[PATCH] Handle absence of DT_DEBUG while debugging ld.so

[Luis Machado <luisgpm@linux.vnet.ibm.com>]

Hi folks,

When trying to debug ld.so, we may find that the DT_DEBUG entry in
the .dynamic section is missing, thus GDB tries to look for the _r_debug
minimal symbol, and eventually finds it. This works OK as long as the
_r_debug minimal symbol is correct and initialized.

In the case where GDB fetches an uninitialized _r_debug minimal symbol
and tries to access its address, we have a memory access error, just
like below:

(gdb) r
Starting program: /foo/lib/ld.so.1
Cannot access memory at address 0x2f648

Thus, we should guard this piece of code against uninitialized addresses
so that GDB can skip this entry and look for another (hopefully) valid
_r_debug symbol.

Tested against HEAD for ppc32/ppc64 with no regressions. OK?

Regards,
Luis

---
2008-07-28  Luis Machado  <luisgpm@br.ibm.com>

	* solib-svr4.c (elf_locate_base): Check if msymbol's
	address is valid.

Index: gdb/solib-svr4.c
===================================================================
--- gdb.orig/solib-svr4.c	2008-07-28 14:35:20.000000000 -0700
+++ gdb/solib-svr4.c	2008-07-28 16:11:42.000000000 -0700
@@ -508,9 +508,14 @@
     return dyn_ptr;
 
   /* This may be a static executable.  Look for the symbol
-     conventionally named _r_debug, as a last resort.  */
+     conventionally named _r_debug, as a last resort and check if
+     the address contained in the minimal symbol is valid before
+     proceeding.  */
   msymbol = lookup_minimal_symbol ("_r_debug", NULL, symfile_objfile);
-  if (msymbol != NULL)
+  if (msymbol != NULL
+      && !target_read_memory (SYMBOL_VALUE_ADDRESS (msymbol),
+			      (gdb_byte *)&dyn_ptr,
+			      TYPE_LENGTH (builtin_type_void_data_ptr)))
     return SYMBOL_VALUE_ADDRESS (msymbol);
 
   /* DT_DEBUG entry not found.  */