Fix PR gdb/20505 - Make vDSO detection work with core files

Loading a core dump that was either generated on a system running
pristine glibc master, or on a Fedora/RHEL system with LD_DEBUG=unused
set in the environment, solib-svr4.c:svr4_current_sos fails to filter
out the vDSO, resulting in:

  (gdb) core-file corefile.core^M
  [New LWP 2362]^M
  warning: Could not load shared library symbols for linux-vdso.so.1.^M
  Do you need "set solib-search-path" or "set sysroot"?^M
  Core was generated by `build-gdb/gdb/testsuite/outputs/gdb.base/corefile/'.^M
  ...

The problem is that gdbarch_vsyscall_range does not support core
inferiors at all.

When live debugging, we're finding the vDSO's start address with
auxv/AT_SYSINFO_EHDR, and then we find the vDSO's size by look for the
corresponding mapping, by parsing /proc/PID/maps.  When debugging a
core dump, we can also determine the starting address from
auxv/AT_SYSINFO_EHDR.  However, we obviously can't read the core
mappings out of the host's /proc.  But we can instead look for a
corresponding load segment in the core's bfd.

gdb/ChangeLog:
2016-08-22  Pedro Alves  <palves@redhat.com>

	PR gdb/20505
	* linux-tdep.c (linux_vsyscall_range_raw): For core inferiors,
	find the vDSO's start address with AT_SYSINFO_EHDR too, and
	determine the vDSO's size by finding the PT_LOAD segment that
	matches AT_SYSINFO_EHDR.

gdb/testsuite/ChangeLog:
2016-08-22  Pedro Alves  <palves@redhat.com>

	PR gdb/20505
	* gdb.base/vdso-warning.exp: Test core dumps too.  Use
	with_test_prefix.  Factor out bits to ...
	(test_no_vdso): ... this new procedure.

1 files changed, 30 insertions, 5 deletions

diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index ab110b0a687..718dc1ab662 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -2287,17 +2287,42 @@ linux_vsyscall_range_raw (struct gdbarch *gdbarch, struct mem_range *range)
   long pid;
   char *data;
 
-  /* Can't access /proc if debugging a core file.  */
-  if (!target_has_execution)
+  if (target_auxv_search (&current_target, AT_SYSINFO_EHDR, &range->start) <= 0)
     return 0;
 
+  /* It doesn't make sense to access the host's /proc when debugging a
+     core file.  Instead, look for the PT_LOAD segment that matches
+     the vDSO.  */
+  if (!target_has_execution)
+    {
+      Elf_Internal_Phdr *phdrs;
+      long phdrs_size;
+      int num_phdrs, i;
+
+      phdrs_size = bfd_get_elf_phdr_upper_bound (core_bfd);
+      if (phdrs_size == -1)
+	return 0;
+
+      phdrs = (Elf_Internal_Phdr *) alloca (phdrs_size);
+      num_phdrs = bfd_get_elf_phdrs (core_bfd, phdrs);
+      if (num_phdrs == -1)
+	return 0;
+
+      for (i = 0; i < num_phdrs; i++)
+	if (phdrs[i].p_type == PT_LOAD
+	    && phdrs[i].p_vaddr == range->start)
+	  {
+	    range->length = phdrs[i].p_memsz;
+	    return 1;
+	  }
+
+      return 0;
+    }
+
   /* We need to know the real target PID to access /proc.  */
   if (current_inferior ()->fake_pid_p)
     return 0;
 
-  if (target_auxv_search (&current_target, AT_SYSINFO_EHDR, &range->start) <= 0)
-    return 0;
-
   pid = current_inferior ()->pid;
 
   /* Note that reading /proc/PID/task/PID/maps (1) is much faster than