On Mon, 09 Jun 2008 19:23:09 +0200, Matthew Legendre wrote:
>
> We're seeing issues when trying to attached to an already stopped process
> on recent utrace kernels (seen on Fedora Core 8 and 9)--waitpid reports
> the arrival of numerous signal 0s,
Being tracked as `stop-attach-then-wait' at:
http://sourceware.org/systemtap/wiki/utrace/tests
While you are right the ptrace-on-utrace emulation is currently incompatible
to ptrace for proper behavior during later ptrace operations you should resume
the job control stop first [attached]. It is a code from a GDB code at:
http://sourceware.org/ml/gdb-patches/2008-05/msg00022.html
Thanks,
Jan
--- test_attached_to_stopped.c 2008-06-10 23:36:54.000000000 +0200
+++ test_attached_to_stopped-jk.c 2008-06-11 19:19:08.000000000 +0200
@@ -19,6 +19,38 @@ void stop_self()
kill(getpid(), SIGSTOP);
}
+/* gdb/linux-nat.c */
+/* Detect `T (stopped)' in `/proc/PID/status'.
+ Other states including `T (tracing stop)' are reported as false. */
+
+static int
+pid_is_stopped (pid_t pid)
+{
+ FILE *status_file;
+ char buf[100];
+ int retval = 0;
+
+ snprintf (buf, sizeof (buf), "/proc/%d/status", (int) pid);
+ status_file = fopen (buf, "r");
+ if (status_file != NULL)
+ {
+ int have_state = 0;
+
+ while (fgets (buf, sizeof (buf), status_file))
+ {
+ if (strncmp (buf, "State:", 6) == 0)
+ {
+ have_state = 1;
+ break;
+ }
+ }
+ if (have_state && strstr (buf, "T (stopped)") != NULL)
+ retval = 1;
+ fclose (status_file);
+ }
+ return retval;
+}
+
int attach_then_run(void (*func)(void))
{
int pid, result;
@@ -32,6 +64,28 @@ int attach_then_run(void (*func)(void))
perror("Ptrace attach error");
exit(-1);
}
+ if (pid_is_stopped(pid)) {
+ /* gdb/linux-nat.c */
+
+ /* The process is definitely stopped. It is in a job control
+ stop, unless the kernel predates the TASK_STOPPED /
+ TASK_TRACED distinction, in which case it might be in a
+ ptrace stop. Make sure it is in a ptrace stop; from there we
+ can kill it, signal it, et cetera.
+
+ First make sure there is a pending SIGSTOP. Since we are
+ already attached, the process can not transition from stopped
+ to running without a PTRACE_CONT; so we know this signal will
+ go into the queue. The SIGSTOP generated by PTRACE_ATTACH is
+ probably already in the queue (unless this kernel is old
+ enough to use TASK_STOPPED for ptrace stops); but since SIGSTOP
+ is not an RT signal, it can only be queued once. */
+ kill (pid, SIGSTOP); /* tgkill() is required for threads! */
+
+ /* Finally, resume the stopped process. This will deliver the SIGSTOP
+ (or a higher priority signal, just like normal PTRACE_ATTACH). */
+ ptrace (PTRACE_CONT, pid, 0, 0);
+ }
return pid;
}
