On Mon, Sep 29, 2014 at 05:27:09PM +0000, Brooks Davis wrote:
> On Mon, Sep 29, 2014 at 05:30:43PM +0200, Luigi Rizzo wrote:
> > 
> > Hi,
> > while trying the netmap-enabled libpcap library with tcpdump, i
> > noticed it fails to return data on a kernel with capsicum (the
> > string "capability mode sandbox enabled" made me suspicious, and
> > removing the cap_*() calls from tcpdump.c seems to make things
> > work again).
> > 
> > Would anyone be able to point me what should be done in the netmap
> > kernel module to make it work with capsicum ?
> > 
> > I am sure the cambridge folks are very interested in this :)
> Without knowing what modifications have been made to libpcap, it's hard
> to say what you need to change, but the short version is that once
> cap_enter is called, you must not attempt to open any file handles as
> that's won't work.  I can't think of any other likely cause.  Are all
> the returns of all open(), socket(), etc calls checked?

Hi Brooks,
thanks for the feedback.

The change (attached, with some debugging code; it dates back to
december and i am trying to upstream it into FreeBSD now) is a set
of methods called to open, dispatch and inject packets.

> In practice that means that either opening files must come earlier, or
> a singling mechanism needs to be added to tcpdump and libpcap to tell
> tcpdump not to enter capability mode when using netmap.

The nm_open() (which includes open and mmap) occurs before the
cap_enter() call, and poll() works fine until we do the
cap_enter()/cap_sandboxed() calls.

I was wondering whether I should somewhat annotate the file descriptor
(in the netmap kernel module) indicating that it is right to access it
after cap_enter(). poll() returns 1 and errno=0
when polling for POLLIN on the netmap file descriptor,
while it should return 0 (there is no traffic queued).

I haven't investigated in detail but it almost looks like the
underlying netmap_poll() in the device driver is not called.


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