25 files changed, 947 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index d219e54..85a4ac3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,44 @@
+2016-12-31  Siddhesh Poyarekar  <siddhesh@sourceware.org>
+
+	* manual/install.texi: Add --enable-tunables option.
+	* INSTALL: Regenerate.
+	* README.tunables: New file.
+	* Makeconfig (CPPFLAGS): Define TOP_NAMESPACE.
+	(before-compile): Generate dl-tunable-list.h early.
+	* config.h.in: Add HAVE_TUNABLES.
+	* config.make.in: Add have-tunables.
+	* configure.ac: Add --enable-tunables option.
+	* configure: Regenerate.
+	* csu/init-first.c (__libc_init_first): Move
+	__libc_init_secure earlier...
+	* csu/init-first.c (LIBC_START_MAIN):... to here.
+	Include dl-tunables.h, libc-internal.h.
+	(LIBC_START_MAIN) [!SHARED]: Initialize tunables for static
+	binaries.
+	* elf/Makefile (dl-routines): Add dl-tunables.
+	* elf/Versions (ld): Add __tunable_set_val to GLIBC_PRIVATE
+	namespace.
+	* elf/dl-support (_dl_nondynamic_init): Unset MALLOC_CHECK_
+	only when !HAVE_TUNABLES.
+	* elf/rtld.c (process_envvars): Likewise.
+	* elf/dl-sysdep.c [HAVE_TUNABLES]: Include dl-tunables.h
+	(_dl_sysdep_start): Call __tunables_init.
+	* elf/dl-tunable-types.h: New file.
+	* elf/dl-tunables.c: New file.
+	* elf/dl-tunables.h: New file.
+	* elf/dl-tunables.list: New file.
+	* malloc/tst-malloc-usable-static.c: New test case.
+	* malloc/Makefile (tests-static): Add it.
+	* malloc/arena.c [HAVE_TUNABLES]: Include dl-tunables.h.
+	Define TUNABLE_NAMESPACE.
+	(DL_TUNABLE_CALLBACK (set_mallopt_check)): New function.
+	(DL_TUNABLE_CALLBACK_FNDECL): New macro.  Use it to define
+	callback functions.
+	(ptmalloc_init): Set tunable values.
+	* scripts/gen-tunables.awk: New file.
+	* sysdeps/mach/hurd/dl-sysdep.c: Include dl-tunables.h.
+	(_dl_sysdep_start): Call __tunables_init.
+
 2016-12-31  Florian Weimer  <fweimer@redhat.com>
 
 	* resolv/resolv.h (RES_BLAST): Deprecate.
diff --git a/INSTALL b/INSTALL
index 104f36b..25619fc 100644
--- a/INSTALL
+++ b/INSTALL
@@ -169,6 +169,11 @@ will be used, and CFLAGS sets optimization options for the compiler.
      By default for x86_64, the GNU C Library is built with the vector
      math library.  Use this option to disable the vector math library.
 
+'--enable-tunables'
+     Tunables support allows additional library parameters to be
+     customized at runtime.  This is an experimental feature and affects
+     startup time and is thus disabled by default.
+
 '--build=BUILD-SYSTEM'
 '--host=HOST-SYSTEM'
      These options are for cross-compiling.  If you specify both options
diff --git a/Makeconfig b/Makeconfig
index 0158eaa..b173e4c 100644
--- a/Makeconfig
+++ b/Makeconfig
@@ -934,6 +934,11 @@ CPPFLAGS = $(config-extra-cppflags) $(CPPUNDEFS) $(CPPFLAGS-config) \
 	   $(foreach lib,$(libof-$(basename $(@F))) \
 			 $(libof-$(<F)) $(libof-$(@F)),$(CPPFLAGS-$(lib))) \
 	   $(CPPFLAGS-$(<F)) $(CPPFLAGS-$(@F)) $(CPPFLAGS-$(basename $(@F)))
+
+ifeq (yes,$(have-tunables))
+CPPFLAGS += -DTOP_NAMESPACE=glibc
+endif
+
 override CFLAGS	= -std=gnu11 -fgnu89-inline $(config-extra-cflags) \
 		  $(filter-out %frame-pointer,$(+cflags)) $(+gccwarn-c) \
 		  $(sysdep-CFLAGS) $(CFLAGS-$(suffix $@)) $(CFLAGS-$(<F)) \
@@ -1108,6 +1113,17 @@ $(common-objpfx)libc-modules.stmp: $(..)scripts/gen-libc-modules.awk \
 
 endif
 
+# Build the tunables list header early since it could be used by any module in
+# glibc.
+ifeq (yes,$(have-tunables))
+before-compile += $(common-objpfx)dl-tunable-list.h
+
+$(common-objpfx)dl-tunable-list.h: $(..)scripts/gen-tunables.awk \
+				   $(..)elf/dl-tunables.list
+	$(AWK) -f $^ > $@.tmp
+	mv $@.tmp $@
+endif
+
 common-generated += libc-modules.h libc-modules.stmp
 
