[committed] Fix segfault in die_cu

[Tom de Vries <tdevries@suse.de>]

Hi,

When running dwz in normal mode, we get an error:
...
$ dwz clang-offload-bundler-10.debug -lnone
dwz: clang-offload-bundler-10.debug: Couldn't find DIE referenced by \
  DW_OP_GNU_implicit_pointer
...
but when forcing low-mem mode, we get a segfault:
...
$ dwz clang-offload-bundler-10.debug -l0
Segmentation fault (core dumped)
...

In normal mode, we hit the error here:
...
         ref = off_htab_lookup (NULL, addr);
         if (ref == NULL)
           {
             error (0, 0, "%s: Couldn't find DIE referenced by %s",
                    dso->filename, get_DW_OP_str (op));
...
but for low-mem mode, this doesn't trigger, because we find the dummy DIE that
has been added by read_exprloc_low_mem_phase1.

Fix this by testing for the dummy DIE in the error condition:
...
-	  if (ref == NULL)
+	  if (ref == NULL || (unlikely (low_mem) && ref->die_tag == 0))
...

Committed to trunk.

Thanks,
- Tom

Fix segfault in die_cu

2020-01-24  Tom de Vries  <tdevries@suse.de>

	PR dwz/25456
	* dwz.c (read_exprloc): Test for dummy DIE in error condition.

---
 dwz.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dwz.c b/dwz.c
index b12645d..a81e42a 100644
--- a/dwz.c
+++ b/dwz.c
@@ -1902,7 +1902,7 @@ read_exprloc (DSO *dso, dw_die_ref die, unsigned char *ptr, size_t len,
 	  else
 	    ptr += 4;
 	  ref = off_htab_lookup (NULL, addr);
-	  if (ref == NULL)
+	  if (ref == NULL || (unlikely (low_mem) && ref->die_tag == 0))
 	    {
 	      error (0, 0, "%s: Couldn't find DIE referenced by %s",
 		     dso->filename, get_DW_OP_str (op));