jm + ptrace   2

Tracedump
a single application IP packet sniffer that captures all TCP and UDP packets of a single Linux process. It consists of the following elements:

* ptrace monitor - tracks bind(), connect() and sendto() syscalls and extracts local port numbers that the traced application uses;
* pcap sniffer - using information from the previous module, it captures IP packets on an AF_PACKET socket (with an appropriate BPF filter attached);
* garbage collector - periodically reads /proc/net/{tcp,udp} files in order to detect the sockets that the application no longer uses.

As the output, tracedump generates a PCAP file with SLL-encapsulated IP packets - readable by eg. Wireshark. This file can be later used for detailed analysis of the networking operations made by the application. For instance, it might be useful for IP traffic classification systems.
debugging  networking  linux  strace  ptrace  tracedump  tracing  tcp  udp  sniffer  ip  tcpdump 
may 2014 by jm
Peek and poke in the age of Linux
Neat demo of using ptrace to inject into a running process, just like the good old days ;)
Some time ago I ran into a production issue where the init process (upstart) stopped behaving properly. Specifically, instead of spawning new processes, it deadlocked in a transitional state. [...] What’s worse, upstart doesn’t allow forcing a state transition and trying to manually create and send DBus events didn’t help either. That meant the sane options we were left with were:
restart the host (not desirable at all in that scenario);
start the process manually and hope auto-respawn will not be needed.
Of course there are also some insane options. Why not cheat like in the old times and just PEEK and POKE the process in the right places? The solution used at the time involved a very ugly script driving gdb which probably summoned satan in some edge cases. But edge cases were not hit and majority of hosts recovered without issues.
debugging  memory  linux  upstart  peek  poke  ptrace  gdb  processes  hacks 
march 2013 by jm

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