 # The name under which the run-time dynamic linker is installed.
diff --git a/README.tunables b/README.tunables
new file mode 100644
index 0000000..df74f3b
--- /dev/null
+++ b/README.tunables
@@ -0,0 +1,85 @@
+			TUNABLE FRAMEWORK
+			=================
+
+Tunables is a feature in the GNU C Library that allows application authors and
+distribution maintainers to alter the runtime library behaviour to match their
+workload.
+
+The tunable framework allows modules within glibc to register variables that
+may be tweaked through an environment variable.  It aims to enforce a strict
+namespace rule to bring consistency to naming of these tunable environment
+variables across the project.  This document is a guide for glibc developers to
+add tunables to the framework.
+
+ADDING A NEW TUNABLE
+--------------------
+
+The TOP_NAMESPACE macro is defined by default as 'glibc'.  If distributions
+intend to add their own tunables, they should do so in a different top
+namespace by overriding the TOP_NAMESPACE macro for that tunable.  Downstream
+implementations are discouraged from using the 'glibc' top namespace for
+tunables they don't already have consensus to push upstream.
+
+There are two steps to adding a tunable:
+
+1. Add a tunable ID:
+
+Modules that wish to use the tunables interface must define the
+TUNABLE_NAMESPACE macro.  Following this, for each tunable you want to
+add, make an entry in elf/dl-tunables.list.  The format of the file is as
+follows:
+
+TOP_NAMESPACE {
+  NAMESPACE1 {
+    TUNABLE1 {
+      # tunable attributes, one per line
+    }
+    # A tunable with default attributes, i.e. string variable.
+    TUNABLE2
+    TUNABLE3 {
+      # its attributes
+    }
+  }
+  NAMESPACE2 {
+    ...
+  }
+}
+
+The list of allowed attributes are:
+
+- type:			Data type.  Defaults to STRING.  Allowed types are:
+			INT_32, SIZE_T and STRING.
+
+- minval:		Optional minimum acceptable value.  For a string type
+			this is the minimum length of the value.
+
+- maxval:		Optional maximum acceptable value.  For a string type
+			this is the maximum length of the value.
+
+- env_alias:		An alias environment variable
+
+- is_secure:		Specify whether the tunable should be read for setuid
+			binaries.  True allows the tunable to be read for
+			setuid binaries while false disables it.  Note that
+			even if this is set as true and the value is read, it
+			may not be used if it does not validate against the
+			acceptable values or is not considered safe by the
+			module.
+
+2. Call either the TUNABLE_SET_VALUE and pass into it the tunable name and a
+   pointer to the variable that should be set with the tunable value.
+   If additional work needs to be done after setting the value, use the
+   TUNABLE_SET_VALUE_WITH_CALLBACK instead and additionally pass a pointer to
+   the function that should be called if the tunable value has been set.
+
+FUTURE WORK
+-----------
+
+The framework currently only allows a one-time initialization of variables
+through environment variables and in some cases, modification of variables via
+an API call.  A future goals for this project include:
+
+- Setting system-wide and user-wide defaults for tunables through some
+  mechanism like a configuration file.
+
+- Allow tweaking of some tunables at runtime
diff --git a/config.h.in b/config.h.in
index 82f95a6..7bfe923 100644
--- a/config.h.in
+++ b/config.h.in
@@ -256,4 +256,7 @@
 /* PowerPC32 uses fctidz for floating point to long long conversions.  */
 #define HAVE_PPC_FCTIDZ 0
 
+/* Build glibc with tunables support.  */
+#define HAVE_TUNABLES 0
+
 #endif
diff --git a/config.make.in b/config.make.in
index 4422025..2f8dae2 100644
--- a/config.make.in
+++ b/config.make.in
@@ -96,6 +96,7 @@ use-nscd = @use_nscd@
 build-hardcoded-path-in-tests= @hardcoded_path_in_tests@
 build-pt-chown = @build_pt_chown@
 enable-lock-elision = @enable_lock_elision@
+have-tunables = @have_tunables@
 
 # Build tools.
 CC = @CC@
diff --git a/configure b/configure
index c88f6fe..d80d738 100755
--- a/configure
+++ b/configure
@@ -666,6 +666,7 @@ libc_cv_ssp
 base_machine
 add_on_subdirs
 add_ons
+have_tunables
 build_pt_chown
 build_nscd
 link_obsolete_rpc
@@ -782,6 +783,7 @@ enable_systemtap
 enable_build_nscd
 enable_nscd
 enable_pt_chown
+enable_tunables
 enable_mathvec
 with_cpu
 '
@@ -1452,6 +1454,7 @@ Optional Features:
   --disable-build-nscd    disable building and installing the nscd daemon
   --disable-nscd          library functions will not contact the nscd daemon
   --enable-pt_chown       Enable building and installing pt_chown
+  --enable-tunables       Enable tunables support
   --enable-mathvec        Enable building and installing mathvec [default
                           depends on architecture]
 
