kern/169350: Extraneous output when tracing writes with output redirect to a file

[W.L. Tai]

>Number:         169350
>Category:       kern
>Synopsis:       Extraneous output when tracing writes with output redirect to a file
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Jun 23 15:30:10 UTC 2012
>Closed-Date:
>Last-Modified:
>Originator:     W.L. Tai
>Release:        9.0 stable
>Organization:
>Environment:
FreeBSD aaa.local 9.0-STABLE FreeBSD 9.0-STABLE #0: Fri Jun 22 20:51:15 HKT 2012     root at aaa.local:/usr/obj/usr/src/sys/TAI  amd64
>Description:
Extraneous output when tracing writes from entry to return with output redirection to a file.
>How-To-Repeat:
Consider the following script: test-bug.d

#!/usr/sbin/dtrace -s
#pragma D option quiet

vfs::vop_read:entry, vfs::vop_write:entry
/execname != "dtrace"/
{
        self->flag = 1;
}

vfs::vop_read:return, vfs::vop_write:return
/self->flag/
{
        this->flag = 1;
        self->flag = 0;
}

vfs::vop_read:return, vfs::vop_write:return
/this->flag/
{
        printf("%s\n", execname);
}

After running

dtrace -s test-bug.d -n 'tick-5s { exit(0); }' > /tmp/test

the file /tmp/test would contains lines with dtrace, which should not be there. Somehow dtrace has inherit the value of this->flag.
>Fix:
Work around:

Either continue to use self->flag or use the predicate

/this->flag && execname != "dtrace"/

>Release-Note:
>Audit-Trail:
>Unformatted: