Implement __libc_early_init
diff mbox series

Message ID 87blunjrr8.fsf@oldenburg2.str.redhat.com
State New
Headers show
Series
  • Implement __libc_early_init
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Commit Message

Florian Weimer Oct. 11, 2019, 2:12 p.m. UTC
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.

Built with build-many-glibcs.py.  Tested on aarch64-linux-gnu,
i686-linux-gnu, powerpc64-linux-gnu, s390x-linux-gnu,
x86_64-linux-gnu.

2019-10-11  Florian Weimer  <fweimer@redhat.com>

	* csu/init-first.c (_init): Remove call to __ctype_init.  Moved to
	__libc_early_init.
	* csu/libc-start.c [!SHARED] (LIBC_START_MAIN): Call
	__libc_early_init.
	* elf/Makefile (routines): Add libc_early_init.
	(dl-routines): Add dl-call-libc-early-init.
	* elf/Versions (libc): Export __libc_early_init under
	GLIBC_PRIVATE.
	* elf/dl-call-libc-early-init.c: New file.
	* elf/dl-load.c (_dl_map_object_from_fd): Record in the namespace
	description if libc.so has been loaded.
	* elf/dl-lookup-direct.c: New file.  Extracted from
	elf/dl-lookup.c.
	* elf/dl-open.c (struct dl_open_args): Add libc_already_loaded.
	(dl_open_worker): Set libc_already_loaded.  Call
	_dl_call_libc_early_init if libc.so has been loaded.
	(_dl_open): Initialize libc_already_loaded.  Reset libc_map on
	failure if necessary.
	* elf/libc-early-init.h: New file.
	* elf/libc_early_init.c: Likewise.
	* elf/rtld.c (dl_main): Call _dl_call_libc_early_init.
	* sysdeps/generic/ldsodefs.h (struct rtld_global): Add libc_map
	member to the namespace description.
	(_dl_lookup_direct): Declare.
	* sysdeps/mach/hurd/i386/init-first.c (posixland_init): Do not
	call __ctype_init.

Patch
diff mbox series

diff --git a/csu/init-first.c b/csu/init-first.c
index e0f489ee6d..8caec305b7 100644
--- a/csu/init-first.c
+++ b/csu/init-first.c
@@ -16,7 +16,6 @@ 
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <fcntl.h>
@@ -80,9 +79,6 @@  _init (int argc, char **argv, char **envp)
 
   __init_misc (argc, argv, envp);
 
-  /* Initialize ctype data.  */
-  __ctype_init ();
-
 #if defined SHARED && !defined NO_CTORS_DTORS_SECTIONS
   __libc_global_ctors ();
 #endif
diff --git a/csu/libc-start.c b/csu/libc-start.c
index 14e2b693f0..6b71a89f1d 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'.  */
diff --git a/elf/Makefile b/elf/Makefile
index dea51ca182..3e08185654 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -25,7 +25,7 @@  headers		= elf.h bits/elfclass.h link.h bits/link.h
 routines	= $(all-dl-routines) dl-support dl-iteratephdr \
 		  dl-addr dl-addr-obj enbl-secure dl-profstub \
 		  dl-origin dl-libc dl-sym dl-sysdep dl-error \
-		  dl-reloc-static-pie
+		  dl-reloc-static-pie libc_early_init
 
 # The core dynamic linking functions are in libc for the static and
 # profiled libraries.
@@ -33,7 +33,8 @@  dl-routines	= $(addprefix dl-,load lookup object reloc deps hwcaps \
 				  runtime init fini debug misc \
 				  version profile tls origin scope \
 				  execstack open close trampoline \
-				  exception sort-maps)
+				  exception sort-maps lookup-direct \
+				  call-libc-early-init)
 ifeq (yes,$(use-ldconfig))
 dl-routines += dl-cache
 endif
diff --git a/elf/Versions b/elf/Versions
index 3b09901f6c..f26d2817c3 100644
--- a/elf/Versions
+++ b/elf/Versions
@@ -26,6 +26,7 @@  libc {
     _dl_open_hook; _dl_open_hook2;
     _dl_sym; _dl_vsym;
     __libc_dlclose; __libc_dlopen_mode; __libc_dlsym; __libc_dlvsym;
+    __libc_early_init;
 
