Someone added to the Wikipedia "ptrace" article claiming that, on Linux, a ptraced process couldn't itself ptrace another process. I'm trying to determine if (and if so why) that's the case. Below is a simple program I contrived to test this. My program fails (the sub sub process doesn't run properly) but I'm pretty convinced it's my error and not something fundamental.
In essence the initial process A forks process B which in turn forks C. A ptraces its child B, B ptraces its child C. Once they're set up, all three processes are written to just print A
,B
, or C
to stdout once every second.
In practice what happens is that A and B work fine, but C prints only once and then gets stuck. Checking with ps -eo pid,cmd,wchan
shows C stuck in kernel function ptrace_stop
while the rest are in hrtimer_nanosleep
where I'd expect all three to be.
Very occasionally all three do work (so the program prints Cs as well as As and Bs), which leads me to believe there's some race condition in the initial setup.
My guesses as to what might be wrong are:
- something to do with A seeing a
SIGCHLD
related to B seeing aSIGCHLD
to do with a signal to C, and wait(2) reporting both as coming from B (but a hacky call of PTRACE_CONT to both pids doesn't fix things)? - C should be ptraced by B - has C inherited the ptrace by A instead (and B's call to ptrace neither errored nor overwrote this)?
Can anyone figure out what I'm doing wrong? Thanks.
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
static void a(){
while(1){
printf ("A\n");
fflush(stdout);
sleep(1);
}
}
static void b(){
while(1){
printf ("B\n");
fflush(stdout);
sleep(1);
}
}
static void c(){
while(1){
printf ("C\n");
fflush(stdout);
sleep(1);
}
}
static void sigchld_handler(int sig){
int result;
pid_t child_pid = wait(NULL); // find who send us this SIGCHLD
printf("SIGCHLD on %d\n", child_pid);
result=ptrace(PTRACE_CONT, child_pid, sig, NULL);
if(result) {
perror("continuing after SIGCHLD");
}
}
int main(int argc,
char **argv){
pid_t mychild_pid;
int result;
printf("pidA = %d\n", getpid());
signal(SIGCHLD, sigchld_handler);
mychild_pid = fork();
if (mychild_pid) {
printf("pidB = %d\n", mychild_pid);
result = ptrace(PTRACE_ATTACH, mychild_pid, NULL, NULL);
if(result==-1){
perror("outer ptrace");
}
a();
}
else {
mychild_pid = fork();
if (mychild_pid) {
printf("pidC = %d\n", mychild_pid);
result = ptrace(PTRACE_ATTACH, mychild_pid, NULL, NULL);
if(result==-1){
perror("inner ptrace");
}
b();
}
else {
c();
}
}
return 0;
}