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
Description: PGP signature