Re: capsicum and netmap ?

On Mon, Sep 29, 2014 at 08:20:08PM +0200, Luigi Rizzo wrote:
> 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.

Ah, that's it.  The problem is that we're limiting the pcap file
descriptors to CAP_READ.  It looks like you'd need to add CAP_EVENT to
that list.  Look for cap_rights_init and cap_rights_limit pairs to find
the right place(s) to modify.

-- Brooks