@@ -3698,6 +3701,19 @@ if test "$build_pt_chown" = yes; then
 
 fi
 
+# Check whether --enable-tunables was given.
+if test "${enable_tunables+set}" = set; then :
+  enableval=$enable_tunables; have_tunables=$enableval
+else
+  have_tunables=no
+fi
+
+
+if test "$have_tunables" = yes; then
+  $as_echo "#define HAVE_TUNABLES 1" >>confdefs.h
+
+fi
+
 # The abi-tags file uses a fairly simplistic model for name recognition that
 # can't distinguish i486-pc-linux-gnu fully from i486-pc-gnu.  So we mutate a
 # $host_os of `gnu*' here to be `gnu-gnu*' just so that it can tell.
diff --git a/configure.ac b/configure.ac
index 2782bfa..22f5cab 100644
--- a/configure.ac
+++ b/configure.ac
@@ -421,6 +421,16 @@ if test "$build_pt_chown" = yes; then
   AC_DEFINE(HAVE_PT_CHOWN)
 fi
 
+AC_ARG_ENABLE([tunables],
+	      [AS_HELP_STRING([--enable-tunables],
+	       [Enable tunables support])],
+	      [have_tunables=$enableval],
+	      [have_tunables=no])
+AC_SUBST(have_tunables)
+if test "$have_tunables" = yes; then
+  AC_DEFINE(HAVE_TUNABLES)
+fi
+
 # The abi-tags file uses a fairly simplistic model for name recognition that
 # can't distinguish i486-pc-linux-gnu fully from i486-pc-gnu.  So we mutate a
 # $host_os of `gnu*' here to be `gnu-gnu*' just so that it can tell.
diff --git a/csu/init-first.c b/csu/init-first.c
index 77c6e1c..465f25b 100644
--- a/csu/init-first.c
+++ b/csu/init-first.c
@@ -72,8 +72,6 @@ _init (int argc, char **argv, char **envp)
   __environ = envp;
 
 #ifndef SHARED
-  __libc_init_secure ();
-
   /* First the initialization which normally would be done by the
      dynamic linker.  */
   _dl_non_dynamic_init ();
diff --git a/csu/libc-start.c b/csu/libc-start.c
index cc59073..15db9b4 100644
--- a/csu/libc-start.c
+++ b/csu/libc-start.c
@@ -21,6 +21,9 @@
 #include <unistd.h>
 #include <ldsodefs.h>
 #include <exit-thread.h>
+#include <libc-internal.h>
+
+#include <elf/dl-tunables.h>
 
 extern void __libc_init_first (int argc, char **argv, char **envp);
 
@@ -174,6 +177,11 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
         }
     }
 
+  /* Initialize very early so that tunables can use it.  */
+  __libc_init_secure ();
+
+  __tunables_init (__environ);
+
   /* Perform IREL{,A} relocations.  */
   apply_irel ();
 
diff --git a/elf/Makefile b/elf/Makefile
index 8a2ce02..de28d99 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -35,6 +35,11 @@ dl-routines	= $(addprefix dl-,load lookup object reloc deps hwcaps \
 ifeq (yes,$(use-ldconfig))
 dl-routines += dl-cache
 endif
+
+ifeq (yes,$(have-tunables))
+dl-routines += dl-tunables
+endif
+
 all-dl-routines = $(dl-routines) $(sysdep-dl-routines)
 # But they are absent from the shared libc, because that code is in ld.so.
 elide-routines.os = $(all-dl-routines) dl-support enbl-secure dl-origin \
diff --git a/elf/Versions b/elf/Versions
index 3d57e36..6abe9db 100644
--- a/elf/Versions
+++ b/elf/Versions
@@ -70,5 +70,8 @@ ld {
 
     # Internal error handling support.  Interposed by libc.so.
     _dl_signal_error; _dl_catch_error;
+
+    # Set value of a tunable.
+    __tunable_set_val;
   }
 }
diff --git a/elf/dl-support.c b/elf/dl-support.c
index c30194c..d350d6d 100644
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -354,8 +354,10 @@ _dl_non_dynamic_init (void)
 	  cp = (const char *) __rawmemchr (cp, '\0') + 1;
 	}
 
+#if !HAVE_TUNABLES
       if (__access ("/etc/suid-debug", F_OK) != 0)
 	__unsetenv ("MALLOC_CHECK_");
+#endif
     }
 
 #ifdef DL_PLATFORM_INIT
diff --git a/elf/dl-sysdep.c b/elf/dl-sysdep.c
index eaa7155..4283767 100644
--- a/elf/dl-sysdep.c
+++ b/elf/dl-sysdep.c
@@ -44,6 +44,8 @@
 #include <hp-timing.h>
 #include <tls.h>
 
+#include <dl-tunables.h>
+
 extern char **_environ attribute_hidden;
 extern char _end[] attribute_hidden;
 
