I need to be able to see if a mount request generated an error in the mountd daemon. I have a custom kernel that has changed the mount() function call to return the error code. So, now I'm using a very simple DTrace script to catch the error codes:

#!/usr/sbin/dtrace -s

pid$1::mount:return
{
        printf("rc = %d", args[1]);
}

pid$1::audit_mountd_mount:return
{
        printf("rc = %d", args[1]);
}

The issue is what happens when I try to run it:

[root@pnfs-4-11 ~]> ./mountd_res.d `pgrep -x mountd`
dtrace: failed to compile script ./mountd_res.d: line 18: args[ ] may not be referenced because probe description pid100824::mount:return matches an unstable set of probes

What I think this means is that I've got multiple declarations of mount() and they all do not return something. Okay, I can narrow down the probe to just the one I want:

[root@pnfs-4-11 ~]> dtrace -l -f mount
   ID   PROVIDER            MODULE                          FUNCTION NAME
 2378 lx-syscall                                               mount entry
 2379 lx-syscall                                               mount return
13708        fbt           genunix                             mount entry
13709        fbt           genunix                             mount return
31190    syscall                                               mount entry
31191    syscall                                               mount return
65944  pid100824            mountd                             mount entry
65945  pid100824         libc.so.1                             mount entry
65946  pid100824            mountd                             mount return
65947  pid100824         libc.so.1                             mount return

And if I adjust my script:

pid$1:mountd:mount:return
{
        printf("rc = %d", args[1]);
}

We see I've fixed this issue!

[root@pnfs-4-11 ~]> ./mountd_res.d `pgrep -x mountd`
dtrace: failed to compile script ./mountd_res.d: line 18: index 1 is out of range for pid100824:mountd:mount:return args[ ]

Okay, I got my syntax wrong for the return code:

pid$1:mountd:mount:return
{
        printf("rc = %d", arg1);
}

And now I see the correct output:

[root@pnfs-4-11 ~]> ./mountd_res.d `pgrep -x mountd`
dtrace: script './mountd_res.d' matched 2 probes
CPU     ID                    FUNCTION:NAME
  0  65946                     mount:return rc = 0
  0  65240        audit_mountd_mount:return rc = 1
  0  65946                     mount:return rc = 0
  0  65240        audit_mountd_mount:return rc = 1