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Subject: [patch] Fixes problem setting breakpoint in dynamic loader
From: PAUL GILLIAM <pgilliam@us.ibm.com>
Reply-To: pgilliam@us.ibm.com
To: gdb-patches@sources.redhat.com
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On PowerPC-64, with 64-bit executables, GDB has been giving this message
for a while:

        warning: Unable to find dynamic linker breakpoint function.
        GDB will be unable to debug shared library initializers
        and track explicitly loaded dynamic code.

This is because "enable_break()" in solib-svr4.c was looking for the
symbol "._dl_debug_state" in the 64-bit dynamic loader and not finding
it.  This should not be a surprise because these 'dot' symbols have not
been used for a while.

The reason that the non-'dot' symbol was also passed by, is that it
points into a data section and the existing code only checked code
sections.

If none of the symbols on the list was found in a code section, the
attached patch looks in data sections.  When it finds one, and the data
section is either '.plt' or '.opd', then the address points to a
function descriptor which is then used to find the corresponding code
address where the breakpoint can be set.

OK to commit?

-=# Paul #=-

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2006-05-24  Paul Gilliam  <pgilliam@us.ibm.com

	* solib-svr4.c (enable_break): Resolve break address when the
	symbol is found in the data section.

diff -Naur old/solib-svr4.c new/solib-svr4.c
--- old/solib-svr4.c	2006-05-24 15:50:42.000000000 -0700
+++ new/solib-svr4.c	2006-05-24 15:51:36.000000000 -0700
@@ -85,16 +85,6 @@
   "rtld_db_dlactivity",
   "_rtld_debug_state",
 
-  /* On the 64-bit PowerPC, the linker symbol with the same name as
-     the C function points to a function descriptor, not to the entry
-     point.  The linker symbol whose name is the C function name
-     prefixed with a '.' points to the function's entry point.  So
-     when we look through this table, we ignore symbols that point
-     into the data section (thus skipping the descriptor's symbol),
-     and eventually try this one, giving us the real entry point
-     address.  */
-  "._dl_debug_state",
-
   NULL
 };
 
@@ -1043,20 +1033,75 @@
       /* Now try to set a breakpoint in the dynamic linker.  */
       for (bkpt_namep = solib_break_names; *bkpt_namep != NULL; bkpt_namep++)
 	{
-          /* On ABI's that use function descriptors, there are usually
-             two linker symbols associated with each C function: one
-             pointing at the actual entry point of the machine code,
-             and one pointing at the function's descriptor.  The
-             latter symbol has the same name as the C function.
-
-             What we're looking for here is the machine code entry
-             point, so we are only interested in symbols in code
-             sections.  */
+	  /* What we're looking for here is the machine code entry point,
+	     so we are only interested in symbols in code sections.
+
+	     On ABI's that use function descriptors, the linker symbol with
+	     the same name as a C funtion points to that functions descriptor.
+	     when those function descriptors are in the code section, they
+	     contain executable code and we can set a breakpoint there. */
 	  sym_addr = bfd_lookup_symbol (tmp_bfd, *bkpt_namep, SEC_CODE);
 	  if (sym_addr != 0)
 	    break;
 	}
 
+      if (sym_addr == 0)
+        {
+	  CORE_ADDR sect_offset;
+	  
+	  /* No symbol was found in a code section, so look in the data
+             sections.  This will only happen when the linker symbol points
+	     to a function descriptor that is in a data section. */
+	  for (bkpt_namep = solib_break_names; *bkpt_namep!=NULL; bkpt_namep++)
+	    {
+	      /* On ABI's that use function descriptors that are in the data
+	         section,
+	      sym_addr = bfd_lookup_symbol (tmp_bfd, *bkpt_namep, SEC_DATA);
+	      if (sym_addr != 0)
+		break;
+	    }
+	  if (sym_addr == 0)
+	    {
+	      target_close (tmp_bfd_target, 0);
+	      goto bkpt_at_symbol;
+	    }
+
+	  /* On some ABI's, the function descriptor we need will be in the
+	     ".plt" section.  In others, it will be in the ".opd" section. */
+	  if (sym_addr + load_addr >= interp_plt_sect_low
+              && sym_addr + load_addr < interp_plt_sect_high)
+	    {
+	      interp_sect = bfd_get_section_by_name (tmp_bfd, ".plt");
+	      sect_offset = interp_plt_sect_low - load_addr;
+	    }
+	  else
+	    {
+	      interp_sect = bfd_get_section_by_name (tmp_bfd, ".opd");
+	      if (interp_sect != 0)
+		{
+		  sect_offset = bfd_section_vma (tmp_bfd, interp_sect);
+		  if (sym_addr < sect_offset)
+		    interp_sect == 0;
+		  else if (sym_addr - sect_offset >=
+				       bfd_section_size (tmp_bfd, interp_sect))
+		    interp_sect == 0;
+		}
+	    }
+	  if (interp_sect != 0)
+	    {
+	      /* Try to convert the function descriptor we found above, into
+		 the address we need.  It will be relocated below by adding
+		 "load_addr" to it. */
+	      char *buf = alloca (sizeof (LONGEST));
+	      if (bfd_get_section_contents (tmp_bfd, interp_sect, buf,
+					    sym_addr - sect_offset,
+					    sizeof (LONGEST)))
+		sym_addr = extract_unsigned_integer (buf, sizeof (LONGEST));
+	      else
+		sym_addr = 0;
+	    }
+        }
+
       /* We're done with both the temporary bfd and target.  Remember,
          closing the target closes the underlying bfd.  */
       target_close (tmp_bfd_target, 0);

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