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

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Date:	Wed, 7 Sep 2011 22:03:19 +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 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.

Best Regards,
Michał Mirosław
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