Harden powerpc64 elf_machine_fixup_plt

[Alan Modra <amodra at gmail dot com>]

IFUNC is difficult to correctly implement on any target needing a GOT
to support position independent code, due to the dependency on order
of dynamic relocations.  ld.so should be changed to apply IFUNC
relocations last, globally, because without that it is actually
impossible to write an IFUNC resolver in C that works in all
situations.  Case in point, vfork in libpthread.so is an IFUNC with
the resolver returning &__libc_vfork.  (system and fork are similar.)
If another shared library, libA say, uses vfork then it is quite
possible that libpthread.so hasn't been dynamically relocated before
the unfortunate libA is dynamically relocated.  In that case the GOT
entry for &__libc_vfork is still zero, so the IFUNC resolver returns
NULL.  LD_BIND_NOW=1 results in libA PLT dynamic relocations being
applied using this NULL value and ld.so segfaults.

This patch hardens ld.so to not segfault on a NULL from an IFUNC
resolver.  It also fixes a problem with undefined weak.  If you leave
the plt entry as-is for undefined weak then if the entry is ever
called it will loop in ld.so rather than segfaulting.

Regression tested powerpc64-linux.

	* sysdeps/powerpc/powerpc64/dl-machine.h (elf_machine_fixup_plt):
	Don't segfault if ifunc resolver returns a NULL.  Do set plt to
	zero for undefined weak.
	(elf_machine_plt_conflict): Similarly.

diff --git a/sysdeps/powerpc/powerpc64/dl-machine.h b/sysdeps/powerpc/powerpc64/dl-machine.h
index 55ac736..0576781 100644
--- a/sysdeps/powerpc/powerpc64/dl-machine.h
+++ b/sysdeps/powerpc/powerpc64/dl-machine.h
@@ -472,19 +472,32 @@ elf_machine_fixup_plt (struct link_map *map, lookup_t sym_map,
   Elf64_FuncDesc *plt = (Elf64_FuncDesc *) reloc_addr;
   Elf64_FuncDesc *rel = (Elf64_FuncDesc *) finaladdr;
   Elf64_Addr offset = 0;
+  Elf64_FuncDesc zero_fd = {0, 0, 0};
 
   PPC_DCBT (&plt->fd_aux);
   PPC_DCBT (&plt->fd_func);
-  PPC_DCBT (&rel->fd_aux);
-  PPC_DCBT (&rel->fd_func);
 
-  /* If sym_map is NULL, it's a weak undefined sym;  Leave the plt zero.  */
+  /* If sym_map is NULL, it's a weak undefined sym;  Set the plt to
+     zero.  finaladdr should be zero already in this case, but guard
+     against invalid plt relocations with non-zero addends.  */
   if (sym_map == NULL)
-    return 0;
+    finaladdr = 0;
+
+  /* Don't die here if finaladdr is zero, die if this plt entry is
+     actually called.  Makes a difference when LD_BIND_NOW=1.
+     finaladdr may be zero for a weak undefined symbol, or when an
+     ifunc resolver returns zero.  */
+  if (finaladdr == 0)
+    rel = &zero_fd;
+  else
+    {
+      PPC_DCBT (&rel->fd_aux);
+      PPC_DCBT (&rel->fd_func);
+    }
 
   /* If the opd entry is not yet relocated (because it's from a shared
      object that hasn't been processed yet), then manually reloc it.  */
-  if (map != sym_map && !sym_map->l_relocated
+  if (finaladdr != 0 && map != sym_map && !sym_map->l_relocated
 #if !defined RTLD_BOOTSTRAP && defined SHARED
       /* Bootstrap map doesn't have l_relocated set for it.  */
       && sym_map != &GL(dl_rtld_map)
@@ -522,6 +535,13 @@ elf_machine_plt_conflict (struct link_map *map, lookup_t sym_map,
 #if _CALL_ELF != 2
   Elf64_FuncDesc *plt = (Elf64_FuncDesc *) reloc_addr;
   Elf64_FuncDesc *rel = (Elf64_FuncDesc *) finaladdr;
+  Elf64_FuncDesc zero_fd = {0, 0, 0};
+
+  if (sym_map == NULL)
+    finaladdr = 0;
+
+  if (finaladdr == 0)
+    rel = &zero_fd;
 
   plt->fd_func = rel->fd_func;
   plt->fd_aux = rel->fd_aux;

-- 
Alan Modra
Australia Development Lab, IBM