Using stapdyn to probe child processes of the target?

[Serhei Makarov <serhei dot etc at gmail dot com>]

Hello all,

I've been trying to observe gcc's execution using Stapdyn. Most of the
actual work is done by a child process (cc1) forked from gcc. (Below
I've attached a script, proctree.stp, for listing processes forked by
a target command.) If I'm using kernel systemtap, I can just target
cc1 directly, for example:

stap -ve 'probe
process("/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/cc1").insn {
counter<<<1 } global counter probe end { printf("%d calls\n",
@count(counter)) }' -c "gcc test/widget3.c"

Instrumenting using kernel uprobes introduces quite a large
performance overhead, so I would prefer Dyninst for this. But the same
script doesn't work with --dyninst option, because the target process
selected using -c is not cc1.

As far as I can tell, my only options for observing cc1 with Stapdyn are:

(a) use -c option : invoke cc1 directly. Feasible in this case, may
not be feasible in other software where the parent/child processes
coordinate using more complex IPC. This also doesn't give a profile of
the full GCC execution (unlike, say, "valgrind --trace-children=yes").
(b) use -x option : attach to cc1 by PID after it starts running. Not
feasible in this case since cc1 is a batch program, not a long-running
daemon.

Am I missing any possibilities? Is it currently possible to use
Stapdyn to attach to processes other than a single target?

All the best,
     Serhei Makarov

PS Here's the proctree.stp script:

global pids

probe begin {
  pids[target()] = 0
}

probe process.begin {
  child = pid(); parent = ppid()
  if (parent in pids) {
    printf("fork %s(%d) -> %s(%d)\n", pexecname(), parent, execname(), child)
    pids[child] = parent
  }
}

This outputs things like:

[serhei@beatrice e-tracer]$ stap proctree.stp -c "gcc test/widget3.c"
Missing separate debuginfos, use: debuginfo-install
kernel-core-4.10.10-200.fc25.x86_64
fork gcc(2725) -> gcc(2728)
fork gcc(2725) -> cc1(2728)
fork gcc(2725) -> gcc(2729)
fork gcc(2725) -> as(2729)
fork gcc(2725) -> gcc(2730)
fork gcc(2725) -> collect2(2730)
fork collect2(2730) -> collect2(2731)
fork collect2(2730) -> ld(2731)

Or, here's a Firefox browsing session:

[serhei@beatrice e-tracer]$ stap proctree.stp -c "firefox --new-instance"
fork firefox(3179) -> firefox(3182)
fork firefox(3179) -> basename(3182)
fork firefox(3179) -> firefox(3183)
fork firefox(3179) -> uname(3183)
fork firefox(3179) -> firefox(3184)
fork firefox(3179) -> pidof(3184)
fork firefox(3179) -> firefox(3185)
fork firefox(3179) -> mkdir(3185)
fork firefox(3179) -> firefox(3186)
fork firefox(3186) -> firefox(3187)
fork firefox(3186) -> firefox(3188)
fork firefox(3186) -> sed(3188)
fork firefox(3179) -> firefox(3189)
fork firefox(3189) -> firefox(3190)
fork firefox(3189) -> firefox(3191)
fork firefox(3189) -> sed(3191)
fork firefox(3179) -> firefox(3192)
fork firefox(3179) -> expr(3192)
fork run-mozilla.sh(3179) -> run-mozilla.sh(3193)
fork run-mozilla.sh(3179) -> basename(3193)
fork run-mozilla.sh(3179) -> run-mozilla.sh(3194)
fork run-mozilla.sh(3179) -> dirname(3194)
fork run-mozilla.sh(3179) -> run-mozilla.sh(3195)
fork run-mozilla.sh(3179) -> uname(3195)
fork firefox(3179) -> firefox(3196)
fork firefox(3179) -> firefox(3198)
fork Gecko_IOThread(3179) -> Gecko_IOThread(3266)
fork Gecko_IOThread(3179) -> firefox(3266)