Hotfix for leaking filedescriptors

I just came across this neat gdb trick. Say you have a badly written program that's leaking file descriptors all over.. and it's in prod (the horror!) and it's imperative that you let it limp along till the patched binary comes around... GDB can close the file descriptors for you!

Here's how:

Leaky Program


Lsof tells it all. Take note of the two open descriptors to /etc/passwd


Playing around with variables in GDB. Can you guess why we chose the magic value 2147483646? For a hint, look at the last gist in this post


What does lsof think?


Bulk close of fds


Shell output from leaky

Dammit, read the output...

I keep forgetting to read the output of  commands

$ gdb apache2 /tmp/gdb/core
GNU gdb (GDB) 7.0.1-debian
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
...
     
warning: The current binary is a PIE (Position Independent Executable), which
GDB does NOT currently support.  Most debugger features will fail if used
in this session.
     
Reading symbols from /usr/sbin/apache2...Reading symbols from /usr/lib/debug/usr/sbin/apache2-mpm-prefork...done.
(no debugging symbols found)...done.
Core was generated by `/usr/sbin/apache2 -k start'.
Program terminated with signal 11, Segmentation fault.
#0  0x00007f9779b25c4f in ?? ()
(gdb) bt
#0  0x00007f9779b25c4f in ?? ()
#1  0x00007fc05b93c8b0 in ?? ()
#2  0x0000000000000007 in ?? ()
#3  0x0000000000000000 in ?? ()
(gdb) where
#0  0x00007f9779b25c4f in ?? ()
#1  0x00007fc05b93c8b0 in ?? ()
#2  0x0000000000000007 in ?? ()
#3  0x0000000000000000 in ?? ()
(gdb) quit
root@acme:~#
root@acme:~# apt-get install apache2-mpm-worker

Turns out that squeeze's gdb doesn't support PIE, pulling in SID's copy works

$ gdb -p 21164
GNU gdb (GDB) 7.2-debian
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
.                                                                                                   Attaching to process 21164
Reading symbols from /usr/lib/apache2/mpm-worker/apache2...Reading symbols from /usr/lib/debug/usr/lib/apache2/mpm-worker/apache2-mpm-worke
r...done.
done.
Reading symbols from /lib/libpcre.so.3...(no debugging symbols found)...done.
Loaded symbols for /lib/libpcre.so.3
Reading symbols from /usr/lib/libaprutil-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/libaprutil-1.so.0
Reading symbols from /usr/lib/libapr-1 .....