resolv: Introduce struct resolv_conf with extended resolver state

This change provides additional resolver configuration state which
is not exposed through the _res ABI.  It reuses the existing
initstamp field in the supposedly-private part of _res.  Some effort
is undertaken to avoid memory safety issues introduced by applications
which directly patch the _res object.

With this commit, only the initstamp field is moved into struct
resolv_conf.  Additional members will be added later, eventually
migrating the entire resolver configuration.

ChangeLog

@@ -1,3 +1,21 @@
+2017-06-30  Florian Weimer  <fweimer@redhat.com>
+
+	Add extended resolver state/configuration (struct resolv_conf).
+	* resolv/resolv_conf.h, resolv/resolv_conf.c: New files.
+	* resolv/res-close.c (__res_iclose): Call __resolv_conf_detach.
+	* resolv/res_init.c (res_vinit_1): Do not initialize initstamp.
+	(__res_vinit): Call __resolv_conf_allocate and
+	__resolv_conf_attach.
+	* resolv/resolv_context.h (struct resolv_context): Add conf member
+	of type struct resolv_conf.
+	* resolv/resolv_context.c (maybe_init): Get initstamp from struct
+	resolv_conf.  Update conf member after initialization.
+	* resolv/Makefile (routines): Add resolv_conf.
+	* resolv/bits/types/res_state.h [_LIBC] (struct __res_state):
+	Rename _u._ext.initstamp to _u._ext.__glibc_extension_index.
+	[!_LIBC] (struct __res_state): Rename _u._ext._initstamp to
+	_u._ext.__glibc_reserved.
+
 2017-06-30  Florian Weimer  <fweimer@redhat.com>
 
 	[BZ #21668]

resolv/Makefile

@@ -29,7 +29,7 @@ headers	:= resolv.h bits/types/res_state.h \
 
 routines := herror inet_addr inet_ntop inet_pton nsap_addr res_init \
 	    res_hconf res_libc res-state res_randomid res-close \
-	    resolv_context
+	    resolv_context resolv_conf
 
 tests = tst-aton tst-leaks tst-inet_ntop
 xtests = tst-leaks2

resolv/bits/types/res_state.h

@@ -47,10 +47,10 @@ struct __res_state {
 			uint16_t		nsinit;
 			struct sockaddr_in6	*nsaddrs[MAXNS];
 #ifdef _LIBC
-			unsigned long long int	initstamp
+			unsigned long long int __glibc_extension_index
 			  __attribute__((packed));
 #else
-			unsigned int		_initstamp[2];
+			unsigned int		__glibc_reserved[2];
 #endif
 		} _ext;
 	} _u;

resolv/res-close.c

@@ -84,6 +84,7 @@
 
 #include <resolv-internal.h>
 #include <resolv_context.h>
+#include <resolv_conf.h>
 #include <not-cancel.h>
 
 /* Close all open sockets.  If FREE_ADDR is true, deallocate any
@@ -111,6 +112,8 @@ __res_iclose (res_state statp, bool free_addr)
             statp->_u._ext.nsaddrs[ns] = NULL;
           }
       }
+  if (free_addr)
+    __resolv_conf_detach (statp);
 }
 libc_hidden_def (__res_iclose)
 

resolv/res_init.c

@@ -102,6 +102,7 @@
 #include <sys/types.h>
 #include <inet/net-internal.h>
 #include <errno.h>
+#include <resolv_conf.h>
 
 static void res_setoptions (res_state, const char *);
 static uint32_t net_mask (struct in_addr);
@@ -137,7 +138,6 @@ res_vinit_1 (res_state statp, bool preinit, FILE *fp, char **buffer)
   bool havesearch = false;
   int nsort = 0;
   char *net;
-  statp->_u._ext.initstamp = __res_initstamp;
 
   if (!preinit)
     {
@@ -457,6 +457,19 @@ __res_vinit (res_state statp, int preinit)
   bool ok = res_vinit_1 (statp, preinit, fp, &buffer);
   free (buffer);
 
