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, 93 insertions, 0 deletions

diff --git a/nss/nss_module.h b/nss/nss_module.h
new file mode 100644
index 0000000..17c46ea
--- /dev/null
+++ b/nss/nss_module.h
@@ -0,0 +1,93 @@
+/* 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/>.  */
+
+#ifndef _NSS_MODULE_H
+#define _NSS_MODULE_H
+
+#include <nss.h>
+#include <stdbool.h>
+
+/* See nss_database.h for a summary of how this relates.  */
+
+/* Typed function pointers for all functions that can be defined by a
+   service module.  */
+struct nss_module_functions
+{
+#undef DEFINE_NSS_FUNCTION
+#define DEFINE_NSS_FUNCTION(f) nss_##f *f;
+#include "function.def"
+};
+
+/* Untyped version of struct nss_module_functions, for consistent
+   processing purposes.  */
+typedef void *nss_module_functions_untyped[sizeof (struct nss_module_functions)
+                                           / sizeof (void *)];
+
+/* Initialization state of a NSS module.  */
+enum nss_module_state
+{
+  nss_module_uninitialized,
+  nss_module_loaded,
+  nss_module_failed,
+};
+
+/* A NSS service module (potentially unloaded).  Client code should
+   use the functions below.  */
+struct nss_module
+{
+  /* Actual type is enum nss_module_state.  Use int due to atomic
+     access.  Used in a double-checked locking idiom.  */
+  int state;
+
+  /* The function pointers in the module.  */
+  union
+  {
+    struct nss_module_functions typed;
+    nss_module_functions_untyped untyped;
+  } functions;
+
+  /* Only used for __libc_freeres unloading.  */
+  void *handle;
+
+  /* The next module in the list. */
+  struct nss_module *next;
+
+  /* The name of the module (as it appears in /etc/nsswitch.conf).  */
+  char name[];
+};
+
+/* Allocates the NSS module NAME (of NAME_LENGTH bytes) and places it
+   into the global list.  If it already exists in the list, return the
+   pre-existing module.  This does not actually load the module.
+   Returns NULL on memory allocation failure.  */
+struct nss_module *__nss_module_allocate (const char *name,
+                                          size_t name_length) attribute_hidden;
+
+/* Ensures that MODULE is in a loaded or failed state.  */
+bool __nss_module_load (struct nss_module *module) attribute_hidden;
+
+/* Ensures that MODULE is loaded and returns a pointer to the function
+   NAME defined in it.  Returns NULL if MODULE could not be loaded, or
+   if the function NAME is not defined in the module.  */
+void *__nss_module_get_function (struct nss_module *module, const char *name)
+  attribute_hidden;
+
+/* Called from __libc_freeres.  */
+void __nss_module_freeres (void) attribute_hidden;
+
+#endif /* NSS_MODULE_H */