     # Internal error handling support.  Interposes the functions in ld.so.
     _dl_signal_exception; _dl_catch_exception;
diff --git a/elf/dl-call-libc-early-init.c b/elf/dl-call-libc-early-init.c
new file mode 100644
index 0000000000..6c3ac5bfe7
--- /dev/null
+++ b/elf/dl-call-libc-early-init.c
@@ -0,0 +1,41 @@ 
+/* Invoke the early initialization function in libc.so.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <assert.h>
+#include <ldsodefs.h>
+#include <libc-early-init.h>
+#include <link.h>
+#include <stddef.h>
+
+void
+_dl_call_libc_early_init (struct link_map *libc_map)
+{
+  /* There is nothing to do if we did not actually load libc.so.  */
+  if (libc_map == NULL)
+    return;
+
+  const ElfW(Sym) *sym
+    = _dl_lookup_direct (libc_map, "__libc_early_init",
+                         0x69682ac, /* dl_new_hash output.  */
+                         "GLIBC_PRIVATE",
+                         0x0963cf85); /* _dl_elf_hash output.  */
+  assert (sym != NULL);
+  __typeof (__libc_early_init) *early_init
+    = DL_SYMBOL_ADDRESS (libc_map, sym);
+  early_init ();
+}
diff --git a/elf/dl-load.c b/elf/dl-load.c
index 24e2819345..704be15d94 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -30,6 +30,7 @@ 
 #include <sys/param.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <gnu/lib-names.h>
 
 /* Type for the buffer we put the ELF header and hopefully the program
    header.  This buffer does not really have to be too large.  In most
@@ -1373,6 +1374,14 @@  cannot enable executable stack as shared object requires");
     add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
 			    + l->l_info[DT_SONAME]->d_un.d_val));
 