+  if (ok)
+    {
+      struct resolv_conf init = { 0 }; /* No data yet.  */
+      struct resolv_conf *conf = __resolv_conf_allocate (&init);
+      if (conf == NULL)
+        ok = false;
+      else
+        {
+          ok = __resolv_conf_attach (statp, conf);
+          __resolv_conf_put (conf);
+        }
+    }
+
   if (!ok)
     {
       /* Deallocate the name server addresses which have been

resolv/resolv_conf.c

@@ -0,0 +1,322 @@
+/* Extended resolver state separate from struct __res_state.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <resolv_conf.h>
+
+#include <alloc_buffer.h>
+#include <assert.h>
+#include <libc-lock.h>
+#include <resolv-internal.h>
+
+/* _res._u._ext.__glibc_extension_index is used as an index into a
+   struct resolv_conf_array object.  The intent of this construction
+   is to make reasonably sure that even if struct __res_state objects
+   are copied around and patched by applications, we can still detect
+   accesses to stale extended resolver state.  */
+#define DYNARRAY_STRUCT resolv_conf_array
+#define DYNARRAY_ELEMENT struct resolv_conf *
+#define DYNARRAY_PREFIX resolv_conf_array_
+#define DYNARRAY_INITIAL_SIZE 0
+#include <malloc/dynarray-skeleton.c>
+
+/* A magic constant for XORing the extension index
+   (_res._u._ext.__glibc_extension_index).  This makes it less likely
+   that a valid index is created by accident.  In particular, a zero
+   value leads to an invalid index.  */
+#define INDEX_MAGIC 0x26a8fa5e48af8061ULL
+
+/* Global resolv.conf-related state.  */
+struct resolv_conf_global
+{
+  /* struct __res_state objects contain the extension index
+     (_res._u._ext.__glibc_extension_index ^ INDEX_MAGIC), which
+     refers to an element of this array.  When a struct resolv_conf
+     object (extended resolver state) is associated with a struct
+     __res_state object (legacy resolver state), its reference count
+     is increased and added to this array.  Conversely, if the
+     extended state is detached from the basic state (during
+     reinitialization or deallocation), the index is decremented, and
+     the array element is overwritten with NULL.  */
+  struct resolv_conf_array array;
+
+};
+
+/* Lazily allocated storage for struct resolv_conf_global.  */
+static struct resolv_conf_global *global;
+
+/* The lock synchronizes access to global and *global.  It also
+   protects the __refcount member of struct resolv_conf.  */
+__libc_lock_define_initialized (static, lock);
+
+/* Ensure that GLOBAL is allocated and lock it.  Return NULL if
+   memory allocation failes.  */
+static struct resolv_conf_global *
+get_locked_global (void)
+{
+  __libc_lock_lock (lock);
+  /* Use relaxed MO through because of load outside the lock in
+     __resolv_conf_detach.  */
+  struct resolv_conf_global *global_copy = atomic_load_relaxed (&global);
+  if (global_copy == NULL)
+    {
+      global_copy = calloc (1, sizeof (*global));
+      if (global_copy == NULL)
+        return NULL;
+      atomic_store_relaxed (&global, global_copy);
+      resolv_conf_array_init (&global_copy->array);
+    }
+  return global_copy;
+}
+
+/* Relinquish the lock acquired by get_locked_global.  */
+static void
+put_locked_global (struct resolv_conf_global *global_copy)
+{
+  __libc_lock_unlock (lock);
+}
+
+/* Decrement the reference counter.  The caller must acquire the lock
+   around the function call.  */
+static void
+conf_decrement (struct resolv_conf *conf)
+{
+  assert (conf->__refcount > 0);
+  if (--conf->__refcount == 0)
+    free (conf);
+}
+
+/* Internal implementation of __resolv_conf_get, without validation
+   against *RESP.  */
+static struct resolv_conf *
+resolv_conf_get_1 (const struct __res_state *resp)
+{
+  /* Not initialized, and therefore no assoicated context.  */
+  if (!(resp->options & RES_INIT))
+    return NULL;
+
+  struct resolv_conf_global *global_copy = get_locked_global ();
+  if (global_copy == NULL)
+    /* A memory allocation failure here means that no associated
+       contexts exists, so returning NULL is correct.  */
+    return NULL;
+  size_t index = resp->_u._ext.__glibc_extension_index ^ INDEX_MAGIC;
+  struct resolv_conf *conf;
+  if (index < resolv_conf_array_size (&global_copy->array))
+    {
+      conf = *resolv_conf_array_at (&global_copy->array, index);
+      assert (conf->__refcount > 0);
+      ++conf->__refcount;
+    }
+  else
+    conf = NULL;
+  put_locked_global (global_copy);
+  return conf;
+}
+
+/* Check that *RESP and CONF match.  Used by __resolv_conf_get.  */
+static bool
+resolv_conf_matches (const struct __res_state *resp,
+                     const struct resolv_conf *conf)
+{
+  return true;
+}
+
+struct resolv_conf *
+__resolv_conf_get (struct __res_state *resp)
+{
+  struct resolv_conf *conf = resolv_conf_get_1 (resp);
+  if (conf == NULL)
+    return NULL;
+  if (resolv_conf_matches (resp, conf))
+    return conf;
+  __resolv_conf_put (conf);
+  return NULL;
+}
+
+void
+__resolv_conf_put (struct resolv_conf *conf)
