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Date:   Mon, 24 Jan 2022 10:35:00 -0800
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Subject: Re: [PATCH] fcntl.2: document F_GET_SEALS on tmpfs peculiarity
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To:     Hugh Dickins <hughd@google.com>, Aleksa Sarai <cyphar@cyphar.com>
Cc:     Kir Kolyshkin <kolyshkin@gmail.com>, linux-man@vger.kernel.org,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        David Herrmann <dh.herrmann@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20220122005251.1441343-1-kolyshkin@gmail.com>
 <20220122090441.ktxh43lpgsd2dxj4@senku>
 <d7ca2658-b63b-7437-9bd0-82bc59c7c981@google.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
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On 1/23/22 20:18, Hugh Dickins wrote:
> On Sat, 22 Jan 2022, Aleksa Sarai wrote:
> 
>> Adding the maintainers of mm/{shmem,memfd}.c and fs/hugetlbfs/ just in
>> case this was not intended behaviour.
> 
> Kir is correct - thanks - and it is intended behaviour.  Not consciously
> intended to make for a difficult manpage, but the implementation was
> intended to be simple, so tmpfs and hugetlbfs do not internally
> distinguish memfd objects from filesystem files - their filesystem
> files simply start off with F_SEAL_SEAL to rule out any sealing.
> 

Thanks Hugh, I agree.

>> On 2022-01-21, Kir Kolyshkin <kolyshkin@gmail.com> wrote:
>>> Currently, from the description of file sealing it can be deduced that
>>> unless the fd is a memfd, all sealing operations fail with EINVAL.
>>>
>>> Apparently, it's not true for tmpfs or hugetlbfs -- F_GET_SEALS returns
>>> 1 (F_SEAL_SEAL) for an fd opened on these filesystems (probably because
>>> those are used to back memfd files).
>>>
>>> Fix the description to mention that peculiarity. Not knowing this can
>>> result in incorrect code logic (see [1], where the code mistook a
>>> descriptor of a file opened on on tmpfs for a memfd).
>>>
>>> While at it, clarify that fcntl does not actually return EINVAL, but
>>> sets errno to it (as it is usually said elsewhere).
>>>
>>> [1] https://github.com/opencontainers/runc/pull/3342
>>>
>>> Cc: Aleksa Sarai <cyphar@cyphar.com>
>>> Cc: David Herrmann <dh.herrmann@gmail.com>
>>> Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
> 
> Acked-by: Hugh Dickins <hughd@google.com>

Acked-by: Mike Kravetz <mike.kravetz@oracle.com>

-- 
Mike Kravetz