+  /* If we have newly loaded libc.so, update the namespace
+     description.  */
+  if (GL(dl_ns)[nsid].libc_map == NULL
+      && l->l_info[DT_SONAME] != NULL
+      && strcmp (((const char *) D_PTR (l, l_info[DT_STRTAB])
+		  + l->l_info[DT_SONAME]->d_un.d_val), LIBC_SO) == 0)
+    GL(dl_ns)[nsid].libc_map = l;
+
   /* _dl_close can only eventually undo the module ID assignment (via
      remove_slotinfo) if this function returns a pointer to a link
      map.  Therefore, delay this step until all possibilities for
diff --git a/elf/dl-lookup-direct.c b/elf/dl-lookup-direct.c
new file mode 100644
index 0000000000..190b826e1e
--- /dev/null
+++ b/elf/dl-lookup-direct.c
@@ -0,0 +1,116 @@ 
+/* Look up a symbol in a single specified object.
+   Copyright (C) 1995-2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ldsodefs.h>
+#include <string.h>
+#include <elf_machine_sym_no_match.h>
+#include <dl-hash.h>
+
+/* This function corresponds to do_lookup_x in elf/dl-lookup.c.  The
+   variant here is simplified because it requires symbol
+   versioning. */
+static const ElfW(Sym) *
+check_match (const struct link_map *const map, const char *const undef_name,
+             const char *version, uint32_t version_hash,
+             const Elf_Symndx symidx)
+{
+  const ElfW(Sym) *symtab = (const void *) D_PTR (map, l_info[DT_SYMTAB]);
+  const ElfW(Sym) *sym = &symtab[symidx];
+
+  unsigned int stt = ELFW(ST_TYPE) (sym->st_info);
+  if (__glibc_unlikely ((sym->st_value == 0 /* No value.  */
+                         && sym->st_shndx != SHN_ABS
+                         && stt != STT_TLS)
+                        || elf_machine_sym_no_match (sym)))
+    return NULL;
+
+  /* Ignore all but STT_NOTYPE, STT_OBJECT, STT_FUNC,
+     STT_COMMON, STT_TLS, and STT_GNU_IFUNC since these are no
+     code/data definitions.  */
+#define ALLOWED_STT \
+  ((1 << STT_NOTYPE) | (1 << STT_OBJECT) | (1 << STT_FUNC) \
+   | (1 << STT_COMMON) | (1 << STT_TLS) | (1 << STT_GNU_IFUNC))
+  if (__glibc_unlikely (((1 << stt) & ALLOWED_STT) == 0))
+    return NULL;
+
+  const char *strtab = (const void *) D_PTR (map, l_info[DT_STRTAB]);
+
+  if (strcmp (strtab + sym->st_name, undef_name) != 0)
+    /* Not the symbol we are looking for.  */
+    return NULL;
+
+  ElfW(Half) ndx = map->l_versyms[symidx] & 0x7fff;
+  if (map->l_versions[ndx].hash != version_hash
+      || strcmp (map->l_versions[ndx].name, version) != 0)
+    /* It's not the version we want.  */
+    return NULL;
+
+  return sym;
+}
+
+
+/* This function corresponds to do_lookup_x in elf/dl-lookup.c.  The
+   variant here is simplified because it does not search object
+   dependencies.  It is optimized for a successful lookup.  */
+const ElfW(Sym) *
+_dl_lookup_direct (struct link_map *map,
+                   const char *undef_name, uint32_t new_hash,
+                   const char *version, uint32_t version_hash)
+{
+  const ElfW(Addr) *bitmask = map->l_gnu_bitmask;
+  if (__glibc_likely (bitmask != NULL))
+    {
+      Elf32_Word bucket = map->l_gnu_buckets[new_hash % map->l_nbuckets];
+      if (bucket != 0)
+        {
+          const Elf32_Word *hasharr = &map->l_gnu_chain_zero[bucket];
+
+          do
+            if (((*hasharr ^ new_hash) >> 1) == 0)
+              {
+                Elf_Symndx symidx = ELF_MACHINE_HASH_SYMIDX (map, hasharr);
+                const ElfW(Sym) *sym = check_match (map, undef_name,
+                                                    version, version_hash,
+                                                    symidx);
+                if (sym != NULL)
+                  return sym;
+              }
+          while ((*hasharr++ & 1u) == 0);
+        }
+    }
+  else
+    {
+      /* Fallback code for lack of GNU_HASH support.  */
+      uint32_t old_hash = _dl_elf_hash (undef_name);
+
+      /* Use the old SysV-style hash table.  Search the appropriate
+         hash bucket in this object's symbol table for a definition
+         for the same symbol name.  */
+      for (Elf_Symndx symidx = map->l_buckets[old_hash % map->l_nbuckets];
+           symidx != STN_UNDEF;
+           symidx = map->l_chain[symidx])
+        {
+          const ElfW(Sym) *sym = check_match (map, undef_name,
+                                              version, version_hash, symidx);
+          if (sym != NULL)
+            return sym;
+        }
+    }
+
+  return NULL;
+}
diff --git a/elf/dl-open.c b/elf/dl-open.c
index a9fd4cbced..98353cdcc3 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -33,6 +33,7 @@ 
 #include <stap-probe.h>
 #include <atomic.h>
 #include <libc-internal.h>
+#include <libc-early-init.h>
 
 #include <dl-dst.h>
 #include <dl-prop.h>
@@ -50,6 +51,14 @@  struct dl_open_args
   struct link_map *map;
   /* Namespace ID.  */
   Lmid_t nsid;
+
+  /* Set to to true 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 needs to 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;
@@ -208,6 +217,11 @@  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;
+
   /* One might be tempted to assert that we are RT_CONSISTENT at this point, but that
      may not be true if this is a recursive call to dlopen.  */
   _dl_debug_initialize (0, args->nsid);
@@ -502,6 +516,11 @@  TLS generation counter wrapped!  Please report this."));
   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
@@ -584,6 +603,7 @@  no more namespaces available for dlmopen()"));
   args.caller_dlopen = caller_dlopen;
   args.map = NULL;
   args.nsid = nsid;
+  args.libc_already_loaded = true; /* No reset below with early failure.  */
   args.argc = argc;
   args.argv = argv;
   args.env = env;
@@ -599,6 +619,11 @@  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)
diff --git a/elf/libc-early-init.h b/elf/libc-early-init.h
new file mode 100644
index 0000000000..02b855754e
--- /dev/null
+++ b/elf/libc-early-init.h
@@ -0,0 +1,35 @@ 
+/* Early initialization of libc.so.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _LIBC_EARLY_INIT_H
+#define _LIBC_EARLY_INIT_H
+
+struct link_map;
+
+/* If LIBC_MAP is not NULL, look up the __libc_early_init symbol in it
+   and call this function.  */
+void _dl_call_libc_early_init (struct link_map *libc_map) attribute_hidden;
+
+/* In the shared case, this function is defined in libc.so and invoked
+   from ld.so (or on the fist static dlopen) after complete relocation
+   of a new loaded libc.so, but before user-defined ELF constructors
+   run.  In the static case, this function is called directly from the
+   startup code.  */
+void __libc_early_init (void);
+
+#endif /* _LIBC_EARLY_INIT_H */
diff --git a/elf/libc_early_init.c b/elf/libc_early_init.c
new file mode 100644
index 0000000000..1ac66d895d
--- /dev/null
+++ b/elf/libc_early_init.c
@@ -0,0 +1,27 @@ 
+/* Early initialization of libc.so, libc.so side.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ctype.h>
+#include <libc-early-init.h>
+
+void
+__libc_early_init (void)
+{
+  /* Initialize ctype data.  */
+  __ctype_init ();
+}
diff --git a/elf/rtld.c b/elf/rtld.c
index 8a6e1a11b2..a29e71cfd1 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -43,6 +43,7 @@ 
 #include <stap-probe.h>
 #include <stackinfo.h>
 #include <not-cancel.h>
+#include <libc-early-init.h>
 
