ld.so: Do not export free/calloc/malloc/realloc functions [BZ #25486]

Exporting functions and relying on symbol interposition from libc.so
makes the choice of implementation dependent on DT_NEEDED order, which
is not what some compiler drivers expect.

This commit replaces one magic mechanism (symbol interposition) with
another one (preprocessor-/compiler-based redirection).  This makes
the hand-over from the minimal malloc to the full malloc more
explicit.

Removing the ABI symbols is backwards-compatible because libc.so is
always in scope, and the dynamic loader will find the malloc-related
symbols there since commit f0b2132b35248c1f4a80f62a2c38cddcc802aa8c
("ld.so: Support moving versioned symbols between sonames
[BZ #24741]").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/elf/rtld.c b/elf/rtld.c
index 553cfbd..51dfaf9 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -534,6 +534,9 @@ _dl_start (void *arg)
      header table in core.  Put the rest of _dl_start into a separate
      function, that way the compiler cannot put accesses to the GOT
      before ELF_DYNAMIC_RELOCATE.  */
+
+  __rtld_malloc_init_stubs ();
+
   {
 #ifdef DONT_USE_BOOTSTRAP_MAP
     ElfW(Addr) entry = _dl_start_final (arg);
@@ -2210,6 +2213,10 @@ ERROR: '%s': cannot process note segment.\n", _dl_argv[0]);
 	  rtld_timer_stop (&relocate_time, start);
 	}
 
+      /* The library defining malloc has already been relocated due to
+	 prelinking.  Resolve the malloc symbols for the dynamic
+	 loader.  */
+      __rtld_malloc_init_real (main_map);
 
       /* Mark all the objects so we know they have been already relocated.  */
       for (struct link_map *l = main_map; l != NULL; l = l->l_next)
@@ -2310,6 +2317,11 @@ ERROR: '%s': cannot process note segment.\n", _dl_argv[0]);
 	 re-relocation, we might call a user-supplied function
 	 (e.g. calloc from _dl_relocate_object) that uses TLS data.  */
 
+      /* The malloc implementation has been relocated, so resolving
+	 its symbols (and potentially calling IFUNC resolvers) is safe
+	 at this point.  */
+      __rtld_malloc_init_real (main_map);
+
       RTLD_TIMING_VAR (start);
       rtld_timer_start (&start);