diff mbox series

[2/4] <nss_action.h>: New abstraction for combining NSS modules and NSS actions

Message ID xnmu4r4wpa.fsf@greed.delorie.com
State Superseded
Headers show
Series nsswitch.conf reloading refactoring | expand

Commit Message

DJ Delorie June 25, 2020, 4:05 a.m. UTC
---
 nss/Makefile           |   3 +-
 nss/nss_action.c       | 114 ++++++++++++++++++++++++
 nss/nss_action.h       |  92 +++++++++++++++++++
 nss/nss_action_parse.c | 197 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 405 insertions(+), 1 deletion(-)
 create mode 100644 nss/nss_action.c
 create mode 100644 nss/nss_action.h
 create mode 100644 nss/nss_action_parse.c

Comments

Carlos O'Donell July 1, 2020, 7:08 p.m. UTC | #1
On 6/25/20 12:05 AM, DJ Delorie via Libc-alpha wrote:

Please post v2.
- See suggested comments.
- See minor code changes requested in nss/nss_action.h
 
> ---
>  nss/Makefile           |   3 +-
>  nss/nss_action.c       | 114 ++++++++++++++++++++++++
>  nss/nss_action.h       |  92 +++++++++++++++++++
>  nss/nss_action_parse.c | 197 +++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 405 insertions(+), 1 deletion(-)
>  create mode 100644 nss/nss_action.c
>  create mode 100644 nss/nss_action.h
>  create mode 100644 nss/nss_action_parse.c
> 
> diff --git a/nss/Makefile b/nss/Makefile
> index 5d357eb51e..464655d045 100644
> --- a/nss/Makefile
> +++ b/nss/Makefile
> @@ -28,7 +28,8 @@ headers			:= nss.h
>  routines		= nsswitch getnssent getnssent_r digits_dots \
>  			  valid_field valid_list_field rewrite_field \
>  			  $(addsuffix -lookup,$(databases)) \
> -			  compat-lookup nss_hash nss_module
> +			  compat-lookup nss_hash nss_module nss_action \
> +			  nss_action_parse

OK. New nss_action and 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
> diff --git a/nss/nss_action.c b/nss/nss_action.c
> new file mode 100644
> index 0000000000..24f1c5e4f9
> --- /dev/null
> +++ b/nss/nss_action.c
> @@ -0,0 +1,114 @@
> +/* NSS actions, elements in a nsswitch.conf configuration line.

OK.

> +   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.  */

OK. This deduplication is important and keeps us bounded. If a roaming
device or network configuration change forces us to change between 1
or 2 or even 3 different configurations those cached configurations will
be reused without taking more memory.

> +
> +struct nss_action_list_wrapper

OK. Wraps the action list.

> +{
> +  /* The next element of the list.x */

s/.x/./g

> +  struct nss_action_list_wrapper *next;

OK. Next element.

> +
> +  /* Number of elements in the list (excluding the terminator).  */
> +  size_t count;

OK.

> +
> +  /* NULL-terminated list of actions.  */
> +  struct nss_action actions[];

OK.

> +};
> +
> +/* Global list of allocated NSS action lists.  */
> +struct nss_action_list_wrapper *nss_actions;

OK. Global list.

> +
> +/* Covers the list.  */

Which list?

Suggest:

/* Lock covers the nss_actions list.  */

> +__libc_lock_define (static, nss_actions_lock);

> +
> +/* Returns true if the actions are equal (same module, same actions
> +   aray).  */

s/aray/array/g

> +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;

OK.

> +}
> +
> +/* Returns true if the COUNT actions at A and B are equal (according
> +   to actions_equal above).  */

Suggest:

/* Returns true if COUNT actions at A and B are equal (according
   to actions_equal above). Caller must ensure that A and 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;
> +}

OK.

> +
> +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);

OK. Look for a list we can use.

> +  if (result == NULL)
> +    {

OK. Didn't find one.

> +      struct nss_action_list_wrapper *wrapper
> +        = malloc (sizeof (*wrapper) + sizeof (*actions) * count);

OK. Allocate one of the right size.

> +      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;

OK. Return the allocated list or NULL on temporary allocation failure.

> +}
> +
> +void __libc_freeres_fn_section
> +__nss_action_freeres (void)
> +{

OK. Shutting down. Don't take the lock and walk the global list freeing
everything.

> +  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;
> +}
> diff --git a/nss/nss_action.h b/nss/nss_action.h
> new file mode 100644
> index 0000000000..7c0d3ef505
> --- /dev/null
> +++ b/nss/nss_action.h
> @@ -0,0 +1,92 @@
> +/* NSS actions, elements in a nsswitch.conf configuration line.

OK.

> +   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>
> +
> +#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;

OK.

> +
> +  /* 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;

OK.

> +};
> +


> +/* Index in actions.  */

Suggest:

/* 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 2 * (2 + status);

Suggest:

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)) & 3;

Suggest:

return (action->action_bits >> nss_actions_bits_index (status)) & NSS_BPL_MASK;

> +}

OK.

> +
> +/* 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 = 3 << offset;

Use NSS_BPL_MASK and then "mask" makes sense.

> +  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;
> +  /* Produces 0b00 .. 00 AA AA AA AA AA.  */
> +  action->action_bits = bits * 0x155;

This needs to be rewritten to be expressed as a function of all the
nss_status entries. That is to say you need to call nss_action_set
for all status and actions. I dislike the above magic. It should be
immediately readable and understandable that we set all the entries.

> +}
> +
> +/* A list of struct nss_action objects in array terminated by an
> +   action with a NULL module.  */
> +typedef struct nss_action *nss_action_list;

OK.

> +
> +/* 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;

OK.

> +
> +/* 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);

OK.