@@ -219,6 +221,8 @@ _dl_sysdep_start (void **start_argptr,
     }
 #endif
 
+  __tunables_init (_environ);
+
 #ifdef DL_SYSDEP_INIT
   DL_SYSDEP_INIT;
 #endif
diff --git a/elf/dl-tunable-types.h b/elf/dl-tunable-types.h
new file mode 100644
index 0000000..d1591b6
--- /dev/null
+++ b/elf/dl-tunable-types.h
@@ -0,0 +1,46 @@
+/* Tunable type information.
+
+   Copyright (C) 2016 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 _TUNABLE_TYPES_H_
+# define _TUNABLE_TYPES_H_
+#include <stddef.h>
+
+typedef void (*tunable_callback_t) (void *);
+
+typedef enum
+{
+  TUNABLE_TYPE_INT_32,
+  TUNABLE_TYPE_SIZE_T,
+  TUNABLE_TYPE_STRING
+} tunable_type_code_t;
+
+typedef struct
+{
+  tunable_type_code_t type_code;
+  int64_t min;
+  int64_t max;
+} tunable_type_t;
+
+typedef union
+{
+  int64_t numval;
+  const char *strval;
+} tunable_val_t;
+
+#endif
diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
new file mode 100644
index 0000000..472747b
--- /dev/null
+++ b/elf/dl-tunables.c
@@ -0,0 +1,320 @@
+/* The tunable framework.  See the README.tunables to know how to use the
+   tunable in a glibc module.
+
+   Copyright (C) 2016 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 <stdint.h>
+#include <stdbool.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <libc-internal.h>
+#include <sysdep.h>
+#include <fcntl.h>
+#include <ldsodefs.h>
+
+#define TUNABLES_INTERNAL 1
+#include "dl-tunables.h"
+
+/* Compare environment names, bounded by the name hardcoded in glibc.  */
+static bool
+is_name (const char *orig, const char *envname)
+{
+  for (;*orig != '\0' && *envname != '\0'; envname++, orig++)
+    if (*orig != *envname)
+      break;
+
+  /* The ENVNAME is immediately followed by a value.  */
+  if (*orig == '\0' && *envname == '=')
+    return true;
+  else
+    return false;
+}
+
+static char **
+get_next_env (char **envp, char **name, size_t *namelen, char **val)
+{
+  while (envp != NULL && *envp != NULL)
+    {
+      char *envline = *envp;
+      int len = 0;
+
+      while (envline[len] != '\0' && envline[len] != '=')
+	len++;
+
+      /* Just the name and no value, go to the next one.  */
+      if (envline[len] == '\0')
+	continue;
+
+      *name = envline;
+      *namelen = len;
+      *val = &envline[len + 1];
+
+      return ++envp;
+    }
+
+  return NULL;
+}
+
+static int
+tunables_unsetenv (char **ep, const char *name)
+{
+  while (*ep != NULL)
+    {
+      size_t cnt = 0;
+
+      while ((*ep)[cnt] == name[cnt] && name[cnt] != '\0')
+	++cnt;
+
+      if (name[cnt] == '\0' && (*ep)[cnt] == '=')
+	{
+	  /* Found it.  Remove this pointer by moving later ones to
+	     the front.  */
+	  char **dp = ep;
+
+	  do
+	    dp[0] = dp[1];
+	  while (*dp++);
+	  /* Continue the loop in case NAME appears again.  */
+	}
+      else
+	++ep;
+    }
+
+  return 0;
+}
+
+/* A stripped down strtoul-like implementation for very early use.  It does not
+   set errno if the result is outside bounds because it gets called before
+   errno may have been set up.  */
+static unsigned long int
+tunables_strtoul (const char *nptr)
+{
+  unsigned long int result = 0;
+  long int sign = 1;
+  unsigned max_digit;
+
+  while (*nptr == ' ' || *nptr == '\t')
+    ++nptr;
+
+  if (*nptr == '-')
+    {
+      sign = -1;
+      ++nptr;
+    }
+  else if (*nptr == '+')
+    ++nptr;
+
+  if (*nptr < '0' || *nptr > '9')
+    return 0UL;
+
+  int base = 10;
+  max_digit = 9;
+  if (*nptr == '0')
+    {
+      if (nptr[1] == 'x' || nptr[1] == 'X')
+	{
+	  base = 16;
+	  nptr += 2;
+	}
+      else
+	{
+	  base = 8;
+	  max_digit = 7;
+	}
+    }
+
+  while (1)
+    {
+      unsigned long int digval;
+      if (*nptr >= '0' && *nptr <= '0' + max_digit)
+        digval = *nptr - '0';
+      else if (base == 16)
+        {
+	  if (*nptr >= 'a' && *nptr <= 'f')
+	    digval = *nptr - 'a' + 10;
+	  else if (*nptr >= 'A' && *nptr <= 'F')
+	    digval = *nptr - 'A' + 10;
+	  else
+	    break;
+	}
+      else
+        break;
+
+      if (result > ULONG_MAX / base
+	  || (result == ULONG_MAX / base && digval > ULONG_MAX % base))
