linux-kernel - Re: [PATCH -next 1/2] mm/memfd: make F_SEAL_FUTURE

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Message-Id: <20181124164229.89c670b6e7a3530ef7b0a40c@linux-foundation.org>
Date:   Sat, 24 Nov 2018 16:42:29 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        LKML <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>, Jann Horn <jannh@...gle.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Linux API <linux-api@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        marcandre.lureau@...hat.com, Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH -next 1/2] mm/memfd: make F_SEAL_FUTURE_WRITE seal more
 robust

On Thu, 22 Nov 2018 15:09:06 -0800 Joel Fernandes <joel@...lfernandes.org> wrote:

> Android uses ashmem for sharing memory regions.  We are looking forward to
> migrating all usecases of ashmem to memfd so that we can possibly remove
> the ashmem driver in the future from staging while also benefiting from
> using memfd and contributing to it.  Note staging drivers are also not ABI
> and generally can be removed at anytime.
> 
> One of the main usecases Android has is the ability to create a region and
> mmap it as writeable, then add protection against making any "future"
> writes while keeping the existing already mmap'ed writeable-region active.
> This allows us to implement a usecase where receivers of the shared
> memory buffer can get a read-only view, while the sender continues to
> write to the buffer.  See CursorWindow documentation in Android for more
> details:
> https://developer.android.com/reference/android/database/CursorWindow
> 
> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> which prevents any future mmap and write syscalls from succeeding while
> keeping the existing mmap active.
> 
> A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
> where we don't need to modify core VFS structures to get the same
> behavior of the seal. This solves several side-effects pointed by Andy.
> self-tests are provided in later patch to verify the expected semantics.
> 
> [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/

This changelog doesn't have the nifty test case code which was in
earlier versions?