8145099: Better error message when SA can't attach to a process

Reviewed-by: jbachorik, stuefe

agent/src/os/linux/LinuxDebuggerLocal.c

@@ -209,9 +209,12 @@ JNIEXPORT void JNICALL Java_sun_jvm_hotspot_debugger_linux_LinuxDebuggerLocal_at
   verifyBitness(env, (char *) &buf);
   CHECK_EXCEPTION;
 
+  char err_buf[200];
   struct ps_prochandle* ph;
-  if ( (ph = Pgrab(jpid)) == NULL) {
-    THROW_NEW_DEBUGGER_EXCEPTION("Can't attach to the process");
+  if ( (ph = Pgrab(jpid, err_buf, sizeof(err_buf))) == NULL) {
+    char msg[230];
+    snprintf(msg, sizeof(msg), "Can't attach to the process: %s", err_buf);
+    THROW_NEW_DEBUGGER_EXCEPTION(msg);
   }
   (*env)->SetLongField(env, this_obj, p_ps_prochandle_ID, (jlong)(intptr_t)ph);
   fillThreadsAndLoadObjects(env, this_obj, ph);

agent/src/os/linux/libproc.h

@@ -82,7 +82,7 @@ typedef int bool;
 struct ps_prochandle;
 
 // attach to a process
-struct ps_prochandle* Pgrab(pid_t pid);
+struct ps_prochandle* Pgrab(pid_t pid, char* err_buf, size_t err_buf_len);
 
 // attach to a core dump
 struct ps_prochandle* Pgrab_core(const char* execfile, const char* corefile);

agent/src/os/linux/ps_proc.c

@@ -215,9 +215,12 @@ static bool ptrace_waitpid(pid_t pid) {
 }
 
 // attach to a process/thread specified by "pid"
-static bool ptrace_attach(pid_t pid) {
+static bool ptrace_attach(pid_t pid, char* err_buf, size_t err_buf_len) {
   if (ptrace(PTRACE_ATTACH, pid, NULL, NULL) < 0) {
-    print_debug("ptrace(PTRACE_ATTACH, ..) failed for %d\n", pid);
+    char buf[200];
+    char* msg = strerror_r(errno, buf, sizeof(buf));
+    snprintf(err_buf, err_buf_len, "ptrace(PTRACE_ATTACH, ..) failed for %d: %s", pid, msg);
+    print_debug("%s\n", err_buf);
     return false;
   } else {
     return ptrace_waitpid(pid);
@@ -339,16 +342,17 @@ static ps_prochandle_ops process_ops = {
 };
 
 // attach to the process. One and only one exposed stuff
-struct ps_prochandle* Pgrab(pid_t pid) {
+struct ps_prochandle* Pgrab(pid_t pid, char* err_buf, size_t err_buf_len) {
   struct ps_prochandle* ph = NULL;
   thread_info* thr = NULL;
 
   if ( (ph = (struct ps_prochandle*) calloc(1, sizeof(struct ps_prochandle))) == NULL) {
-     print_debug("can't allocate memory for ps_prochandle\n");
+     snprintf(err_buf, err_buf_len, "can't allocate memory for ps_prochandle");
+     print_debug("%s\n", err_buf);
      return NULL;
   }
 
-  if (ptrace_attach(pid) != true) {
+  if (ptrace_attach(pid, err_buf, err_buf_len) != true) {
      free(ph);
      return NULL;
   }
@@ -371,7 +375,7 @@ struct ps_prochandle* Pgrab(pid_t pid) {
   thr = ph->threads;
   while (thr) {
      // don't attach to the main thread again
-     if (ph->pid != thr->lwp_id && ptrace_attach(thr->lwp_id) != true) {
+    if (ph->pid != thr->lwp_id && ptrace_attach(thr->lwp_id, err_buf, err_buf_len) != true) {
         // even if one attach fails, we get return NULL
         Prelease(ph);
         return NULL;