Avoid crash on missing dwz file

If DWARF contains a reference to a "dwz" file, but there is no
.gnu_debugaltlink section, then gdb will crash.  This happens because
dwarf2_get_dwz_file will return NULL, but some callers do not expect
this.

This patch changes dwarf2_get_dwz_file so that callers can require a
dwz file.  Then, it updates the callers that are attempting to process
references to the dwz file to require one.

This includes a new testcase.  The dwarf.exp changes don't handle the
new forms exactly correctly -- they are only handled well enough to
let this test case complete.

gdb/ChangeLog
2021-03-06  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (dwarf2_get_dwz_file): Add 'require' parameter.
	* dwarf2/read.c (dwarf2_get_dwz_file): Add 'require' parameter.
	(get_abbrev_section_for_cu, read_attribute_value)
	(get_debug_line_section): Update.
	* dwarf2/macro.c (dwarf_decode_macro_bytes): Update.

gdb/testsuite/ChangeLog
2021-03-06  Tom Tromey  <tom@tromey.com>

	* lib/dwarf.exp (_handle_DW_FORM): Treat DW_FORM_GNU_ref_alt and
	DW_FORM_GNU_strp_alt like DW_FORM_sec_offset.
	* gdb.dwarf2/dwznolink.exp: New file.

3 files changed, 21 insertions, 12 deletions

diff --git a/gdb/dwarf2/macro.c b/gdb/dwarf2/macro.c
index afe2f91..2ecebe6 100644
--- a/gdb/dwarf2/macro.c
+++ b/gdb/dwarf2/macro.c
@@ -509,7 +509,8 @@ dwarf_decode_macro_bytes (dwarf2_per_objfile *per_objfile,
 		    || macinfo_type == DW_MACRO_undef_sup
 		    || section_is_dwz)
 		  {
-		    dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd);
+		    dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd,
+							 true);
 
 		    body = dwz->read_string (objfile, str_offset);
 		  }
@@ -693,7 +694,8 @@ dwarf_decode_macro_bytes (dwarf2_per_objfile *per_objfile,
 
 	    if (macinfo_type == DW_MACRO_import_sup)
 	      {
-		dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd);
+		dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd,
+						     true);
 
 		dwz->macro.read (objfile);
 
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index e9cd929..65e2047 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -2285,7 +2285,7 @@ dwz_search_other_debugdirs (std::string &filename, bfd_byte *buildid,
 /* See dwarf2read.h.  */
 
 struct dwz_file *
-dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
+dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd, bool require)
 {
   bfd_size_type buildid_len_arg;
   size_t buildid_len;
@@ -2301,7 +2301,11 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
   if (data == NULL)
     {
       if (bfd_get_error () == bfd_error_no_error)
-	return NULL;
+	{
+	  if (!require)
+	    return nullptr;
+	  error (_("could not read '.gnu_debugaltlink' section"));
+	}
       error (_("could not read '.gnu_debugaltlink' section: %s"),
 	     bfd_errmsg (bfd_get_error ()));
     }
@@ -6308,7 +6312,7 @@ get_abbrev_section_for_cu (struct dwarf2_per_cu_data *this_cu)
   dwarf2_per_bfd *per_bfd = this_cu->per_bfd;
 
   if (this_cu->is_dwz)
-    abbrev = &dwarf2_get_dwz_file (per_bfd)->abbrev;
+    abbrev = &dwarf2_get_dwz_file (per_bfd, true)->abbrev;
   else
     abbrev = &per_bfd->abbrev;
 
@@ -20654,7 +20658,7 @@ read_attribute_value (const struct die_reader_specs *reader,
       /* FALLTHROUGH */
     case DW_FORM_GNU_strp_alt:
       {
-	dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd);
+	dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd, true);
 	LONGEST str_offset = cu_header->read_offset (abfd, info_ptr,
 						     &bytes_read);
 
@@ -21252,7 +21256,7 @@ get_debug_line_section (struct dwarf2_cu *cu)
     section = &cu->dwo_unit->dwo_file->sections.line;
   else if (cu->per_cu->is_dwz)
     {
-      dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd);
+      dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd, true);
 
       section = &dwz->line;
     }
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index c7f6a11..b974fe4 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -633,11 +633,14 @@ struct signatured_type
   struct dwo_unit *dwo_unit;
 };
 
-/* Open the separate '.dwz' debug file, if needed.  Return NULL if
-   there is no .gnu_debugaltlink section in the file.  Error if there
-   is such a section but the file cannot be found.  */
-
-extern dwz_file *dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd);
+/* Open the separate '.dwz' debug file, if needed.  If there is no
+   .gnu_debugaltlink section in the file, then the result depends on
+   REQUIRE: if REQUIRE is true, then error; if REQUIRE is false,
+   return NULL.  Always error if there is such a section but the file
+   cannot be found.  */
+
+extern dwz_file *dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd,
+				      bool require = false);
 
 /* Return the type of the DIE at DIE_OFFSET in the CU named by
    PER_CU.  */