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

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Date:   Wed, 14 Aug 2019 15:18:05 -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 Wed, 14 Aug 2019 18:09:36 +0300 Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org> wrote:

> On Mon, Aug 12, 2019 at 02:19:24PM -0700, Andrew Morton wrote:
> 
> Hi, Andrew
> 
> >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
> 
> That's to allow only offset to be passed, mapping length is less than 4Gb.
> I have verified all list of mmap for sockets and all of them contain dummy
> cb sock_no_mmap() except the following:
> 
> xsk_mmap()
> tcp_mmap()
> packet_mmap()
> 
> xsk_mmap() - it's what this fix is needed for.
> tcp_mmap() - doesn't have obvious issues with pgoff - no any references on it.
> packet_mmap() - return -EINVAL if it's even set.

Great, thanks.

> 
> >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?
> It should go to linux-next, no one has been using it till this patch
> with 32 bits as w/o this fix af_xdp sockets can't be used at all.
> It unblocks af_xdp socket usage for 32bit systems.
> 
> 
> That's example of potential next commit message:
> Subject: mm: mmap: increase sockets maximum memory size pgoff for 32bits
> 
> The AF_XDP sockets umem mapping interface uses XDP_UMEM_PGOFF_FILL_RING
> and XDP_UMEM_PGOFF_COMPLETION_RING offsets.  These offsets are established
> already and are part of the configuration interface.
> 
> But for 32-bit systems, using AF_XDP socket configuration, these values
> are too large to pass the maximum allowed file size verification.  The
> offsets can be tuned off, but instead of changing the existing interface,
> let's extend the max allowed file size for sockets.
> 
> No one has been using it till this patch with 32 bits as w/o this fix
> af_xdp sockets can't be used at all, so it unblocks af_xdp socket usage
> for 32bit systems.
> 
> All list of mmap cbs for sockets were verified on side effects and
> all of them contain dummy cb - sock_no_mmap() at this moment, except the
> following:
> 
> xsk_mmap() - it's what this fix is needed for.
> tcp_mmap() - doesn't have obvious issues with pgoff - no any references on it.
> packet_mmap() - return -EINVAL if it's even set.
>
> ...
>
> Is it ok to be replicated in PATCH v2 or this explanation is enough here
> to use v1?

I have replaced the changlog in my tree with the above, thanks.