+	return ULONG_MAX;
+      result *= base;
+      result += digval;
+      ++nptr;
+    }
+
+  return result * sign;
+}
+
+/* Initialize the internal type if the value validates either using the
+   explicit constraints of the tunable or with the implicit constraints of its
+   type.  */
+static void
+tunable_set_val_if_valid_range (tunable_t *cur, const char *strval,
+				int64_t default_min, int64_t default_max)
+{
+  int64_t val = tunables_strtoul (strval);
+
+  int64_t min = cur->type.min;
+  int64_t max = cur->type.max;
+
+  if (min == max)
+    {
+      min = default_min;
+      max = default_max;
+    }
+
+  if (val >= min && val <= max)
+    {
+      cur->val.numval = val;
+      cur->strval = strval;
+    }
+}
+
+/* Validate range of the input value and initialize the tunable CUR if it looks
+   good.  */
+static void
+tunable_initialize (tunable_t *cur, const char *strval)
+{
+  switch (cur->type.type_code)
+    {
+    case TUNABLE_TYPE_INT_32:
+	{
+	  tunable_set_val_if_valid_range (cur, strval, INT32_MIN, INT32_MAX);
+	  break;
+	}
+    case TUNABLE_TYPE_SIZE_T:
+	{
+	  tunable_set_val_if_valid_range (cur, strval, 0, SIZE_MAX);
+	  break;
+	}
+    case TUNABLE_TYPE_STRING:
+	{
+	  cur->val.strval = cur->strval = strval;
+	  break;
+	}
+    default:
+      __builtin_unreachable ();
+    }
+}
+
+/* Disable a tunable if it is set.  */
+static void
+disable_tunable (tunable_id_t id, char **envp)
+{
+  const char *env_alias = tunable_list[id].env_alias;
+
+  if (env_alias != NULL)
+    tunables_unsetenv (envp, tunable_list[id].env_alias);
+}
+
+/* Disable the glibc.malloc.check tunable in SETUID/SETGID programs unless
+   the system administrator overrides it by creating the /etc/suid-debug
+   file.  This is a special case where we want to conditionally enable/disable
+   a tunable even for setuid binaries.  We use the special version of access()
+   to avoid setting ERRNO, which is a TLS variable since TLS has not yet been
+   set up.  */
+static inline void
+__always_inline
+maybe_disable_malloc_check (void)
+{
+  if (__libc_enable_secure && __access_noerrno ("/etc/suid-debug", F_OK) != 0)
+    disable_tunable (TUNABLE_ENUM_NAME(glibc, malloc, check), __environ);
+}
+
+/* Initialize the tunables list from the environment.  For now we only use the
+   ENV_ALIAS to find values.  Later we will also use the tunable names to find
+   values.  */
+void
+__tunables_init (char **envp)
+{
+  char *envname = NULL;
+  char *envval = NULL;
+  size_t len = 0;
+
+  maybe_disable_malloc_check ();
+
+  while ((envp = get_next_env (envp, &envname, &len, &envval)) != NULL)
+    {
+      for (int i = 0; i < sizeof (tunable_list) / sizeof (tunable_t); i++)
+	{
+	  tunable_t *cur = &tunable_list[i];
+
+	  /* Skip over tunables that have either been set already or should be
+	     skipped.  */
+	  if (cur->strval != NULL || cur->env_alias == NULL
+	      || (__libc_enable_secure && !cur->is_secure))
+	    continue;
+
+	  const char *name = cur->env_alias;
+
+	  /* We have a match.  Initialize and move on to the next line.  */
+	  if (is_name (name, envname))
+	    {
+	      tunable_initialize (cur, envval);
+	      break;
+	    }
+	}
+    }
+}
+
+/* Set the tunable value.  This is called by the module that the tunable exists
+   in. */
+void
+__tunable_set_val (tunable_id_t id, void *valp, tunable_callback_t callback)
+{
+  tunable_t *cur = &tunable_list[id];
+
+  /* Don't do anything if our tunable was not set during initialization or if
+     it failed validation.  */
+  if (cur->strval == NULL)
+    return;
+
+  if (valp == NULL)
+    goto cb;
+
+  switch (cur->type.type_code)
+    {
+    case TUNABLE_TYPE_INT_32:
+	{
+	  *((int32_t *) valp) = (int32_t) cur->val.numval;
+	  break;
+	}
+    case TUNABLE_TYPE_SIZE_T:
+	{
+	  *((size_t *) valp) = (size_t) cur->val.numval;
+	  break;
+	}
+    case TUNABLE_TYPE_STRING:
+	{
+	  *((const char **)valp) = cur->val.strval;
+	  break;
+	}
+    default:
+      __builtin_unreachable ();
+    }
+
+cb:
+  if (callback)
