PR symtab/14466: Work around PR libc/13097 "linux-vdso.so.1"

With upstream glibc, GDB prints:

  warning: Could not load shared library symbols for linux-vdso.so.1.
  Do you need "set solib-search-path" or "set sysroot"?

A bug's been filed for glibc a few years back:

  http://sourceware.org/bugzilla/show_bug.cgi?id=13097

but it's still not resolved.  It's not clear whether there's even
consensus that this is indeed a glibc bug.  It would actually be nice
if GDB also listed the vDSO in the shared library list, but there are
some design considerations with that:

 - the vDSO is mapped by the kernel, not userspace, therefore we
   should load its symbols right from the process's start of life,
   even before glibc / the userspace loader sets up the initial DSO
   list.  The program might even be using a custom loader or no
   loader.

 - that kind of hints at that solib.c should handle retrieving shared
   library lists from more than one source, and that symfile-mem.c's
   loading of the vDSO would be converted to load and relocate the
   vDSO's bfd behind the target_so_ops interface.

 - and then, once glibc links in the vDSO to its DSO list, we'd need
   to either:

    a) somehow hand over the vDSO from one target_so_ops to the other

    b) simply keep hiding glibc's entry.

And then b) seems the simplest.

With that in mind, this patch simply discards the vDSO from glibc's
reported shared library list.

We can match the vDSO address range with the addresses found iterating
the dynamic linker list, to tell which dynamic linker entry is the
vDSO.

Tested on x86_64 Fedora 20.

gdb/
2014-10-10  Jan Kratochvil  <jan.kratochvil@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	PR symtab/14466
	* solib-svr4.c (svr4_read_so_list): Rename to ...
	(svr4_current_sos_1): ... this and change the function comment.
	(svr4_current_sos): New function.

gdb/testsuite/
2014-10-10  Jan Kratochvil  <jan.kratochvil@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	PR symtab/14466
	* gdb.base/vdso-warning.c: New file.
	* gdb.base/vdso-warning.exp: New file.

1 files changed, 79 insertions, 2 deletions

diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index 5c87019..5313bf0 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -1461,10 +1461,11 @@ svr4_current_sos_direct (struct svr4_info *info)
   return head;
 }
 
-/* Implement the "current_sos" target_so_ops method.  */
+/* Implement the main part of the "current_sos" target_so_ops
+   method.  */
 
 static struct so_list *
-svr4_current_sos (void)
+svr4_current_sos_1 (void)
 {
   struct svr4_info *info = get_svr4_info ();
 
@@ -1477,6 +1478,82 @@ svr4_current_sos (void)
   return svr4_current_sos_direct (info);
 }
 
+/* Implement the "current_sos" target_so_ops method.  */
+
+static struct so_list *
+svr4_current_sos (void)
+{
+  struct so_list *so_head = svr4_current_sos_1 ();
+  struct mem_range vsyscall_range;
+
+  /* Filter out the vDSO module, if present.  Its symbol file would
+     not be found on disk.  The vDSO/vsyscall's OBJFILE is instead
+     managed by symfile-mem.c:add_vsyscall_page.  */
+  if (gdbarch_vsyscall_range (target_gdbarch (), &vsyscall_range)
+      && vsyscall_range.length != 0)
+    {
+      struct so_list **sop;
+
+      sop = &so_head;
+      while (*sop != NULL)
+	{
+	  struct so_list *so = *sop;
+
+	  /* We can't simply match the vDSO by starting address alone,
+	     because lm_info->l_addr_inferior (and also l_addr) do not
+	     necessarily represent the real starting address of the
+	     ELF if the vDSO's ELF itself is "prelinked".  The l_ld
+	     field (the ".dynamic" section of the shared object)
+	     always points at the absolute/resolved address though.
+	     So check whether that address is inside the vDSO's
+	     mapping instead.
+
+	     E.g., on Linux 3.16 (x86_64) the vDSO is a regular
+	     0-based ELF, and we see:
+
+	      (gdb) info auxv
+	      33  AT_SYSINFO_EHDR  System-supplied DSO's ELF header 0x7ffff7ffb000
+	      (gdb)  p/x *_r_debug.r_map.l_next
+	      $1 = {l_addr = 0x7ffff7ffb000, ..., l_ld = 0x7ffff7ffb318, ...}
+
+	     And on Linux 2.6.32 (x86_64) we see:
+
+	      (gdb) info auxv
+	      33  AT_SYSINFO_EHDR  System-supplied DSO's ELF header 0x7ffff7ffe000
+	      (gdb) p/x *_r_debug.r_map.l_next
+	      $5 = {l_addr = 0x7ffff88fe000, ..., l_ld = 0x7ffff7ffe580, ... }
+
+	     Dumping that vDSO shows:
+
+	      (gdb) info proc mappings
+	      0x7ffff7ffe000  0x7ffff7fff000  0x1000  0  [vdso]
+	      (gdb) dump memory vdso.bin 0x7ffff7ffe000 0x7ffff7fff000
+	      # readelf -Wa vdso.bin
+	      [...]
+		Entry point address: 0xffffffffff700700
+	      [...]
+	      Section Headers:
+		[Nr] Name     Type    Address	       Off    Size
+		[ 0]	      NULL    0000000000000000 000000 000000
+		[ 1] .hash    HASH    ffffffffff700120 000120 000038
+		[ 2] .dynsym  DYNSYM  ffffffffff700158 000158 0000d8
+	      [...]
+		[ 9] .dynamic DYNAMIC ffffffffff700580 000580 0000f0
+	  */
+	  if (address_in_mem_range (so->lm_info->l_ld, &vsyscall_range))
+	    {
+	      *sop = so->next;
+	      free_so (so);
+	      break;
+	    }
+
+	  sop = &so->next;
+	}
+    }
+
+  return so_head;
+}
+
 /* Get the address of the link_map for a given OBJFILE.  */
 
 CORE_ADDR