elf: Implement __libc_early_init

This function is defined in libc.so, and the dynamic loader calls
right after relocation has been finished, before any ELF constructors
or the preinit function is invoked.  It is also used in the static
build for initializing parts of the static libc.

To locate __libc_early_init, a direct symbol lookup function is used,
_dl_lookup_direct.  It does not search the entire symbol scope and
consults merely a single link map.  This function could also be used
to implement lookups in the vDSO (as an optimization).

A per-namespace variable (libc_map) is added for locating libc.so,
to avoid repeated traversals of the search scope.  It is similar to
GL(dl_initfirst).  An alternative would have been to thread a context
argument from _dl_open down to _dl_map_object_from_fd (where libc.so
is identified).  This could have avoided the global variable, but
the change would be larger as a result.  It would not have been
possible to use this to replace GL(dl_initfirst) because that global
variable is used to pass the function pointer past the stack switch
from dl_main to the main program.  Replacing that requires adding
a new argument to _dl_init, which in turn needs changes to the
architecture-specific libc.so startup code written in assembler.

__libc_early_init should not be used to replace _dl_var_init (as
it exists today on some architectures).  Instead, _dl_lookup_direct
should be used to look up a new variable symbol in libc.so, and
that should then be initialized from the dynamic loader, immediately
after the object has been loaded in _dl_map_object_from_fd (before
relocation is run).  This way, more IFUNC resolvers which depend on
these variables will work.

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

1 files changed, 5 insertions, 0 deletions

diff --git a/csu/libc-start.c b/csu/libc-start.c
index 12468c5a89..ccc743c9d1 100644
--- a/csu/libc-start.c
+++ b/csu/libc-start.c
@@ -22,6 +22,7 @@
 #include <ldsodefs.h>
 #include <exit-thread.h>
 #include <libc-internal.h>
+#include <elf/libc-early-init.h>
 
 #include <elf/dl-tunables.h>
 
@@ -238,6 +239,10 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
     __cxa_atexit ((void (*) (void *)) rtld_fini, NULL, NULL);
 
 #ifndef SHARED
+  /* Perform early initialization.  In the shared case, this function
+     is called from the dynamic loader as early as possible.  */
+  __libc_early_init ();
+
   /* Call the initializer of the libc.  This is only needed here if we
      are compiling for the static library in which case we haven't
      run the constructors in `_dl_start_user'.  */