netdev - Re: [PATCH 08/11] netlink: implement memory mapped sendmsg()

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Date:	Thu, 8 Sep 2011 20:08:23 +0200
From:	Michał Mirosław <mirq@...e.qmqm.pl>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	davem@...emloft.net, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 08/11] netlink: implement memory mapped sendmsg()

On Thu, Sep 08, 2011 at 11:33:09AM +0200, Patrick McHardy wrote:
> Am 07.09.2011 22:03, schrieb Michał Mirosław:
> > On Wed, Sep 07, 2011 at 05:22:00PM +0200, Patrick McHardy wrote:
> >> On 04.09.2011 18:18, Michał Mirosław wrote:
> >>> On Sat, Sep 03, 2011 at 07:26:08PM +0200, kaber@...sh.net wrote:
> >>>> From: Patrick McHardy <kaber@...sh.net>
> >>>>
> >>>> Add support for memory mapped sendmsg() to netlink. Userspace queued to
> >>>> be processed frames into the TX ring and invokes sendmsg with
> >>>> msg.iov.iov_base = NULL to trigger processing of all pending messages.
> >>>>
> >>>> Since the kernel usually performs full message validation before beginning
> >>>> processing, userspace must be prevented from modifying the message
> >>>> contents while the kernel is processing them. In order to do so, the
> >>>> frames contents are copied to an allocated skb in case the the ring is
> >>>> mapped more than once or the file descriptor is shared (f.i. through
> >>>> AF_UNIX file descriptor passing).
> >>>>
> >>>> Otherwise an skb without a data area is allocated, the data pointer set
> >>>> to point to the data area of the ring frame and the skb is processed.
> >>>> Once the skb is freed, the destructor releases the frame back to userspace
> >>>> by setting the status to NL_MMAP_STATUS_UNUSED.
> >>>
> >>> Is this protected from threads? Like: one thread waits on sendmsg() and
> >>> another (same process) changes the buffer.
> >> Yes, if the ring is mapped multiple times (or the file descriptor
> >> is changed), the contents are copied to an allocated skb.
> > 
> > I mean:
> > 
> > [1] mmap()
> > [1] fill buffers
> > [1] pthread_create() [creates: 2]
> > [1] sendmsg() starts
> > [2] modify buffers
> > [1] sendmsg() returns
> > 
> > So: no multiple mmaps, and no touching of the fd. I haven't dug into
> > filesystem layer to see if threads affect file->f_count, but there
> > sure are no multiple mappings here.
> If CLONE_VM is given to clone(), the mapping is visible in both
> threads and thus we have multiple mappings (vma_ops->open() is
> invoked through clone()). Without CLONE_VM, the second thread
> can't access the ring unless it mmap()s it itself, in case we'd
> also have multiple mappings.

I made a quick look into kernel/fork.c, and it looks to me that if CLONE_VM
is set, then vma->open() is actually avoided --- it's called via dup_mm()
and dup_mmap() only if CLONE_VM is not there and the VMA needs to be copied.

Best Regards,
Michał Mirosław
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