<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>

nss/Makefile

@@ -30,7 +30,8 @@ routines		= nsswitch getnssent getnssent_r digits_dots \
 			  $(addsuffix -lookup,$(databases)) \
 			  compat-lookup nss_hash nss_files_fopen \
 			  nss_readline nss_parse_line_result \
-			  nss_fgetent_r nss_module
+			  nss_fgetent_r nss_module nss_action \
+			  nss_action_parse
 
 # These are the databases that go through nss dispatch.
 # Caution: if you add a database here, you must add its real name

nss/nss_action.c

@@ -0,0 +1,116 @@
+/* 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/>.  */
+
+#include <nss_action.h>
+
+#include <string.h>
+#include <libc-lock.h>
+
+/* Maintain a global list of NSS action lists.  Since most databases
+   use the same list of actions, this list is usually short.
+   Deduplication in __nss_action_allocate ensures that the list does
+   not grow without bounds.  */
+
+struct nss_action_list_wrapper
+{
+  /* The next element of the list.  */
+  struct nss_action_list_wrapper *next;
+
+  /* Number of elements in the list (excluding the terminator).  */
+  size_t count;
+
+  /* NULL-terminated list of actions.  */
+  struct nss_action actions[];
+};
+
+/* Toplevel list of allocated NSS action lists.  */
+static struct nss_action_list_wrapper *nss_actions;
+
+/* Lock covers the nss_actions list.  */
+__libc_lock_define (static, nss_actions_lock);
+
+/* Returns true if the actions are equal (same module, same actions
+   array).  */
+static bool
+actions_equal (const struct nss_action *a, const struct nss_action *b)
+{
+  return a->module == b->module && a->action_bits == b->action_bits;
+}
+
+
+/* Returns true if COUNT actions at A and B are equal (according to
+   actions_equal above). Caller must ensure that either A or B have at
+   least COUNT actions.  */
+static bool
+action_lists_equal (const struct nss_action *a, const struct nss_action *b,
+                    size_t count)
+{
+  for (size_t i = 0; i < count; ++i)
+    if (!actions_equal (a + i, b + i))
+      return false;
+  return true;
+}
+
+/* Returns a pre-allocated action list for COUNT actions at ACTIONS,
+   or NULL if no such list exists.  */
+static nss_action_list
+find_allocated (struct nss_action *actions, size_t count)
+{
+  for (struct nss_action_list_wrapper *p = nss_actions; p != NULL; p = p->next)
+    if (p->count == count && action_lists_equal (p->actions, actions, count))
+      return p->actions;
+  return NULL;
+}
+
+nss_action_list
+__nss_action_allocate (struct nss_action *actions, size_t count)
+{
+  nss_action_list result = NULL;
+  __libc_lock_lock (nss_actions_lock);
+
+  result = find_allocated (actions, count);
+  if (result == NULL)
+    {
+      struct nss_action_list_wrapper *wrapper
+        = malloc (sizeof (*wrapper) + sizeof (*actions) * count);
+      if (wrapper != NULL)
+        {
+          wrapper->next = nss_actions;
+          wrapper->count = count;
+          memcpy (wrapper->actions, actions, sizeof (*actions) * count);
+          nss_actions = wrapper;
+          result = wrapper->actions;
+        }
+    }
+
+  __libc_lock_unlock (nss_actions_lock);
+  return result;
+}
+
+void __libc_freeres_fn_section
+__nss_action_freeres (void)
+{
+  struct nss_action_list_wrapper *current = nss_actions;
+  while (current != NULL)
+    {
+      struct nss_action_list_wrapper *next = current->next;
+      free (current);
+      current = next;
+    }
+  nss_actions = NULL;
+}

