elf: Add _dl_find_object function

It can be used to speed up the libgcc unwinder, and the internal
_dl_find_dso_for_object function (which is used for caller
identification in dlopen and related functions, and in dladdr).

_dl_find_object is in the internal namespace due to bug 28503.
If libgcc switches to _dl_find_object, this namespace issue will
be fixed.  It is located in libc for two reasons: it is necessary
to forward the call to the static libc after static dlopen, and
there is a link ordering issue with -static-libgcc and libgcc_eh.a
because libc.so is not a linker script that includes ld.so in the
glibc build tree (so that GCC's internal -lc after libgcc_eh.a does
not pick up ld.so).

It is necessary to do the i386 customization in the
sysdeps/x86/bits/dl_find_object.h header shared with x86-64 because
otherwise, multilib installations are broken.

The implementation uses software transactional memory, as suggested
by Torvald Riegel.  Two copies of the supporting data structures are
used, also achieving full async-signal-safety.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/elf/rtld.c b/elf/rtld.c
index e78001c2a4..edfa95b70c 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -51,6 +51,7 @@
 #include <dl-tunables.h>
 #include <get-dynamic-info.h>
 #include <dl-execve.h>
+#include <dl-find_object.h>
 
 #include <assert.h>
 
@@ -582,6 +583,10 @@ _dl_start (void *arg)
 
   __rtld_malloc_init_stubs ();
 
+  /* Do not use an initializer for these members because it would
+     intefere with __rtld_static_init.  */
+  GLRO (dl_find_object) = &_dl_find_object;
+
   {
 #ifdef DONT_USE_BOOTSTRAP_MAP
     ElfW(Addr) entry = _dl_start_final (arg);
@@ -2335,6 +2340,9 @@ dl_main (const ElfW(Phdr) *phdr,
 	  rtld_timer_stop (&relocate_time, start);
 	}
 
+      /* Set up the object lookup structures.  */
+      _dl_find_object_init ();
+
       /* The library defining malloc has already been relocated due to
 	 prelinking.  Resolve the malloc symbols for the dynamic
 	 loader.  */
@@ -2443,6 +2451,9 @@ dl_main (const ElfW(Phdr) *phdr,
 	 re-relocation, we might call a user-supplied function
 	 (e.g. calloc from _dl_relocate_object) that uses TLS data.  */
 
+      /* Set up the object lookup structures.  */
+      _dl_find_object_init ();
+
       /* The malloc implementation has been relocated, so resolving
 	 its symbols (and potentially calling IFUNC resolvers) is safe
 	 at this point.  */