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POcalling ptrace inside a ptraced Linux process
 

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    <p>Someone added to the <a href="http://en.wikipedia.org/wiki/Ptrace" rel="noreferrer">Wikipedia "ptrace" article</a> 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.</p> <p>In essence the initial process <strong>A</strong> forks process <strong>B</strong> which in turn forks <strong>C</strong>. <strong>A</strong> ptraces its child <strong>B</strong>, <strong>B</strong> ptraces its child <strong>C</strong>. Once they're set up, all three processes are written to just print <code>A</code>,<code>B</code>, or <code>C</code> to stdout once every second. </p> <p>In practice what happens is that <strong>A</strong> and <strong>B</strong> work fine, but <strong>C</strong> prints only once and then gets stuck. Checking with <code>ps -eo pid,cmd,wchan</code> shows <strong>C</strong> stuck in kernel function <code>ptrace_stop</code> while the rest are in <code>hrtimer_nanosleep</code> where I'd expect all three to be. </p> <p>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. </p> <p>My <em>guesses</em> as to what might be wrong are:</p> <ul> <li>something to do with <strong>A</strong> seeing a <code>SIGCHLD</code> related to <strong>B</strong> seeing a <code>SIGCHLD</code> to do with a signal to <strong>C</strong>, and wait(2) reporting both as coming from <strong>B</strong> (but a hacky call of PTRACE_CONT to both pids doesn't fix things)?</li> <li><strong>C</strong> should be ptraced by <strong>B</strong> - has <strong>C</strong> inherited the ptrace by <strong>A</strong> instead (and <strong>B</strong>'s call to ptrace neither errored nor overwrote this)?</li> </ul> <p>Can anyone figure out what I'm doing wrong? Thanks.</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;string.h&gt; #include &lt;unistd.h&gt; #include &lt;signal.h&gt; #include &lt;sys/ptrace.h&gt; #include &lt;sys/wait.h&gt; 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; } </code></pre>
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