[PATCH 1/2] gdb: check for empty offsets vector in inherit_abstract_dies

[Simon Marchi <simon.marchi@polymtl.ca>]

When building GDB with clang and --enable-ubsan, I get:

  UNRESOLVED: gdb.dwarf2/frame-inlined-in-outer-frame.exp: starti prompt

The cause being:

    $ ./gdb --data-directory=data-directory -nx -q -readnow testsuite/outputs/gdb.dwarf2/frame-inlined-in-outer-frame/frame-inlined-in-outer-frame
    Reading symbols from testsuite/outputs/gdb.dwarf2/frame-inlined-in-outer-frame/frame-inlined-in-outer-frame...
    Expanding full symbols from testsuite/outputs/gdb.dwarf2/frame-inlined-in-outer-frame/frame-inlined-in-outer-frame...
    /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:11954:47: runtime error: applying non-zero offset 8 to null pointer

I found this to happen with ld-linux on at least Arch Linux and Ubuntu
22.04:

    $ ./gdb --data-directory=data-directory -nx -q -readnow -iex "set debuginfod enabled on" /lib64/ld-linux-x86-64.so.2
    Reading symbols from /lib64/ld-linux-x86-64.so.2...
    Reading symbols from /home/simark/.cache/debuginfod_client/22bd7a2c03d8cfc05ef7092bfae5932223189bc1/debuginfo...
    Expanding full symbols from /home/simark/.cache/debuginfod_client/22bd7a2c03d8cfc05ef7092bfae5932223189bc1/debuginfo...
    /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:11954:47: runtime error: applying non-zero offset 8 to null pointer

The problem happens when doing this:

    sect_offset *offsetp = offsets.data () + 1

When `offsets` is an empty vector, `offsets.data ()` returns nullptr.
Fix it by wrapping that in a `!offsets.empty ()` check.

Change-Id: I6d29ba2fe80ba4308f68effd9c57d4ee8d67c29f
---
 gdb/dwarf2/read.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 89ba9122e910..bf52354c4260 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -11949,17 +11949,22 @@ inherit_abstract_dies (struct die_info *die, struct dwarf2_cu *cu)
 	corresponding_abstract_child = corresponding_abstract_child->sibling;
     }
 
-  std::sort (offsets.begin (), offsets.end ());
-  sect_offset *offsets_end = offsets.data () + offsets.size ();
-  for (sect_offset *offsetp = offsets.data () + 1;
-       offsetp < offsets_end;
-       offsetp++)
-    if (offsetp[-1] == *offsetp)
-      complaint (_("Multiple children of DIE %s refer "
-		   "to DIE %s as their abstract origin"),
-		 sect_offset_str (die->sect_off), sect_offset_str (*offsetp));
+  if (!offsets.empty ())
+    {
+      std::sort (offsets.begin (), offsets.end ());
+      sect_offset *offsets_end = offsets.data () + offsets.size ();
+      for (sect_offset *offsetp = offsets.data () + 1;
+	   offsetp < offsets_end;
+	   offsetp++)
+	if (offsetp[-1] == *offsetp)
+	  complaint (_("Multiple children of DIE %s refer "
+		       "to DIE %s as their abstract origin"),
+		     sect_offset_str (die->sect_off),
+		     sect_offset_str (*offsetp));
+    }
 
   sect_offset *offsetp = offsets.data ();
+  sect_offset *offsets_end = offsets.data () + offsets.size ();
   die_info *origin_child_die = origin_die->child;
   while (origin_child_die != nullptr && origin_child_die->tag != 0)
     {

base-commit: 75436c534bfd7f548a13b5f926c3bd234b23b8d0
-- 
2.38.0