linux - Ptrace parent process -

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i'm trying monitor/redirect syscalls in own process. ld_preload doesn't work when fwrite calls write inside libc, , got/plt hooks seem have same problem. i'm looking solution based on ptrace, can't fork() , run main app child because app communicates parent via signals.

there thread 2006 suggests tracer can on thread group that's different tracee, doesn't seem work in practice: http://yarchive.net/comp/linux/ptrace_self_attach.html

pid = fork(); if (pid == 0) {     prctl(pr_set_ptracer, getppid());     raise(sigstop); } else {     sleep(1);     ptrace(ptrace_seize, pid, null, null);     (;;) {         int status;         int ret = waitpid(pid, &status, 0);         warn("wait=%d:", ret);         ret = ptrace(ptrace_syscall, pid, null, null);         warn("ptrace=%d:", ret);             } }

the problem i'm facing ptrace(ptrace_syscall) expects tracee in ptrace-wait state, i.e. must have raised sigstop , tracer needs wait() it. since relation inversed in case (tracer child of tracee) ptrace_syscall returns esrch.

how strace away tracing existing pid ?

i'm bit unclear on you're asking here. sounds have attaching part resolved (which difficult problem resolve). if case, getting process stop not problem. send process signal. process stop , send trap can decide signal. @ point can call ptrace(ptrace_syscall, pid, 0, 0). both start in syscall trace mode, , prevent signal arriving @ debugee (thus not introducing unexpected signals process).


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