Fix PR 21337: segfault when re-reading symbols.

Fix issue exposed by commit 3e29f34.

The basic issue is that section data referenced through an objfile
pointer can also be referenced via the program-space data pointer,
although via a separate mapping mechanism, which is set up by
update_section_map.  Thus once section data attached to an objfile
pointer is released, the section map associated with the program-space
data pointer must be marked dirty to ensure that update_section_map is
called to prevent stale data being referenced.  For the matter at hand
this marking is being done via a call to objfiles_changed.

Before commit 3e29f34 objfiles_changed could be called after all of
the objfile pointers were processed in reread_symbols since section
data references via the program-space data pointer would not occur in
the calls of read_symbols performed by reread_symbols.

With commit 3e29f34 MIPS target specific calls to find_pc_section were
added to the code for DWARF information processing, which is called
via read_symbols.  Thus in reread_symbols the call to objfiles_changed
needs to be called before calling read_symbols, otherwise stale
section data can be referenced.

Thanks to Luis Machado for providing text for the main comment
associated with the change.

gdb/
2017-06-28  Doug Gilmore  <Doug.Gilmore@imgtec.com>
    PR gdb/21337
    * symfile.c (reread_symbols): Call objfiles_changed just before
    read_symbols.

gdb/testsuite/
2017-06-28  Doug Gilmore  <Doug.Gilmore@imgtec.com>
    PR gdb/21337
    * gdb.base/reread-readsym.exp: New file.
    * gdb.base/reread-readsym.c: New file.

1 files changed, 20 insertions, 3 deletions

diff --git a/gdb/symfile.c b/gdb/symfile.c
index aa53415..9cbd6e5 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -2587,6 +2587,9 @@ reread_symbols (void)
 	  /* Free the obstacks for non-reusable objfiles.  */
 	  psymbol_bcache_free (objfile->psymbol_cache);
 	  objfile->psymbol_cache = psymbol_bcache_init ();
+
+	  /* NB: after this call to obstack_free, objfiles_changed
+	     will need to be called (see discussion below).  */
 	  obstack_free (&objfile->objfile_obstack, 0);
 	  objfile->sections = NULL;
 	  objfile->compunit_symtabs = NULL;
@@ -2639,6 +2642,23 @@ reread_symbols (void)
 	  clear_complaints (&symfile_complaints, 1, 1);
 
 	  objfile->flags &= ~OBJF_PSYMTABS_READ;
+
+	  /* We are about to read new symbols and potentially also
+	     DWARF information.  Some targets may want to pass addresses
+	     read from DWARF DIE's through an adjustment function before
+	     saving them, like MIPS, which may call into
+	     "find_pc_section".  When called, that function will make
+	     use of per-objfile program space data.
+
+	     Since we discarded our section information above, we have
+	     dangling pointers in the per-objfile program space data
+	     structure.  Force GDB to update the section mapping
+	     information by letting it know the objfile has changed,
+	     making the dangling pointers point to correct data
+	     again.  */
+
+	  objfiles_changed ();
+
 	  read_symbols (objfile, 0);
 
 	  if (!objfile_has_symbols (objfile))
@@ -2671,9 +2691,6 @@ reread_symbols (void)
 
   if (!new_objfiles.empty ())
     {
-      /* Notify objfiles that we've modified objfile sections.  */
-      objfiles_changed ();
-
       clear_symtab_users (0);
 
       /* clear_objfile_data for each objfile was called before freeing it and