DNS using Name Service Switch module and Casper

[Vasily Postnicov]

Turns out, if you do not specify either -4 or -6 to ping, unsandboxed
getaddrinfo() will be called in /usr/src/sbin/ping/main.c, line 139.
(what's the point in sandboxing then, lol?) This somehow affects
sandboxing.

Look at the screenshot, it explains where fork() gets stuck.
https://photos.app.goo.gl/T1B3Fo1hg6z7r3vZ6

Oh yes, my module works if you specify -4 to ping command.

пт, 8 янв. 2021 г. в 20:58, Vasily Postnicov <shamaz.mazum at gmail.com>:
>
> Nevermind my last question. ZeroMQ is written on C++. Here is shown how you can execute everything with almost empty main.
>
> https://stackoverflow.com/questions/38717534/how-do-i-start-a-c-thread-at-program-startup
>
> For C the only way is to use __attribute__((constructor)) AFAIK
>
> пт, 8 янв. 2021 г., 20:17 Vasily Postnicov <shamaz.mazum at gmail.com>:
>>
>> I have noticed that after I kill stuck ping, the process spawned with
>> cap_init() remains. I cannot even kill it with SIGKILL. This is the
>> output of procstat on such a process.
>>
>>
>> vasily   969   0.0  0.1   26428   6532 v0  I    22:43    0:00.00 ping
>> vonbraun.local
>> vasily   983   0.0  0.1   26428   6532 v0  I    22:43    0:00.00 ping
>> resurrected.local
>> vasily  1024   0.0  0.1   26428   6532 v0  I    22:49    0:00.00 ping
>> resurrected.local
>> vasily  1028   0.0  0.1   26428   6532 v0  I    22:49    0:00.00 ping
>> resurrected.local
>> root    1089   0.0  0.0   12976   2512 v1  S+   22:58    0:00.01 grep ping
>>   PID    TID COMM                TDNAME              KSTACK
>>  1028 100579 ping                -                   mi_switch+0x155
>> sleepq_switch+0x109 sleepq_catch_signals+0x266 sleepq_wait_sig+0x9
>> _sleep+0x2aa umtxq_sleep+0x19e do_lock_umutex+0x744
>> __umtx_op_wait_umutex+0x49 sys__umtx_op+0x7a amd64_syscall+0x12e
>> fast_syscall_common+0xf8
>>
>> I checked ZeroMQ on which my NSS module is based. It does not use
>> pthread_atfork(), but uses lots of other unusual pthread functions,
>> like pthread_setaffinity_np() or pthread_setschedparam(). Do not know
>> if it matters. Also I do not quite understand when the code in my
>> module is executed. It should be executed after the capsicumized
>> sandbox is created, should it not? And I got hang in the process of
>> creating the sandbox. So I do not understand how my code affects this
>> process :)
>>
>>
>> пт, 8 янв. 2021 г. в 18:45, Mark Johnston <markj at freebsd.org>:
>> >
>> > On Wed, Jan 06, 2021 at 07:08:14PM +0300, Vasily Postnicov wrote:
>> > > That's what I found.
>> > >
>> > > At first, ping calls cap_init() in capdns_setup(). cap_init() forks a
>> > > process, then the parent returns and the child calls casper_main_loop().
>> > > The child and the parent both have a socket to communicate.
>> > > casper_main_loop() calls zygote_init() and that one blocks on fork(). I do
>> > > not know how it could be. How can fork() block?
>> >
>> > Does you module somehow use pthread_atfork()?
>> >
>> > > The parent process later calls cap_service_open() and that function calls
>> > > cap_xfer_nvlist(). Because the child process is stuck somewhere in
>> > > zygote_init() it never sends an nvlist back. So ping blocks.
>> >
>> > Can you show output from "procstat -kk <pid>" when this hang occurs?
>> >
>> > > All this is figured out by inserting printf()'s. LLDB refuses to run ping
>> > > with 'error: Child exec failed'.
>> >
>> > Presumably it needs to be run as root since ping(8) is a setuid
>> > executable.
>> >
>> > > вт, 5 янв. 2021 г. в 17:43, Mark Johnston <markj at freebsd.org>:
>> > >
>> > > > On Tue, Jan 05, 2021 at 10:02:37AM +0300, Vasily Postnicov wrote:
>> > > > > Hello. I wrote a simple daemon called ZeroDNS which provides
>> > > > functionality
>> > > > > similar to multicast DNS, namely it discovers other participating
>> > > > machines
>> > > > > over the LAN and stores their hostname and IPv4 address pairs.
>> > > > >
>> > > > > Here is a NSS module which allows the system to use information from that
>> > > > > daemon:
>> > > > > https://github.com/shamazmazum/nss-zero-dns
>> > > > >
>> > > > > You need to modify /etc/nsswitch.conf, changing the line 'hosts: files
>> > > > dns'
>> > > > > to 'hosts: files dns zerodns'.
>> > > > >
>> > > > > It all works on FreeBSD 12.2-RELEASE, but sometimes not on 13.0-CURRENT.
>> > > > > For example, ping(8) just blocks when trying to ping a host whose name is
>> > > > > resolvable with ZeroDNS. Turns out that programs built with casper
>> > > > support
>> > > > > (like ping(8) and some others) stop working with my NSS module (they just
>> > > > > block trying to resolve the name).
>> > > >
>> > > > Presumably it's the casper process (i.e., cap_dns) that uses your
>> > > > module?  If the main ping process is blocked trying to resolve a name,
>> > > > it's waiting for the cap_dns process - where exactly is it getting
>> > > > stuck?
>> > > >
>> > > > > Is there some kind of manual on how to write casper-compatible NSS
>> > > > modules?
>> > > >