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

From: David Hildenbrand <david@redhat.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: David Hildenbrand <david@redhat.com>,
	Michal Hocko <mhocko@suse.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
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	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
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	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
Date: Sun, 14 Feb 2021 10:58:44 +0100	[thread overview]
Message-ID: <052DACE9-986B-424C-AF8E-D6A4277DE635@redhat.com> (raw)
In-Reply-To: <20210214091954.GM242749@kernel.org>

> Am 14.02.2021 um 10:20 schrieb Mike Rapoport <rppt@kernel.org>:
> 
> 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.
> 

Let‘s agree to disagree.

> 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".

No, not quite. But let‘s agree to disagree.

> 
> 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.
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