Darwin: Handle unrelocated dyld.

On Darwin, debugging an helloworld program with GDB does
not work and ends with:

  (gdb) set startup-with-shell off
  (gdb) start
  Temporary breakpoint 1 at 0x100000fb4: file /tmp/helloworld.c, line 1.
  Starting program: /private/tmp/helloworld
  [New Thread 0x2703 of process 18906]
  [New Thread 0x2603 of process 18906]

  [1]+  Stopped                 ./gdb/gdb /tmp/helloworld

When debugging with lldb, instead of having the STOP signal, we can
see that a breakpoint is not set to a proper location:

  Warning:
  Cannot insert breakpoint -1.
  Cannot access memory at address 0xf726

  Command aborted.

The inserted breakpoint is the one used when GDB has to stop the target
when a shared library is loaded or unloaded. The notifier address used
for adding the breakpoint is wrong thus the above failure.
This notifier address is an offset relative to dyld base address, so
the value calculation has to be updated to reflect this.

This was tested on High Sierra by trying to run a simple "hello world"
program.

gdb/ChangeLog:

	PR gdb/20981:
        * solib-darwin.c (darwin_get_dyld_bfd): New function.
        (darwin_solib_get_all_image_info_addr_at_init): Update call.
        (darwin_handle_solib_event): New function.
        (darwin_solib_create_inferior_hook): Handle unrelocated dyld.

Change-Id: I7dde5008c9158f17b78dc89bd7f4bd8a12d4a6e1

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2018-09-19  Xavier Roirand  <roirand@adacore.com>
+
+	PR gdb/20981:
+	* solib-darwin.c (darwin_get_dyld_bfd): New function.
+	(darwin_solib_get_all_image_info_addr_at_init): Update call.
+	(darwin_solib_create_inferior_hook): Handle unrelocated dyld.
+
 2018-09-19  John Baldwin  <jhb@FreeBSD.org>
 
 	* fbsd-tdep.c (fbsd_print_sockaddr_in): Style fix.

gdb/solib-darwin.c

@@ -429,23 +429,21 @@ gdb_bfd_mach_o_fat_extract (bfd *abfd, bfd_format format,
   return gdb_bfd_ref_ptr (result);
 }
 
-/* Extract dyld_all_image_addr when the process was just created, assuming the
-   current PC is at the entry of the dynamic linker.  */
+/* Return the BFD for the program interpreter.  */
 
-static void
-darwin_solib_get_all_image_info_addr_at_init (struct darwin_info *info)
+static gdb_bfd_ref_ptr
+darwin_get_dyld_bfd ()
 {
   char *interp_name;
-  CORE_ADDR load_addr = 0;
 
   /* This method doesn't work with an attached process.  */
   if (current_inferior ()->attach_flag)
-    return;
+    return NULL;
 
   /* Find the program interpreter.  */
   interp_name = find_program_interpreter ();
   if (!interp_name)
-    return;
+    return NULL;
 
   /* Create a bfd for the interpreter.  */
   gdb_bfd_ref_ptr dyld_bfd (gdb_bfd_open (interp_name, gnutarget, -1));
@@ -459,6 +457,18 @@ darwin_solib_get_all_image_info_addr_at_init (struct darwin_info *info)
       else
 	dyld_bfd.release ();
     }
+  return dyld_bfd;
+}
+
+/* Extract dyld_all_image_addr when the process was just created, assuming the
+   current PC is at the entry of the dynamic linker.  */
+
+static void
+darwin_solib_get_all_image_info_addr_at_init (struct darwin_info *info)
+{
+  CORE_ADDR load_addr = 0;
+  gdb_bfd_ref_ptr dyld_bfd = darwin_get_dyld_bfd ();
+
   if (dyld_bfd == NULL)
     return;
 
@@ -528,10 +538,6 @@ darwin_solib_create_inferior_hook (int from_tty)
       return;
     }
 
-  /* Add the breakpoint which is hit by dyld when the list of solib is
-     modified.  */
-  create_solib_event_breakpoint (target_gdbarch (), info->all_image.notifier);
-
   if (info->all_image.count != 0)
     {
       /* Possible relocate the main executable (PIE).  */
@@ -558,6 +564,49 @@ darwin_solib_create_inferior_hook (int from_tty)
       if (vmaddr != load_addr)
 	objfile_rebase (symfile_objfile, load_addr - vmaddr);
     }
+
+  /* Set solib notifier (to reload list of shared libraries).  */
+  CORE_ADDR notifier = info->all_image.notifier;
+
+  if (info->all_image.count == 0)
+    {
+      /* Dyld hasn't yet relocated itself, so the notifier address may
+	 be incorrect (as it has to be relocated).  */
+      CORE_ADDR start = bfd_get_start_address (exec_bfd);
+      if (start == 0)
+	notifier = 0;
+      else
+        {
+          gdb_bfd_ref_ptr dyld_bfd = darwin_get_dyld_bfd ();
+          if (dyld_bfd != NULL)
+            {
+              CORE_ADDR dyld_bfd_start_address;
+              CORE_ADDR dyld_relocated_base_address;
+              CORE_ADDR pc;
+
+              dyld_bfd_start_address = bfd_get_start_address (dyld_bfd.get());
+
+              /* We find the dynamic linker's base address by examining
+                 the current pc (which should point at the entry point
+                 for the dynamic linker) and subtracting the offset of
+                 the entry point.  */
+
+              pc = regcache_read_pc (get_current_regcache ());
+              dyld_relocated_base_address = pc - dyld_bfd_start_address;
+
+              /* We get the proper notifier relocated address by
+                 adding the dyld relocated base address to the current
+                 notifier offset value.  */
+
+              notifier += dyld_relocated_base_address;
+            }
+        }
+    }
+
+  /* Add the breakpoint which is hit by dyld when the list of solib is
+     modified.  */
+  if (notifier != 0)
+    create_solib_event_breakpoint (target_gdbarch (), notifier);
 }
 
 static void