[Bug tapsets/18143] New: target_set tapset does not track threads created with clone()

["maxvt at bu dot edu" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18143

            Bug ID: 18143
           Summary: target_set tapset does not track threads created with
                    clone()
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tapsets
          Assignee: systemtap at sourceware dot org
          Reporter: maxvt at bu dot edu

Created attachment 8197
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8197&action=edit
tentative patch

I'm using systemtap to track system calls done by a specific process and its
descendants (e.g., sshd). Using ancient systemtap, the script works perfectly.
Using 2.6/0.159, no child process activity is picked up. After a little
debugging, it seems child processes are not being added to the target set.

Example probes:
probe syscall.execve {
    if (target_set_pid(pid())) {
        printf("t=%d p=%d (%s) execve (%s)\n", getjc(), pid(), execname(),
argstr)
    }
}

probe syscall.fork.return {
    if (target_set_pid(pid())) {
        printf("t=%d p=%d (%s) fork %s into %s\n", getjc(), pid(), execname(),
name, retstr)
    }
}

Expected output (original pid of sshd is 609, spawning threads 7336 and 7339):
$ stap -x 609 example.stp
t=49957 p=7336 (sshd) execve ("/usr/sbin/sshd" "-D" "-R")
t=50033 p=7339 (sshd) execve ("/bin/bash" )

Actual output:
none

Guess at the cause:
Looking at bug 16667 [1]: "The syscall.fork probe alias was broken up into
syscall.fork, syscall.vfork, and syscall.clone aliases." The target_set tapset
hooks syscall.fork which, reading comment 2 to the bug, tracked fork, fork, and
clone calls. But the change that closed the bug separated the three calls, so
now the tapset code tracks fork but not vfork, nor clone.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=16667

A tentative patch is attached.

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