Remove file handles with gdb(1)

A process holding an open handle to a file and eventually removing that file while the process still runs often results in a stale file handle.

What that mean is that resources occupied by the file instead of being freed are actually not. Root cause of this is the process still holding an open file handle to the now removed file.

Possible solutions to this issue are:

  • Use lsof(8) to find the removed file, then kill the process holding it's file handle open
  • Use gdb(1) to attach to the process, then simply close the file handle

First, let's find the files being removed using lsof(8). Considering that our file was located somewhere in /var we could use the command below to find it.

$ sudo lsof /var

Next, we attach to the process holding the file handle:

$ sudo gdb attach <pid>

Find the files that are opened by the process. Either look at the result from lsof(8), or check /proc/<pid>/fd:

$ sudo ls -l /proc/19080/fd
total 0
lrwx------ 1 root root 64 May 17 14:30 0 -> socket:[5378591]
l-wx------ 1 root root 64 May 17 14:30 1 -> /var/log/kern.log
l-wx------ 1 root root 64 May 17 14:30 2 -> /var/log/messages
lrwx------ 1 root root 64 May 17 14:30 3 -> /dev/xconsole
lr-x------ 1 root root 64 May 17 14:30 4 -> /proc/kmsg
l-wx------ 1 root root 64 May 17 14:30 5 -> /var/log/syslog
l-wx------ 1 root root 64 May 17 14:30 6 -> /var/log/auth.log

Now we can close the file descriptor from the gdb(1) session that we have, which is already attached to our process.

NOTE: using "call" instead of "p" works fine too.

gdb> p close(2)
$1 = 0

Let's redirect the file descriptor to a different file now.

gdb> p fopen("/tmp/file", "w")
$2 = 20746416
(gdb) p fileno($2)
$3 = 7
gdb> quit
Written on June 18, 2015