+{
+  if (conf == NULL)
+    return;
+
+  __libc_lock_lock (lock);
+  conf_decrement (conf);
+  __libc_lock_unlock (lock);
+}
+
+struct resolv_conf *
+__resolv_conf_allocate (const struct resolv_conf *init)
+{
+  /* Allocate the buffer.  */
+  void *ptr;
+  struct alloc_buffer buffer = alloc_buffer_allocate
+    (sizeof (struct resolv_conf),
+     &ptr);
+  struct resolv_conf *conf
+    = alloc_buffer_alloc (&buffer, struct resolv_conf);
+  if (conf == NULL)
+    /* Memory allocation failure.  */
+    return NULL;
+  assert (conf == ptr);
+
+  /* Initialize the contents.  */
+  conf->__refcount = 1;
+  conf->initstamp = __res_initstamp;
+
+  assert (!alloc_buffer_has_failed (&buffer));
+  return conf;
+}
+
+/* Update *RESP from the extended state.  */
+static __attribute__ ((nonnull (1, 2), warn_unused_result)) bool
+update_from_conf (struct __res_state *resp, const struct resolv_conf *conf)
+{
+  /* The overlapping parts of both configurations should agree after
+     initialization.  */
+  assert (resolv_conf_matches (resp, conf));
+  return true;
+}
+
+/* Decrement the configuration object at INDEX and free it if the
+   reference counter reaches 0.  *GLOBAL_COPY must be locked and
+   remains so.  */
+static void
+decrement_at_index (struct resolv_conf_global *global_copy, size_t index)
+{
+  if (index < resolv_conf_array_size (&global_copy->array))
+    {
+      /* Index found.  Deallocate the struct resolv_conf object once
+         the reference counter reaches.  Free the array slot.  */
+      struct resolv_conf **slot
+        = resolv_conf_array_at (&global_copy->array, index);
+      struct resolv_conf *conf = *slot;
+      if (conf != NULL)
+        {
+          conf_decrement (conf);
+          /* Clear the slot even if the reference count is positive.
+             Slots are not shared.  */
+          *slot = NULL;
+        }
+    }
+}
+
+bool
+__resolv_conf_attach (struct __res_state *resp, struct resolv_conf *conf)
+{
+  assert (conf->__refcount > 0);
+
+  struct resolv_conf_global *global_copy = get_locked_global ();
+  if (global_copy == NULL)
+    {
+      free (conf);
+      return false;
+    }
+
+  /* Try to find an unused index in the array.  */
+  size_t index;
+  {
+    size_t size = resolv_conf_array_size (&global_copy->array);
+    bool found = false;
+    for (index = 0; index < size; ++index)
+      {
+        struct resolv_conf **p
+          = resolv_conf_array_at (&global_copy->array, index);
+        if (*p == NULL)
+          {
+            *p = conf;
+            found = true;
+            break;
+          }
+      }
+
+    if (!found)
+      {
+        /* No usable index found.  Increase the array size.  */
+        resolv_conf_array_add (&global_copy->array, conf);
+        if (resolv_conf_array_has_failed (&global_copy->array))
+          {
+            put_locked_global (global_copy);
+            __set_errno (ENOMEM);
+            return false;
+          }
+        /* The new array element was added at the end.  */
+        index = size;
+      }
+  }
+
+  /* We have added a new reference to the object.  */
+  ++conf->__refcount;
+  assert (conf->__refcount > 0);
+  put_locked_global (global_copy);
+
+  if (!update_from_conf (resp, conf))
+    {
+      /* Drop the reference we acquired.  Reacquire the lock.  The
+         object has already been allocated, so it cannot be NULL this
+         time.  */
+      global_copy = get_locked_global ();
+      decrement_at_index (global_copy, index);
+      put_locked_global (global_copy);
+      return false;
+    }
+  resp->_u._ext.__glibc_extension_index = index ^ INDEX_MAGIC;
+
+  return true;
+}
+
+void
+__resolv_conf_detach (struct __res_state *resp)
+{
+  if (atomic_load_relaxed (&global) == NULL)
+    /* Detach operation after a shutdown, or without any prior
+       attachment.  We cannot free the data (and there might not be
+       anything to free anyway).  */
+    return;
+
+  struct resolv_conf_global *global_copy = get_locked_global ();
+  size_t index = resp->_u._ext.__glibc_extension_index ^ INDEX_MAGIC;
+  decrement_at_index (global_copy, index);
+
+  /* Clear the index field, so that accidental reuse is less
+     likely.  */
+  resp->_u._ext.__glibc_extension_index = 0;
+
+  put_locked_global (global_copy);
+}
+
+/* Deallocate the global data.  */
+static void __attribute__ ((section ("__libc_thread_freeres_fn")))
+freeres (void)
+{
+  /* No locking because this function is supposed to be called when
+     the process has turned single-threaded.  */
+  if (global == NULL)
+    return;
+
+  /* Note that this frees only the array itself.  The pointed-to
+     configuration objects should have been deallocated by res_nclose
+     and per-thread cleanup functions.  */
+  resolv_conf_array_free (&global->array);
+
+  free (global);
+
+  /* Stop potential future __resolv_conf_detach calls from accessing
+     deallocated memory.  */
+  global = NULL;
+}
+text_set_element (__libc_subfreeres, freeres);