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 */

nss/nss_action_parse.c

@@ -0,0 +1,197 @@
+/* Parse a service line from nsswitch.conf.
+   Copyright (c) 1996-2020 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_action.h"
+#include "nss_module.h"
+
+#include <ctype.h>
+#include <string.h>
+#include <stdbool.h>
+
+/* Staging area during parsing.  */
+#define DYNARRAY_STRUCT action_list
+#define DYNARRAY_ELEMENT struct nss_action
+#define DYNARRAY_PREFIX action_list_
+#include <malloc/dynarray-skeleton.c>
+
+/* Skip whitespace in line[].  */
+#define SKIP_WS() \
+  while (line[0] != '\0' && isspace (line[0]))	\
+    ++line;
+
+/* Read the source names:
+        `( <source> ( "[" "!"? (<status> "=" <action> )+ "]" )? )*'
+   */
+static bool
+nss_action_parse (const char *line, struct action_list *result)
+{
+  while (1)
+    {
+      SKIP_WS ();
+      if (line[0] == '\0')
+        /* No more sources specified.  */
+        return true;
+
+      /* Read <source> identifier.  */
+      const char *name = line;
+      while (line[0] != '\0' && !isspace (line[0]) && line[0] != '[')
+        ++line;
+      if (name == line)
+        return true;
+
+      struct nss_action new_service
+        = { .module = __nss_module_allocate (name, line - name), };
+      if (new_service.module == NULL)
+        {
+          /* Memory allocation error.  */
+          action_list_mark_failed (result);
+          return false;
+        }
+      nss_action_set_all (&new_service, NSS_ACTION_CONTINUE);
+      nss_action_set (&new_service, NSS_STATUS_SUCCESS, NSS_ACTION_RETURN);
+      nss_action_set (&new_service, NSS_STATUS_RETURN, NSS_ACTION_RETURN);
+
+      SKIP_WS ();
+
+      if (line[0] == '[')
+        {
+          /* Read criterions.  */
+
+	  /* Skip the '['.  */
+	  ++line;
+	  SKIP_WS ();
+
+          do
+            {
+              int not;
+              enum nss_status status;
+              lookup_actions action;
+
+              /* Grok ! before name to mean all statuses but that one.  */
+              not = line[0] == '!';
+              if (not)
+                ++line;
+
+              /* Read status name.  */
+              name = line;
+              while (line[0] != '\0' && !isspace (line[0]) && line[0] != '='
+                     && line[0] != ']')
+                ++line;
+
+              /* Compare with known statuses.  */
+              if (line - name == 7)
+                {
+                  if (__strncasecmp (name, "SUCCESS", 7) == 0)
+                    status = NSS_STATUS_SUCCESS;
+                  else if (__strncasecmp (name, "UNAVAIL", 7) == 0)
+                    status = NSS_STATUS_UNAVAIL;
+                  else
+                    return false;
+                }
+              else if (line - name == 8)
+                {
+                  if (__strncasecmp (name, "NOTFOUND", 8) == 0)
+                    status = NSS_STATUS_NOTFOUND;
+                  else if (__strncasecmp (name, "TRYAGAIN", 8) == 0)
+                    status = NSS_STATUS_TRYAGAIN;
+                  else
+                    return false;
+                }
+              else
+		return false;
+
+	      SKIP_WS ();
+              if (line[0] != '=')
+                return false;
+
+	      /* Skip the '='.  */
+	      ++line;
+	      SKIP_WS ();
+              name = line;
+              while (line[0] != '\0' && !isspace (line[0]) && line[0] != '='
+                     && line[0] != ']')
+                ++line;
+
+              if (line - name == 6 && __strncasecmp (name, "RETURN", 6) == 0)
+                action = NSS_ACTION_RETURN;
+              else if (line - name == 8
+                       && __strncasecmp (name, "CONTINUE", 8) == 0)
+                action = NSS_ACTION_CONTINUE;
+              else if (line - name == 5
+                       && __strncasecmp (name, "MERGE", 5) == 0)
+                action = NSS_ACTION_MERGE;
+              else
+                return false;
+
+              if (not)
+                {
+                  /* Save the current action setting for this status,
+                     set them all to the given action, and reset this one.  */
+                  const lookup_actions save
+                    = nss_action_get (&new_service, status);
+                  nss_action_set_all (&new_service, action);
+                  nss_action_set (&new_service, status, save);
+                }
+              else
+                nss_action_set (&new_service, status, action);
+
+	      SKIP_WS ();
+            }
+          while (line[0] != ']');
+
+          /* Skip the ']'.  */
+          ++line;
+        }
+
+      action_list_add (result, new_service);
+    }
+}
+
+nss_action_list
+ __nss_action_parse (const char *line)
+{
+  struct action_list list;
+  action_list_init (&list);
+  if (nss_action_parse (line, &list))
+    {
+      size_t size = action_list_size (&list);
+      nss_action_list result
+        = malloc (sizeof (*result) * (size + 1));
+      if (result == NULL)
+        {
+          action_list_free (&list);
+          return NULL;
+        }
+      memcpy (result, action_list_begin (&list), sizeof (*result) * size);
+      /* Sentinel.  */
+      result[size].module = NULL;
+      return result;
+    }
+  else if (action_list_has_failed (&list))
+    {
+      /* Memory allocation error.  */
+      __set_errno (ENOMEM);
+      return NULL;
+    }
+  else
+    {
+      /* Parse error.  */
+      __set_errno (EINVAL);
+      return NULL;
+    }
+}