Re: Using stapdyn to probe child processes of the target?

[Arkady <arkady dot miasnikov at gmail dot com>]

If the timing is important you can add some code to the script. For
example you can call to processAttach() in the script when a certain
function in the GCC starts a new process.

On Fri, Jun 16, 2017 at 7:05 AM, Arkady <arkady.miasnikov@gmail.com> wrote:
> I have never tried user space hooking. My immediate idea is to run gcc
> under strace, filter all fork-exec pairs (a Python script here?) and
> attach stap to all child PIDs
>
> On Fri, Jun 16, 2017 at 12:47 AM, Serhei Makarov <serhei.etc@gmail.com> wrote:
>> 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)