> +
> +/* Called from __libc_freeres.  */
> +void __nss_action_freeres (void) attribute_hidden;

OK.

> +
> +
> +#endif /* _NSS_ACTION_H */
> diff --git a/nss/nss_action_parse.c b/nss/nss_action_parse.c
> new file mode 100644
> index 0000000000..ada3acbb08
> --- /dev/null
> +++ b/nss/nss_action_parse.c
> @@ -0,0 +1,197 @@
> +/* Parse a service line from nsswitch.conf.

OK.

> +   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>
> +
> +
> +/* Read the source names:
> +        `( <source> ( "[" "!"? (<status> "=" <action> )+ "]" )? )*'

OK.

> +   */
> +static bool
> +nss_action_parse (const char *line, struct action_list *result)
> +{
> +  while (1)
> +    {
> +      while (isspace (line[0]))
> +        ++line;
> +      if (line[0] == '\0')
> +        /* No source specified.  */
> +        return result;
> +
> +      /* 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), };

OK. Got a new module. Allocate space for it.

> +      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);

OK. Set some actions by default.

> +
> +      while (isspace (line[0]))
> +        ++line;
> +
> +      if (line[0] == '[')
> +        {
> +          /* Read criterions.  */
> +          do
> +            ++line;
> +          while (line[0] != '\0' && isspace (line[0]));
> +
> +          do
> +            {
> +              int not;
> +              enum nss_status status;
> +              lookup_actions action;
> +
> +              /* Grok ! before name to mean all statii 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 statii.  */
> +              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;
> +
> +              while (isspace (line[0]))
> +                ++line;
> +              if (line[0] != '=')
> +                return false;
> +              do
> +                ++line;
> +              while (isspace (line[0]));
> +
> +              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;

OK.

> +              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);

OK.

> +                }
> +              else
> +                nss_action_set (&new_service, status, action);
> +
> +              /* Skip white spaces.  */
> +              while (isspace (line[0]))
> +                ++line;
> +            }
> +          while (line[0] != ']');
> +
> +          /* Skip the ']'.  */
> +          ++line;
> +        }
> +
> +      action_list_add (result, new_service);

OK.

> +    }
> +}
> +
> +nss_action_list
> + __nss_action_parse (const char *line)

OK. Interfact to the rest of NSS.

> +{
> +  struct action_list list;
> +  action_list_init (&list);

OK. Setup DYNARRAY.

> +  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);

OK.

> +      return NULL;
> +    }
> +  else
> +    {
> +      /* Parse error.  */
> +      __set_errno (EINVAL);

OK.

> +      return NULL;
> +    }
> +}
>
diff mbox series

Patch

diff --git a/nss/Makefile b/nss/Makefile
index 5d357eb51e..464655d045 100644
--- a/nss/Makefile
+++ b/nss/Makefile
@@ -28,7 +28,8 @@  headers			:= nss.h
 routines		= nsswitch getnssent getnssent_r digits_dots \
 			  valid_field valid_list_field rewrite_field \
 			  $(addsuffix -lookup,$(databases)) \
-			  compat-lookup nss_hash nss_module
+			  compat-lookup nss_hash 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
diff --git a/nss/nss_action.c b/nss/nss_action.c
new file mode 100644
index 0000000000..24f1c5e4f9
--- /dev/null
+++ b/nss/nss_action.c
@@ -0,0 +1,114 @@ 
+/* 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.x */
+  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[];
+};
+
+/* Global list of allocated NSS action lists.  */
+struct nss_action_list_wrapper *nss_actions;
+
+/* Covers the list.  */
+__libc_lock_define (static, nss_actions_lock);
+
+/* Returns true if the actions are equal (same module, same actions
+   aray).  */
+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 the COUNT actions at A and B are equal (according
+   to actions_equal above).  */
+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;
+}
diff --git a/nss/nss_action.h b/nss/nss_action.h
new file mode 100644
index 0000000000..7c0d3ef505
--- /dev/null
+++ b/nss/nss_action.h
@@ -0,0 +1,92 @@ 
+/* 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>
+
+#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;
+};
+
+/* Index in actions.  */
+static inline int
+nss_actions_bits_index (enum nss_status status)
+{
+  return 2 * (2 + 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)) & 3;
+}
+
+/* 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 = 3 << 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;
+  /* Produces 0b00 .. 00 AA AA AA AA AA.  */
+  action->action_bits = bits * 0x155;
+}
+
+/* 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 */
diff --git a/nss/nss_action_parse.c b/nss/nss_action_parse.c
new file mode 100644
index 0000000000..ada3acbb08
--- /dev/null
+++ b/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>
+
+
+/* Read the source names:
+        `( <source> ( "[" "!"? (<status> "=" <action> )+ "]" )? )*'
+   */
+static bool
+nss_action_parse (const char *line, struct action_list *result)
+{
+  while (1)
+    {
+      while (isspace (line[0]))
+        ++line;
+      if (line[0] == '\0')
+        /* No source specified.  */
+        return result;
+
+      /* 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);
+
+      while (isspace (line[0]))
+        ++line;
+
+      if (line[0] == '[')
+        {
+          /* Read criterions.  */
+          do
+            ++line;
+          while (line[0] != '\0' && isspace (line[0]));
+
+          do
+            {
+              int not;
+              enum nss_status status;
+              lookup_actions action;
+
+              /* Grok ! before name to mean all statii 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 statii.  */
+              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;
+
+              while (isspace (line[0]))
+                ++line;
+              if (line[0] != '=')
+                return false;
+              do
+                ++line;
+              while (isspace (line[0]));
+
+              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 white spaces.  */
+              while (isspace (line[0]))
+                ++line;
+            }
+          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;
+    }
+}