+    callback (&cur->val);
+}
diff --git a/elf/dl-tunables.h b/elf/dl-tunables.h
new file mode 100644
index 0000000..a3f5472
--- /dev/null
+++ b/elf/dl-tunables.h
@@ -0,0 +1,88 @@
+/* The tunable framework.  See the README to know how to use the tunable in
+   a glibc module.
+
+   Copyright (C) 2016 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 _TUNABLES_H_
+#define _TUNABLES_H_
+
+#if !HAVE_TUNABLES
+static inline void
+__always_inline
+__tunables_init (char **unused __attribute_unused)
+{
+  /* This is optimized out if tunables are not enabled.  */
+}
+#else
+
+# include <stddef.h>
+# include "dl-tunable-types.h"
+
+/* A tunable.  */
+struct _tunable
+{
+  const char *name;			/* Internal name of the tunable.  */
+  tunable_type_t type;			/* Data type of the tunable.  */
+  tunable_val_t val;			/* The value.  */
+  const char *strval;			/* The string containing the value,
+					   points into envp.  */
+  bool is_secure;			/* Whether the tunable must be read
+					   even for setuid binaries.  Note that
+					   even if the tunable is read, it may
+					   not get used by the target module if
+					   the value is considered unsafe.  */
+  /* Compatibility elements.  */
+  const char *env_alias;		/* The compatibility environment
+					   variable name.  */
+};
+
+typedef struct _tunable tunable_t;
+
+/* Full name for a tunable is top_ns.tunable_ns.id.  */
+# define TUNABLE_NAME_S(top,ns,id) #top "." #ns "." #id
+
+# define TUNABLE_ENUM_NAME(top,ns,id) TUNABLE_ENUM_NAME1 (top,ns,id)
+# define TUNABLE_ENUM_NAME1(top,ns,id) top ## _ ## ns ## _ ## id
+
+# include "dl-tunable-list.h"
+
+extern void __tunables_init (char **);
+extern void __tunable_set_val (tunable_id_t, void *, tunable_callback_t);
+
+/* Check if the tunable has been set to a non-default value and if it is, copy
+   it over into __VAL.  */
+# define TUNABLE_SET_VAL(__id,__val) \
+({									      \
+  __tunable_set_val							      \
+   (TUNABLE_ENUM_NAME (TOP_NAMESPACE, TUNABLE_NAMESPACE, __id), (__val),      \
+    NULL);								      \
+})
+
+/* Same as TUNABLE_SET_VAL, but also call the callback function __CB.  */
+# define TUNABLE_SET_VAL_WITH_CALLBACK(__id,__val,__cb) \
+({									      \
+  __tunable_set_val							      \
+   (TUNABLE_ENUM_NAME (TOP_NAMESPACE, TUNABLE_NAMESPACE, __id), (__val),      \
+    DL_TUNABLE_CALLBACK (__cb));					      \
+})
+
+/* Namespace sanity for callback functions.  Use this macro to keep the
+   namespace of the modules clean.  */
+# define DL_TUNABLE_CALLBACK(__name) _dl_tunable_ ## __name
+#endif
+#endif
diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
new file mode 100644
index 0000000..11504c4
--- /dev/null
+++ b/elf/dl-tunables.list
@@ -0,0 +1,69 @@
+# Copyright (C) 2016 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/>.
+
+# Allowed attributes for tunables:
+#
+# type: Defaults to STRING
+# minval: Optional minimum acceptable value
+# maxval: Optional maximum acceptable value
+# env_alias: An alias environment variable
+# is_secure: Specify whether the environment variable should be read for
+# setuid binaries.
+
+glibc {
+  malloc {
+    check {
+      type: INT_32
+      minval: 0
+      maxval: 3
+      env_alias: MALLOC_CHECK_
+      is_secure: true
+    }
+    top_pad {
+      type: SIZE_T
+      env_alias: MALLOC_TOP_PAD_
+    }
+    perturb {
+      type: INT_32
+      minval: 0
+      maxval: 0xff
+      env_alias: MALLOC_PERTURB_
+    }
+    mmap_threshold {
+      type: SIZE_T
+      env_alias: MALLOC_MMAP_THRESHOLD_
+    }
+    trim_threshold {
+      type: SIZE_T
+      env_alias: MALLOC_TRIM_THRESHOLD_
+    }
+    mmap_max {
+      type: INT_32
+      env_alias: MALLOC_MMAP_MAX_
+    }
+    arena_max {
+      type: SIZE_T
+      env_alias: MALLOC_ARENA_MAX
+      minval: 1
+    }
+    arena_test {
+      type: SIZE_T
+      env_alias: MALLOC_ARENA_TEST
+      minval: 1
+    }
+  }
+}
diff --git a/elf/rtld.c b/elf/rtld.c
index 4ec25d7..a60ead6 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -2510,7 +2510,9 @@ process_envvars (enum mode *modep)
 
