How to Backtrace Functions?

[dkovacevic]

When getaddrinfo() is called, I'd like to be able to stop the request from completing before res_nquery is called, stemming from a "permissions" concern.

On DragonflyBSD, NetBSD and FreeBSD, the "name-service switch dispatcher" configured in /etc/nsswitch.conf is used to select which sources for hostname lookups to use and what order to use them.

"...is used to select which sources..." - is used by what to select which sources?

I've looked at https://www.freebsd.org/cgi/man.cgi?query=nsdispatch, and it looks like a better place to implement security related code than res_nquery, but the impression I have of the "name-service switch dispatcher routine" is that the decision to go through with dispatch has already been made sometime before that point.

Maybe a better question / line of thinking: when I call getaddrinfo(), how do I trace that call back, one step at a time, to res_nquery?

 

[dkovacevic]

If I put a ton of print statements in libc code, and then compile the FreeBSD kernel, will I be able to see those statements in the root console?

 

[sko]

Maybe a better question / line of thinking: when I call getaddrinfo(), how do I trace that call back, one step at a time, to res_nquery?

This should be exactly what DTrace(1) was made for: Probing the running code/system and understanding what it does (or fails to do).

FreeBSD Handbook on Dtrace: https://www.freebsd.org/doc/handbook/dtrace.html
The DTrace guide on dtrace.org: http://dtrace.org/guide/

 

[dkovacevic]

This should be exactly what DTrace(1) was made for: Probing the running code/system and understanding what it does (or fails to do).

FreeBSD Handbook on Dtrace: https://www.freebsd.org/doc/handbook/dtrace.html
The DTrace guide on dtrace.org: http://dtrace.org/guide/

That looks useful, thank you.

 

[_martin]

Dtrace is definitely good, but good old ltrace(1) is your friend here too. It gives you general idea what is being called.