalin Marinas <catalin.marinas@arm.com>,
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        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
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Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to
 create "secret" memory areas
Message-ID: <20210211230910.GL242749@kernel.org>
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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?
 
> > > AFAIKS, we would need MFD_SECRET and disallow
> > > MFD_ALLOW_SEALING and MFD_HUGETLB.
> > 
> > So here we start to multiplex.
> 
> Yes. And as Michal said, maybe we can support combinations in the future.

Isn't there a general agreement that syscall multiplexing is not a good
thing?
memfd_create already has flags validation that does not look very nice.
Adding there only MFD_SECRET will make it a bit less nice, but when we'll
grow new functionality into secretmem that will become horrible.
 
-- 
Sincerely yours,
Mike.