linux-kernel - Re: [RFC PATCH 0/3] permit write-sealed memfd read-only shared mappings

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Message-ID: <20230421090126.tmem27kfqamkdaxo@quack3>
Date:   Fri, 21 Apr 2023 11:01:26 +0200
From:   Jan Kara <jack@...e.cz>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFC PATCH 0/3] permit write-sealed memfd read-only shared
 mappings

Hi!

On Mon 03-04-23 23:28:29, Lorenzo Stoakes wrote:
> This patch series is in two parts:-
> 
> 1. Currently there are a number of places in the kernel where we assume
>    VM_SHARED implies that a mapping is writable. Let's be slightly less
>    strict and relax this restriction in the case that VM_MAYWRITE is not
>    set.
> 
>    This should have no noticeable impact as the lack of VM_MAYWRITE implies
>    that the mapping can not be made writable via mprotect() or any other
>    means.
> 
> 2. Align the behaviour of F_SEAL_WRITE and F_SEAL_FUTURE_WRITE on mmap().
>    The latter already clears the VM_MAYWRITE flag for a sealed read-only
>    mapping, we simply extend this to F_SEAL_WRITE too.
> 
>    For this to have effect, we must also invoke call_mmap() before
>    mapping_map_writable().
> 
> As this is quite a fundamental change on the assumptions around VM_SHARED
> and since this causes a visible change to userland (in permitting read-only
> shared mappings on F_SEAL_WRITE mappings), I am putting forward as an RFC
> to see if there is anything terribly wrong with it.

So what I miss in this series is what the motivation is. Is it that you need
to map F_SEAL_WRITE read-only? Why?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR