linux-kernel - Re: [PATCH v2 bpf-next] mm: mmap: increase sockets maximum memory size pgoff for 32bits

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Date:   Mon, 12 Aug 2019 14:19:24 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
Cc:     bjorn.topel@...el.com, linux-mm@...ck.org,
        xdp-newbies@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org, ast@...nel.org,
        magnus.karlsson@...el.com
Subject: Re: [PATCH v2 bpf-next] mm: mmap: increase sockets maximum memory
 size pgoff for 32bits

On Mon, 12 Aug 2019 15:43:26 +0300 Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org> wrote:

> The AF_XDP sockets umem mapping interface uses XDP_UMEM_PGOFF_FILL_RING
> and XDP_UMEM_PGOFF_COMPLETION_RING offsets. The offsets seems like are
> established already and are part of configuration interface.
> 
> But for 32-bit systems, while AF_XDP socket configuration, the values
> are to large to pass maximum allowed file size verification.
> The offsets can be tuned ofc, but instead of changing existent
> interface - extend max allowed file size for sockets.

What are the implications of this?  That all code in the kernel which
handles mapped sockets needs to be audited (and tested) for correctly
handling mappings larger than 4G on 32-bit machines?  Has that been
done?  Are we confident that we aren't introducing user-visible buggy
behaviour into unsuspecting legacy code?

Also...  what are the user-visible runtime effects of this change? 
Please send along a paragraph which explains this, for the changelog. 
Does this patch fix some user-visible problem?  If so, should be code
be backported into -stable kernels?