       if (__access ("/etc/suid-debug", F_OK) != 0)
 	{
+#if !HAVE_TUNABLES
 	  unsetenv ("MALLOC_CHECK_");
+#endif
 	  GLRO(dl_debug_mask) = 0;
 	}
 
diff --git a/malloc/Makefile b/malloc/Makefile
index b8efcd6..4e4104e 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -37,6 +37,7 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
 tests-static := \
 	 tst-interpose-static-nothread \
 	 tst-interpose-static-thread \
+	 tst-malloc-usable-static
 
 tests += $(tests-static)
 test-srcs = tst-mtrace
@@ -158,6 +159,7 @@ endif
 
 tst-mcheck-ENV = MALLOC_CHECK_=3
 tst-malloc-usable-ENV = MALLOC_CHECK_=3
+tst-malloc-usable-static-ENV = $(tst-malloc-usable-ENV)
 
 # Uncomment this for test releases.  For public releases it is too expensive.
 #CPPFLAGS-malloc.o += -DMALLOC_DEBUG=1
diff --git a/malloc/arena.c b/malloc/arena.c
index eed4247..234035f 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -19,6 +19,11 @@
 
 #include <stdbool.h>
 
+#if HAVE_TUNABLES
+# define TUNABLE_NAMESPACE malloc
+#endif
+#include <elf/dl-tunables.h>
+
 /* Compile-time constants.  */
 
 #define HEAP_MIN_SIZE (32 * 1024)
@@ -204,6 +209,34 @@ __malloc_fork_unlock_child (void)
   __libc_lock_init (list_lock);
 }
 
+#if HAVE_TUNABLES
+static inline int do_set_mallopt_check (int32_t value);
+void
+DL_TUNABLE_CALLBACK (set_mallopt_check) (void *valp)
+{
+  int32_t value = *(int32_t *) valp;
+  do_set_mallopt_check (value);
+  if (check_action != 0)
+    __malloc_check_init ();
+}
+
+# define DL_TUNABLE_CALLBACK_FNDECL(__name, __type) \
+static inline int do_ ## __name (__type value);				      \
+void									      \
+DL_TUNABLE_CALLBACK (__name) (void *valp)				      \
+{									      \
+  __type value = *(__type *) valp;					      \
+  do_ ## __name (value);						      \
+}
+
+DL_TUNABLE_CALLBACK_FNDECL (set_mmap_threshold, size_t)
+DL_TUNABLE_CALLBACK_FNDECL (set_mmaps_max, int32_t)
+DL_TUNABLE_CALLBACK_FNDECL (set_top_pad, size_t)
+DL_TUNABLE_CALLBACK_FNDECL (set_perturb_byte, int32_t)
+DL_TUNABLE_CALLBACK_FNDECL (set_trim_threshold, size_t)
+DL_TUNABLE_CALLBACK_FNDECL (set_arena_max, size_t)
+DL_TUNABLE_CALLBACK_FNDECL (set_arena_test, size_t)
+#else
 /* Initialization routine. */
 #include <string.h>
 extern char **_environ;
@@ -238,6 +271,7 @@ next_env_entry (char ***position)
 
   return result;
 }
+#endif
 
 
 #ifdef SHARED
@@ -272,6 +306,24 @@ ptmalloc_init (void)
 #endif
 
   thread_arena = &main_arena;
+
+#if HAVE_TUNABLES
+  /* Ensure initialization/consolidation and do it under a lock so that a
+     thread attempting to use the arena in parallel waits on us till we
+     finish.  */
+  __libc_lock_lock (main_arena.mutex);
+  malloc_consolidate (&main_arena);
+
+  TUNABLE_SET_VAL_WITH_CALLBACK (check, NULL, set_mallopt_check);
+  TUNABLE_SET_VAL_WITH_CALLBACK (top_pad, NULL, set_top_pad);
+  TUNABLE_SET_VAL_WITH_CALLBACK (perturb, NULL, set_perturb_byte);
+  TUNABLE_SET_VAL_WITH_CALLBACK (mmap_threshold, NULL, set_mmap_threshold);
+  TUNABLE_SET_VAL_WITH_CALLBACK (trim_threshold, NULL, set_trim_threshold);
+  TUNABLE_SET_VAL_WITH_CALLBACK (mmap_max, NULL, set_mmaps_max);
+  TUNABLE_SET_VAL_WITH_CALLBACK (arena_max, NULL, set_arena_max);
+  TUNABLE_SET_VAL_WITH_CALLBACK (arena_test, NULL, set_arena_test);
+  __libc_lock_unlock (main_arena.mutex);
+#else
   const char *s = NULL;
   if (__glibc_likely (_environ != NULL))
     {
@@ -340,6 +392,8 @@ ptmalloc_init (void)
       if (check_action != 0)
         __malloc_check_init ();
     }
+#endif
+
 #if HAVE_MALLOC_INIT_HOOK
   void (*hook) (void) = atomic_forced_read (__malloc_initialize_hook);
   if (hook != NULL)
diff --git a/malloc/tst-malloc-usable-static.c b/malloc/tst-malloc-usable-static.c
new file mode 100644
index 0000000..8907db0
--- /dev/null
+++ b/malloc/tst-malloc-usable-static.c
@@ -0,0 +1 @@
+#include <malloc/tst-malloc-usable.c>
diff --git a/manual/install.texi b/manual/install.texi
index d02e870..d412962 100644
--- a/manual/install.texi
+++ b/manual/install.texi
@@ -200,6 +200,11 @@ configure with @option{--disable-werror}.
 By default for x86_64, @theglibc{} is built with the vector math library.
 Use this option to disable the vector math library.
 