 #include <assert.h>
 
@@ -2314,6 +2315,9 @@  ERROR: '%s': cannot process note segment.\n", _dl_argv[0]);
       rtld_timer_accum (&relocate_time, start);
     }
 
+  /* Relocation is complete.  Perform early libc initialization.  */
+  _dl_call_libc_early_init (GL(dl_ns)[LM_ID_BASE].libc_map);
+
   /* Do any necessary cleanups for the startup OS interface code.
      We do these now so that no calls are made after rtld re-relocation
      which might be resolved to different functions than we expect.
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index f3ba13ee68..2c05bff1e9 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -329,6 +329,11 @@  struct rtld_global
        allocated by rtld.  Later it keeps the size of the map.  It might be
        reset if in _dl_close if the last global object is removed.  */
     size_t _ns_global_scope_alloc;
+
+    /* Once libc.so has been loaded into the namespace, this points to
+       its link map.  */
+    struct link_map *libc_map;
+
     /* Search table for unique objects.  */
     struct unique_sym_table
     {
@@ -927,6 +932,19 @@  extern lookup_t _dl_lookup_symbol_x (const char *undef,
      attribute_hidden;
 
 
+/* Restricted version of _dl_lookup_symbol_x.  Searches MAP (and only
+   MAP) for the symbol UNDEF_NAME, with GNU hash NEW_HASH (computed
+   with dl_new_hash), symbol version VERSION, and symbol version hash
+   VERSION_HASH (computed with _dl_elf_hash).  Returns a pointer to
+   the symbol table entry in MAP on success, or NULL on failure.  MAP
+   must have symbol versioning information, or otherwise the result is
+   undefined.  */
+const ElfW(Sym) *_dl_lookup_direct (struct link_map *map,
+				    const char *undef_name,
+				    uint32_t new_hash,
+				    const char *version,
+				    uint32_t version_hash) attribute_hidden;
+
 /* Add the new link_map NEW to the end of the namespace list.  */
 extern void _dl_add_to_namespace_list (struct link_map *new, Lmid_t nsid)
      attribute_hidden;
diff --git a/sysdeps/mach/hurd/i386/init-first.c b/sysdeps/mach/hurd/i386/init-first.c
index f1f1c40389..2fb913e922 100644
--- a/sysdeps/mach/hurd/i386/init-first.c
+++ b/sysdeps/mach/hurd/i386/init-first.c
@@ -17,7 +17,6 @@ 
    <https://www.gnu.org/licenses/>.  */
 
 #include <assert.h>
-#include <ctype.h>
 #include <hurd.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -84,9 +83,6 @@  posixland_init (int argc, char **argv, char **envp)
 #endif
   __init_misc (argc, argv, envp);
 
-  /* Initialize ctype data.  */
-  __ctype_init ();
-
 #if defined SHARED && !defined NO_CTORS_DTORS_SECTIONS
   __libc_global_ctors ();
 #endif