[PATCH v4 08/10] csu: Move static pie self relocation later [BZ #27072]

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

IFUNC resolvers may depend on tunables and cpu feature setup so
move static pie self relocation after those.

It is hard to guarantee that the ealy startup code does not rely
on relocations so this is a bit fragile. It would be more robust
to handle RELATIVE relocs early and only IRELATIVE relocs later,
but the current relocation processing code cannot do that.

The early startup code before relocation processing includes

  _dl_aux_init (auxvec);
  __libc_init_secure ();
  __tunables_init (__environ);
  ARCH_INIT_CPU_FEATURES ();

These are simple enough that RELATIVE relocs can be avoided.

__ehdr_start may require RELATIVE relocation so it was moved
later, fortunately ehdr and phdr are not used in the early code.

Fixes bug 27072.
---
 csu/libc-start.c | 44 +++++++++++++++++++++++++-------------------
 1 file changed, 25 insertions(+), 19 deletions(-)

diff --git a/csu/libc-start.c b/csu/libc-start.c
index 1e90dcb0a7..c2b59431a3 100644
--- a/csu/libc-start.c
+++ b/csu/libc-start.c
@@ -146,8 +146,6 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
   int result;
 
 #ifndef SHARED
-  _dl_relocate_static_pie ();
-
   char **ev = &argv[argc + 1];
 
   __environ = ev;
@@ -169,24 +167,7 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
   }
 #  endif
   _dl_aux_init (auxvec);
-  if (GL(dl_phdr) == NULL)
 # endif
-    {
-      /* Starting from binutils-2.23, the linker will define the
-         magic symbol __ehdr_start to point to our own ELF header
-         if it is visible in a segment that also includes the phdrs.
-         So we can set up _dl_phdr and _dl_phnum even without any
-         information from auxv.  */
-
-      extern const ElfW(Ehdr) __ehdr_start
-	__attribute__ ((weak, visibility ("hidden")));
-      if (&__ehdr_start != NULL)
-        {
-          assert (__ehdr_start.e_phentsize == sizeof *GL(dl_phdr));
-          GL(dl_phdr) = (const void *) &__ehdr_start + __ehdr_start.e_phoff;
-          GL(dl_phnum) = __ehdr_start.e_phnum;
-        }
-    }
 
   /* Initialize very early so that tunables can use it.  */
   __libc_init_secure ();
@@ -195,6 +176,11 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
 
   ARCH_INIT_CPU_FEATURES ();
 
+  /* Do static pie self relocation after tunables and cpu features
+     are setup for ifunc resolvers. Before this point relocations
+     must be avoided.  */
+  _dl_relocate_static_pie ();
+
   /* Perform IREL{,A} relocations.  */
   ARCH_SETUP_IREL ();
 
@@ -206,6 +192,26 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
      hwcap and platform fields available in the TCB.  */
   ARCH_APPLY_IREL ();
 
+# ifdef HAVE_AUX_VECTOR
+  if (GL(dl_phdr) == NULL)
+# endif
+    {
+      /* Starting from binutils-2.23, the linker will define the
+         magic symbol __ehdr_start to point to our own ELF header
+         if it is visible in a segment that also includes the phdrs.
+         So we can set up _dl_phdr and _dl_phnum even without any
+         information from auxv.  */
+
+      extern const ElfW(Ehdr) __ehdr_start
+	__attribute__ ((weak, visibility ("hidden")));
+      if (&__ehdr_start != NULL)
+        {
+          assert (__ehdr_start.e_phentsize == sizeof *GL(dl_phdr));
+          GL(dl_phdr) = (const void *) &__ehdr_start + __ehdr_start.e_phoff;
+          GL(dl_phnum) = __ehdr_start.e_phnum;
+        }
+    }
+
   /* Set up the stack checker's canary.  */
   uintptr_t stack_chk_guard = _dl_setup_stack_chk_guard (_dl_random);
 # ifdef THREAD_SET_STACK_GUARD
-- 
2.17.1