resolv/resolv_conf.h

@@ -0,0 +1,69 @@
+/* Extended resolver state separate from struct __res_state.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef RESOLV_STATE_H
+#define RESOLV_STATE_H
+
+#include <stdbool.h>
+#include <stddef.h>
+
+/* Extended resolver state associated with res_state objects.  Client
+   code can reach this state through a struct resolv_context
+   object.  */
+struct resolv_conf
+{
+  /* Used to propagate the effect of res_init across threads.  This
+     member is mutable and prevents sharing of the same struct
+     resolv_conf object among multiple struct __res_state objects.  */
+  unsigned long long int initstamp;
+
+  /* Reference counter.  The object is deallocated once it reaches
+     zero.  For internal use within resolv_conf only.  */
+  size_t __refcount;
+};
+
+/* The functions below are for use by the res_init resolv.conf parser
+   and the struct resolv_context facility.  */
+
+struct __res_state;
+
+/* Return the extended resolver state for *RESP, or NULL if it cannot
+   be determined.  A call to this function must be paired with a call
+   to __resolv_conf_put.  */
+struct resolv_conf *__resolv_conf_get (struct __res_state *) attribute_hidden;
+
+/* Converse of __resolv_conf_get.  */
+void __resolv_conf_put (struct resolv_conf *) attribute_hidden;
+
+/* Allocate a new struct resolv_conf object and copy the
+   pre-configured values from *INIT.  Return NULL on allocation
+   failure.  The object must be deallocated using
+   __resolv_conf_put.  */
+struct resolv_conf *__resolv_conf_allocate (const struct resolv_conf *init)
+  attribute_hidden __attribute__ ((nonnull (1), warn_unused_result));
+
+/* Associate an existing extended resolver state with *RESP.  Return
+   false on allocation failure.  In addition, update *RESP with the
+   overlapping non-extended resolver state.  */
+bool __resolv_conf_attach (struct __res_state *, struct resolv_conf *)
+  attribute_hidden;
+
+/* Detach the extended resolver state from *RESP.  */
+void __resolv_conf_detach (struct __res_state *resp) attribute_hidden;
+
+#endif /* RESOLV_STATE_H */

resolv/resolv_context.c

@@ -17,9 +17,11 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <resolv_context.h>
+#include <resolv_conf.h>
 #include <resolv-internal.h>
 
