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, 25 insertions, 0 deletions

diff --git a/elf/dl-open.c b/elf/dl-open.c
index 7b3b177aa6..ac7ab495e4 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -34,6 +34,7 @@
 #include <atomic.h>
 #include <libc-internal.h>
 #include <array_length.h>
+#include <libc-early-init.h>
 
 #include <dl-dst.h>
 #include <dl-prop.h>
@@ -57,6 +58,13 @@ struct dl_open_args
      (non-negative).  */
   unsigned int original_global_scope_pending_adds;
 
+  /* Set to true by dl_open_worker if libc.so was already loaded into
+     the namespace at the time dl_open_worker was called.  This is
+     used to determine whether libc.so early initialization has
+     already been done before, and whether to roll back the cached
+     libc_map value in the namespace in case of a dlopen failure.  */
+  bool libc_already_loaded;
+
   /* Original parameters to the program and the current environment.  */
   int argc;
   char **argv;
@@ -500,6 +508,11 @@ dl_open_worker (void *a)
 	args->nsid = call_map->l_ns;
     }
 
+  /* The namespace ID is now known.  Keep track of whether libc.so was
+     already loaded, to determine whether it is necessary to call the
+     early initialization routine (or clear libc_map on error).  */
+  args->libc_already_loaded = GL(dl_ns)[args->nsid].libc_map != NULL;
+
   /* Retain the old value, so that it can be restored.  */
   args->original_global_scope_pending_adds
     = GL (dl_ns)[args->nsid]._ns_global_scope_pending_adds;
@@ -734,6 +747,11 @@ dl_open_worker (void *a)
   if (relocation_in_progress)
     LIBC_PROBE (reloc_complete, 3, args->nsid, r, new);
 
+  /* If libc.so was not there before, attempt to call its early
+     initialization routine.  */
+  if (!args->libc_already_loaded)
+    _dl_call_libc_early_init (GL(dl_ns)[args->nsid].libc_map);
+
 #ifndef SHARED
   DL_STATIC_INIT (new);
 #endif
@@ -828,6 +846,8 @@ no more namespaces available for dlmopen()"));
   args.caller_dlopen = caller_dlopen;
   args.map = NULL;
   args.nsid = nsid;
+  /* args.libc_already_loaded is always assigned by dl_open_worker
+     (before any explicit/non-local returns).  */
   args.argc = argc;
   args.argv = argv;
   args.env = env;
@@ -856,6 +876,11 @@ no more namespaces available for dlmopen()"));
   /* See if an error occurred during loading.  */
   if (__glibc_unlikely (exception.errstring != NULL))
     {
+      /* Avoid keeping around a dangling reference to the libc.so link
+	 map in case it has been cached in libc_map.  */
+      if (!args.libc_already_loaded)
+	GL(dl_ns)[nsid].libc_map = NULL;
+
       /* Remove the object from memory.  It may be in an inconsistent
 	 state if relocation failed, for example.  */
       if (args.map)