<nss_action.h>: New abstraction for combining NSS modules and NSS actions

nss_action manages a set of lists of actions; these are the portions
of the lines in nsswitch.conf to the right of the colons, like
"dns [!UNAVAIL=return] files".  Each permutation of actions and
conditionals is cached for reuse, which limits memory growth, and
refers to the static list of modules managed by nss_modules.

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

1 files changed, 108 insertions, 0 deletions

diff --git a/nss/nss_action.h b/nss/nss_action.h
new file mode 100644
index 0000000000..fa280afd05
--- /dev/null
+++ b/nss/nss_action.h
@@ -0,0 +1,108 @@
+/* NSS actions, elements in a nsswitch.conf configuration line.
+   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_ACTION_H
+#define _NSS_ACTION_H
+
+#include <stddef.h>
+
+/* See nss_database.h for a summary of how this relates.  */
+
+#include "nsswitch.h" /* For lookup_actions.  */
+
+struct nss_module;
+
+/* A NSS action pairs a service module with the action for each result
+   state.  */
+struct nss_action
+{
+  /* The service module that provides the functionality (potentially
+     not yet loaded).  */
+  struct nss_module *module;
+
+  /* Action according to result.  Two bits for each lookup_actions
+     value (from nsswitch.h), indexed by enum nss_status (from nss.h).  */
+  unsigned int action_bits;
+};
+
+/* Value to add to first nss_status value to get zero.  */
+#define NSS_STATUS_BIAS 2
+/* Number of bits per lookup action.  */
+#define NSS_BPL 2
+#define NSS_BPL_MASK ((1 << NSS_BPL) - 1)
+
+/* Index in actions of an NSS status.  Note that in nss/nss.h the
+   status starts at -2, and we shift that up to zero by adding 2.
+   Thus for example NSS_STATUS_TRYAGAIN, which is -2, would index into
+   the 0th bit place as expected.  */
+static inline int
+nss_actions_bits_index (enum nss_status status)
+{
+  return NSS_BPL * (NSS_STATUS_BIAS + status);
+}
+
+/* Returns the lookup_action value for STATUS in ACTION.  */
+static inline lookup_actions
+nss_action_get (const struct nss_action *action, enum nss_status status)
+{
+  return ((action->action_bits >> nss_actions_bits_index (status))
+	  & NSS_BPL_MASK);
+}
+
+/* Sets the lookup_action value for STATUS in ACTION.  */
+static inline void
+nss_action_set (struct nss_action *action,
+                enum nss_status status, lookup_actions actions)
+{
+  int offset = nss_actions_bits_index (status);
+  unsigned int mask = NSS_BPL_MASK << offset;
+  action->action_bits = ((action->action_bits & ~mask)
+                         | ((unsigned int) actions << offset));
+}
+
+static inline void
+nss_action_set_all (struct nss_action *action, lookup_actions actions)
+{
+  unsigned int bits = actions & NSS_BPL_MASK;
+  action->action_bits = (   bits
+			 | (bits << (NSS_BPL * 1))
+			 | (bits << (NSS_BPL * 2))
+			 | (bits << (NSS_BPL * 3))
+			 | (bits << (NSS_BPL * 4))
+			 );
+}
+
+/* A list of struct nss_action objects in array terminated by an
+   action with a NULL module.  */
+typedef struct nss_action *nss_action_list;
+
+/* Returns a pointer to an allocated NSS action list that has COUNT
+   actions that matches the array at ACTIONS.  */
+nss_action_list __nss_action_allocate (struct nss_action *actions,
+                                       size_t count) attribute_hidden;
+
+/* Returns a pointer to a list allocated by __nss_action_allocate, or
+   NULL on error.  ENOMEM means a (temporary) memory allocation error,
+   EINVAL means that LINE is syntactically invalid.  */
+nss_action_list __nss_action_parse (const char *line);
+
+/* Called from __libc_freeres.  */
+void __nss_action_freeres (void) attribute_hidden;
+
+
+#endif /* _NSS_ACTION_H */