[RFA/solib-svr4] use AT_BASE auxiliary entry to compute load base address

[Joel Brobecker <brobecker@adacore.com>]

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Hello,

I came across an unexpected issue on sparc-solaris 2.9, where I tried
to attach to an Ada program. When I later did a "continue", the debugger
error'ed out saying:

    (gdb) c
    Continuing.
    Warning:
    Cannot insert breakpoint -1.
    Error accessing memory address 0xff36159c: I/O error.

Breakpoint -1 here corresponds to the shlib breakpoint:

    (gdb) maintenance info breakpoints 
    Num Type           Disp Enb Address    What
    -1  shlib events   keep y   0xff36159c 

I think that the problem is that we're computing the wrong base load
address in solib-svr4.c. As it turned out, there was a "TODO" hint
added by Daniel that suggested using the AT_BASE auxiliary entry.
The attached patch attempts to do that.

2007-09-12  Joel Brobecker  <brobecker@adacore.com>

        * solib-svr4.c: Add include of "auxv.h".
        (enable_break): Use the AT_BASE auxiliary entry if available.
        * Makefile.in (solib-svr4.o): Update dependencies.

Tested on x86-linux, no regression. Currently testing on sparc-solaris,
but it's taking a loooong time because sigstep is keeps timing out to
death. It fixes the issue above, and I'm confident the results will be OK.

OK to apply?

Thanks,
-- 
Joel

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Index: solib-svr4.c
===================================================================
RCS file: /cvs/src/src/gdb/solib-svr4.c,v
retrieving revision 1.75
diff -u -p -r1.75 solib-svr4.c
--- solib-svr4.c	23 Aug 2007 18:08:38 -0000	1.75
+++ solib-svr4.c	12 Sep 2007 21:03:39 -0000
@@ -41,6 +41,7 @@
 #include "bfd-target.h"
 #include "elf-bfd.h"
 #include "exec.h"
+#include "auxv.h"
 
 static struct link_map_offsets *svr4_fetch_link_map_offsets (void);
 static int svr4_have_link_map_offsets (void);
@@ -1008,11 +1009,6 @@ enable_break (void)
          be trivial on GNU/Linux).  Therefore, we have to try an alternate
          mechanism to find the dynamic linker's base address.  */
 
-      /* TODO drow/2006-09-12: This is somewhat fragile, because it
-	 relies on read_pc.  On both Solaris and GNU/Linux we can use
-	 the AT_BASE auxilliary entry, which GDB now knows how to
-	 access, to find the base address.  */
-
       tmp_fd = solib_open (buf, &tmp_pathname);
       if (tmp_fd >= 0)
 	tmp_bfd = bfd_fopen (tmp_pathname, gnutarget, FOPEN_RB, tmp_fd);
@@ -1048,9 +1044,19 @@ enable_break (void)
 	  so = so->next;
 	}
 
+      /* If we were not able to find the base address of the loader
+         from our so_list, then try using the AT_BASE auxilliary entry.  */
+      if (!load_addr_found)
+        if (target_auxv_search (&current_target, AT_BASE, &load_addr) > 0)
+          load_addr_found = 1;
+
       /* Otherwise 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.  */
+	 dynamic linker) and subtracting the offset of the entry point.
+
+         This is more fragile than the previous approaches, but is a good
+         fallback method because it has actually been working well in
+         most cases.  */
       if (!load_addr_found)
 	{
 	  load_addr = (read_pc ()
Index: Makefile.in
===================================================================
RCS file: /cvs/src/src/gdb/Makefile.in,v
retrieving revision 1.936
diff -u -p -r1.936 Makefile.in
--- Makefile.in	5 Sep 2007 00:14:02 -0000	1.936
+++ Makefile.in	12 Sep 2007 21:03:07 -0000
@@ -2622,7 +2622,7 @@ solib-svr4.o: solib-svr4.c $(defs_h) $(e
 	$(elf_mips_h) $(symtab_h) $(bfd_h) $(symfile_h) $(objfiles_h) \
 	$(gdbcore_h) $(target_h) $(inferior_h) $(gdb_assert_h) \
 	$(solist_h) $(solib_h) $(solib_svr4_h) $(bfd_target_h) $(elf_bfd_h) \
-	$(exec_h)
+	$(exec_h) $(auxv_h)
 solib-target.o: solib-target.c $(defs_h) $(objfiles_h) $(solist_h) \
 	$(symtab_h) $(symfile_h) $(target_h) $(vec_h) $(xml_support_h) \
 	$(gdb_string_h)