nss: Introduce <nss_module.h>

This provides the struct nss_module type, which combines the old
struct service_library type with the known_function tree, by
statically allocating space for all function pointers.

struct nss_module is fairly large (536 bytes), but it will be
shared across NSS databases.  The old known_function handling
had non-some per-function overhead (at least 32 bytes per looked-up
function, but more for long function anmes), so overall, this is not
too bad.  Resolving all functions at load time simplifies locking,
and the repeated lookups should be fast because the caches are hot
at this point.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

1 files changed, 304 insertions, 0 deletions

diff --git a/nss/nss_module.c b/nss/nss_module.c
new file mode 100644
index 0000000000..8de8db09c3
--- /dev/null
+++ b/nss/nss_module.c
@@ -0,0 +1,304 @@
+/* Global list of NSS service modules.
+   Copyright (c) 2020 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 <nss_module.h>
+
+#include <array_length.h>
+#include <assert.h>
+#include <atomic.h>
+#include <dlfcn.h>
+#include <gnu/lib-names.h>
+#include <libc-lock.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#ifdef LINK_OBSOLETE_NSL
+# define DEFAULT_CONFIG    "compat [NOTFOUND=return] files"
+# define DEFAULT_DEFCONFIG "nis [NOTFOUND=return] files"
+#else
+# define DEFAULT_CONFIG    "files"
+# define DEFAULT_DEFCONFIG "files"
+#endif
+
+/* Suffix after .so of NSS service modules.  This is a bit of magic,
+   but we assume LIBNSS_FILES_SO looks like "libnss_files.so.2" and we
+   want a pointer to the ".2" part.  We have no API to extract this
+   except through the auto-generated lib-names.h and some static
+   pointer manipulation.  The "-1" accounts for the trailing NUL
+   included in the sizeof.  */
+static const char *const __nss_shlib_revision
+	= LIBNSS_FILES_SO + sizeof("libnss_files.so") - 1;
+
+/* A single-linked list used to implement a mapping from service names
+   to NSS modules.  (Most systems only use five or so modules, so a
+   list is sufficient here.)  Elements of this list are never freed
+   during normal operation.  */
+static struct nss_module *nss_module_list;
+
+/* Covers the list and also loading of individual NSS service
+   modules.  */
+__libc_lock_define (static, nss_module_list_lock);
+
+#if defined USE_NSCD && (!defined DO_STATIC_NSS || defined SHARED)
+/* Nonzero if this is the nscd process.  */
+static bool is_nscd;
+/* The callback passed to the init functions when nscd is used.  */
+static void (*nscd_init_cb) (size_t, struct traced_file *);
+#endif
+
+/* Allocate the service NAME with length NAME_LENGTH.  If the service
+   is already allocated in the nss_module_list cache then we return a
+   pointer to the struct nss_module, otherwise we try to allocate a
+   new struct nss_module entry and add it to the global
+   nss_modules_list cache.  If we fail to allocate the entry we return
+   NULL.  Failure to allocate the entry is always transient.  */
+struct nss_module *
+__nss_module_allocate (const char *name, size_t name_length)
+{
+  __libc_lock_lock (nss_module_list_lock);
+
+  struct nss_module *result = NULL;
+  for (struct nss_module *p = nss_module_list; p != NULL; p = p->next)
+    if (strncmp (p->name, name, name_length) == 0
+        && p->name[name_length] == '\0')
+      {
+        /* Return the previously existing object.  */
+        result = p;
+        break;
+      }
+
+  if (result == NULL)
+    {
+      /* Allocate a new list entry if the name was not found in the
+         list.  */
+      result = malloc (sizeof (*result) + name_length + 1);
+      if (result != NULL)
+        {
+          result->state = nss_module_uninitialized;
+          memcpy (result->name, name, name_length);
+          result->name[name_length] = '\0';
+          result->handle = NULL;
+          result->next = nss_module_list;
+          nss_module_list = result;
+        }
+    }
+
+  __libc_lock_unlock (nss_module_list_lock);
+  return result;
+}
+
+/* Long enough to store the name of any function in the
+   nss_function_name_array list below, as getprotobynumber_r is the
+   longest entry in that list.  */
+typedef char function_name[sizeof("getprotobynumber_r")];
+
+static const function_name nss_function_name_array[] =
+  {
+#undef DEFINE_NSS_FUNCTION
+#define DEFINE_NSS_FUNCTION(x) #x,
+#include "function.def"
+  };
+
+/* Internal implementation of __nss_module_load.  */
+static bool
+module_load (struct nss_module *module)
+{
+  void *handle;
+  {
+    char *shlib_name;
+    if (__asprintf (&shlib_name, "libnss_%s.so%s",
+                    module->name, __nss_shlib_revision) < 0)
+      /* This is definitely a temporary failure.  Do not update
+         module->state.  This will trigger another attempt at the next
+         call.  */
+      return false;
+
+    handle = __libc_dlopen (shlib_name);
+    free (shlib_name);
+  }
+
+  /* Failing to load the module can be caused by several different
+     scenarios.  One such scenario is that the module has been removed
+     from the disk.  In which case the in-memory version is all that