 #include <assert.h>
+#include <errno.h>
 #include <stdlib.h>
 #include <stdio.h>
 
@@ -52,19 +54,37 @@ static __thread struct resolv_context *current attribute_tls_model_ie;
 /* Initialize *RESP if RES_INIT is not yet set in RESP->options, or if
    res_init in some other thread requested re-initializing.  */
 static __attribute__ ((warn_unused_result)) bool
-maybe_init (struct __res_state *resp, bool preinit)
+maybe_init (struct resolv_context *ctx, bool preinit)
 {
+  struct __res_state *resp = ctx->resp;
   if (resp->options & RES_INIT)
     {
-      if (__res_initstamp != resp->_u._ext.initstamp)
+      /* If there is no associated resolv_conf object despite the
+         initialization, something modified *ctx->resp.  Do not
+         override those changes.  */
+      if (ctx->conf != NULL && ctx->conf->initstamp != __res_initstamp)
         {
           if (resp->nscount > 0)
+            /* This call will detach the extended resolver state.  */
             __res_iclose (resp, true);
-          return __res_vinit (resp, 1) == 0;
+          /* And this call will attach it again.  */
+          if (__res_vinit (resp, 1) < 0)
+            {
+              /* The configuration no longer matches after failed
+                 initialization.  */
+              __resolv_conf_put (ctx->conf);
+              ctx->conf = NULL;
+              return false;
+            }
+          /* Delay the release of the old configuration until this
+             point, so that __res_vinit can reuse it if possible.  */
+          __resolv_conf_put (ctx->conf);
+          ctx->conf = __resolv_conf_get (ctx->resp);
         }
       return true;
     }
 
+  assert (ctx->conf == NULL);
   if (preinit)
     {
       if (!resp->retrans)
@@ -75,7 +95,11 @@ maybe_init (struct __res_state *resp, bool preinit)
       if (!resp->id)
         resp->id = res_randomid ();
     }
-  return __res_vinit (resp, preinit) == 0;
+
+  if (__res_vinit (resp, preinit) < 0)
+    return false;
+  ctx->conf = __resolv_conf_get (ctx->resp);
+  return true;
 }
 
 /* Allocate a new context object and initialize it.  The object is put
@@ -87,6 +111,7 @@ context_alloc (struct __res_state *resp)
   if (ctx == NULL)
     return NULL;
   ctx->resp = resp;
+  ctx->conf = __resolv_conf_get (resp);
   ctx->__refcount = 1;
   ctx->__from_res = true;
   ctx->__next = current;
@@ -98,8 +123,11 @@ context_alloc (struct __res_state *resp)
 static void
 context_free (struct resolv_context *ctx)
 {
+  int error_code = errno;
   current = ctx->__next;
+  __resolv_conf_put (ctx->conf);
   free (ctx);
+  __set_errno (error_code);
 }
 
 /* Reuse the current context object.  */
@@ -130,7 +158,7 @@ context_get (bool preinit)
   struct resolv_context *ctx = context_alloc (&_res);
   if (ctx == NULL)
     return NULL;
-  if (!maybe_init (ctx->resp, preinit))
+  if (!maybe_init (ctx, preinit))
     {
       context_free (ctx);
       return NULL;

resolv/resolv_context.h

@@ -40,15 +40,22 @@
 #ifndef _RESOLV_CONTEXT_H
 #define _RESOLV_CONTEXT_H
 
+#include <bits/types/res_state.h>
+#include <resolv/resolv_conf.h>
 #include <stdbool.h>
 #include <stddef.h>
-#include <bits/types/res_state.h>
 
 /* Temporary resolver state.  */
 struct resolv_context
 {
   struct __res_state *resp;     /* Backing resolver state.   */
 
+  /* Extended resolver state.  This is set to NULL if the
+     __resolv_context_get functions are unable to locate an associated
+     extended state.  In this case, the configuration data in *resp
+     has to be used; otherwise, the data from *conf should be
+     preferred (because it is a superset).  */
+  struct resolv_conf *conf;
 
   /* The following fields are for internal use within the
      resolv_context module.  */