linux-kernel - Re: Sealed memfd & no-fault mmap

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Date:   Thu, 3 Jun 2021 13:07:56 -0700
From:   Ming Lin <mlin@...nel.org>
To:     Simon Ser <contact@...rsion.fr>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Herrmann <dh.herrmann@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <greg@...ah.com>,
        "tytso@....edu" <tytso@....edu>
Subject: Re: Sealed memfd & no-fault mmap

On 6/3/2021 6:01 AM, Simon Ser wrote:

> 
> Regarding the requirements for Wayland:
> 
> - The baseline requirement is being able to avoid SIGBUS for read-only mappings
>    of shm files.
> - Wayland clients can expand their shm files. However the compositor doesn't
>    need to immediately access the new expanded region. The client will tell the
>    compositor what the new shm file size is, and the compositor will re-map it.
> - Ideally, MAP_NOSIGBUS would work on PROT_WRITE + MAP_SHARED mappings (of
>    course, the no-SIGBUS behavior would be restricted to that mapping). The
>    use-case is writing back to client buffers e.g. for screen capture. From the
>    earlier discussions it seems like this would be complicated to implement.
>    This means we'll need to come up with a new libwayland API to allow
>    compositors to opt-in to the read-only mappings. This is sub-optimal but
>    seems doable.
> - Ideally, MAP_SIGBUS wouldn't be restricted to shm. There are use-cases for
>    using it on ordinary files too, e.g. for sharing ICC profiles. But from the
>    earlier replies it seems very unlikely that this will become possible, and
>    making it work only on shm files would already be fantastic.

In the new version of the patches, MAP_NOSIGBUS is not restricted to shmem.
It can be used on ordinary files.