+     we have, and if the module->state indidates it is loaded then we
+     can use it.  */
+  if (handle == NULL)
+    {
+      /* dlopen failure.  We do not know if this a temporary or
+         permanent error.  See bug 22041.  Update the state using the
+         double-checked locking idiom.  */
+
+      __libc_lock_lock (nss_module_list_lock);
+      bool result = result;
+      switch ((enum nss_module_state) atomic_load_acquire (&module->state))
+        {
+        case nss_module_uninitialized:
+          atomic_store_release (&module->state, nss_module_failed);
+          result = false;
+          break;
+        case nss_module_loaded:
+          result = true;
+          break;
+        case nss_module_failed:
+          result = false;
+          break;
+        }
+      __libc_lock_unlock (nss_module_list_lock);
+      return result;
+    }
+
+  nss_module_functions_untyped pointers;
+
+  /* Look up and store locally all the function pointers we may need
+     later.  Doing this now means the data will not change in the
+     future.  */
+  for (size_t idx = 0; idx < array_length (nss_function_name_array); ++idx)
+    {
+      char *function_name;
+      if (__asprintf (&function_name, "_nss_%s_%s",
+                      module->name, nss_function_name_array[idx]) < 0)
+        {
+          /* Definitely a temporary error.  */
+          __libc_dlclose (handle);
+          return false;
+        }
+      pointers[idx] = __libc_dlsym (handle, function_name);
+      free (function_name);
+#ifdef PTR_MANGLE
+      PTR_MANGLE (pointers[idx]);
+#endif
+    }
+
+# ifdef USE_NSCD
+  if (is_nscd)
+    {
+      /* Call the init function when nscd is used.  */
+      size_t initlen = (5 + strlen (module->name)
+			+ strlen ("_init") + 1);
+      char init_name[initlen];
+
+      /* Construct the init function name.  */
+      __stpcpy (__stpcpy (__stpcpy (init_name,
+				    "_nss_"),
+			  module->name),
+		"_init");
+
+      /* Find the optional init function.  */
+      void (*ifct) (void (*) (size_t, struct traced_file *))
+	= __libc_dlsym (handle, init_name);
+      if (ifct != NULL)
+	{
+	  void (*cb) (size_t, struct traced_file *) = nscd_init_cb;
+#  ifdef PTR_DEMANGLE
+	  PTR_DEMANGLE (cb);
+#  endif
+	  ifct (cb);
+	}
+    }
+# endif
+
+  /* Install the function pointers, following the double-checked
+     locking idiom.  Delay this after all processing, in case loading
+     the module triggers unwinding.  */
+  __libc_lock_lock (nss_module_list_lock);
+  switch ((enum nss_module_state) atomic_load_acquire (&module->state))
+    {
+    case nss_module_uninitialized:
+    case nss_module_failed:
+      memcpy (module->functions.untyped, pointers,
+              sizeof (module->functions.untyped));
+      module->handle = handle;
+      /* Synchronizes with unlocked __nss_module_load atomic_load_acquire.  */
+      atomic_store_release (&module->state, nss_module_loaded);
+      break;
+    case nss_module_loaded:
+      /* If the module was already loaded, close our own handle.  This
+         does not actually unload the modules, only the reference
+         counter is decremented for the loaded module.  */
+      __libc_dlclose (handle);
+      break;
+    }
+  __libc_lock_unlock (nss_module_list_lock);
+  return true;
+}
+
+/* Force the module identified by MODULE to be loaded.  We return
+   false if the module could not be loaded, true otherwise.  Loading
+   the module requires looking up all the possible interface APIs and
+   caching the results.  */
+bool
+__nss_module_load (struct nss_module *module)
+{
+  switch ((enum nss_module_state) atomic_load_acquire (&module->state))
+    {
+    case nss_module_uninitialized:
+      return module_load (module);
+    case nss_module_loaded:
+      /* Loading has already succeeded.  */
+      return true;
+    case nss_module_failed:
+      /* Loading previously failed.  */
+      return false;
+    }
+  __builtin_unreachable ();
+}
+
+static int
+name_search (const void *left, const void *right)
+{
+  return strcmp (left, right);
+}
+
+/* Load module MODULE (if it isn't already) and return a pointer to
+   the module's implementation of NAME, otherwise return NULL on
+   failure or error.  */
+void *
+__nss_module_get_function (struct nss_module *module, const char *name)
+{
+  if (!__nss_module_load (module))
+    return NULL;
+
+  function_name *name_entry = bsearch (name, nss_function_name_array,
+                                       array_length (nss_function_name_array),
+                                       sizeof (function_name), name_search);
+  assert (name_entry != NULL);
+  size_t idx = name_entry - nss_function_name_array;
+  void *fptr = module->functions.untyped[idx];
+#ifdef PTR_DEMANGLE
+  PTR_DEMANGLE (fptr);
+#endif
+  return fptr;
+}
+
+void __libc_freeres_fn_section
+__nss_module_freeres (void)
+{
+  struct nss_module *current = nss_module_list;
+  while (current != NULL)
+    {
+      if (current->state == nss_module_loaded)
+        __libc_dlclose (current->handle);
+
+      struct nss_module *next = current->next;
+      free (current);
+      current = next;
+    }
+  nss_module_list = NULL;
+}