+@item --enable-tunables
+Tunables support allows additional library parameters to be customized at
+runtime.  This is an experimental feature and affects startup time and is thus
+disabled by default.
+
 @item --build=@var{build-system}
 @itemx --host=@var{host-system}
 These options are for cross-compiling.  If you specify both options and
diff --git a/scripts/gen-tunables.awk b/scripts/gen-tunables.awk
new file mode 100644
index 0000000..b65b5a4
--- /dev/null
+++ b/scripts/gen-tunables.awk
@@ -0,0 +1,157 @@
+# Generate dl-tunable-list.h from dl-tunables.list
+
+BEGIN {
+  tunable=""
+  ns=""
+  top_ns=""
+}
+
+# Skip over blank lines and comments.
+/^#/ {
+  next
+}
+
+/^[ \t]*$/ {
+  next
+}
+
+# Beginning of either a top namespace, tunable namespace or a tunable, decided
+# on the current value of TUNABLE, NS or TOP_NS.
+$2 == "{" {
+  if (top_ns == "") {
+    top_ns = $1
+  }
+  else if (ns == "") {
+    ns = $1
+  }
+  else if (tunable == "") {
+    tunable = $1
+  }
+  else {
+    printf ("Unexpected occurrence of '{': %s:%d\n", FILENAME, FNR)
+    exit 1
+  }
+
+  next
+}
+
+# End of either a top namespace, tunable namespace or a tunable.
+$1 == "}" {
+  if (tunable != "") {
+    # Tunables definition ended, now fill in default attributes.
+    if (!types[top_ns][ns][tunable]) {
+      types[top_ns][ns][tunable] = "STRING"
+    }
+    if (!minvals[top_ns][ns][tunable]) {
+      minvals[top_ns][ns][tunable] = "0"
+    }
+    if (!maxvals[top_ns][ns][tunable]) {
+      maxvals[top_ns][ns][tunable] = "0"
+    }
+    if (!env_alias[top_ns][ns][tunable]) {
+      env_alias[top_ns][ns][tunable] = "NULL"
+    }
+    if (!is_secure[top_ns][ns][tunable]) {
+      is_secure[top_ns][ns][tunable] = "false"
+    }
+
+    tunable = ""
+  }
+  else if (ns != "") {
+    ns = ""
+  }
+  else if (top_ns != "") {
+    top_ns = ""
+  }
+  else {
+    printf ("syntax error: extra }: %s:%d\n", FILENAME, FNR)
+    exit 1
+  }
+  next
+}
+
+# Everything else, which could either be a tunable without any attributes or a
+# tunable attribute.
+{
+  if (ns == "") {
+    printf("Line %d: Invalid tunable outside a namespace: %s\n", NR, $0)
+    exit 1
+  }
+
+  if (tunable == "") {
+    # We encountered a tunable without any attributes, so note it with a
+    # default.
+    types[top_ns][ns][$1] = "STRING"
+    next
+  }
+
+  # Otherwise, we have encountered a tunable attribute.
+  split($0, arr, ":")
+  attr = gensub(/^[ \t]+|[ \t]+$/, "", "g", arr[1])
+  val = gensub(/^[ \t]+|[ \t]+$/, "", "g", arr[2])
+
+  if (attr == "type") {
+    types[top_ns][ns][tunable] = val
+  }
+  else if (attr == "minval") {
+    minvals[top_ns][ns][tunable] = val
+  }
+  else if (attr == "maxval") {
+    maxvals[top_ns][ns][tunable] = val
+  }
+  else if (attr == "env_alias") {
+    env_alias[top_ns][ns][tunable] = sprintf("\"%s\"", val)
+  }
+  else if (attr == "is_secure") {
+    if (val == "true" || val == "false") {
+      is_secure[top_ns][ns][tunable] = val
+    }
+    else {
+      printf("Line %d: Invalid value (%s) for is_secure: %s, ", NR, val,
+	     $0)
+      print("Allowed values are 'true' or 'false'")
+      exit 1
+    }
+  }
+}
+
+END {
+  if (ns != "") {
+    print "Unterminated namespace.  Is a closing brace missing?"
+    exit 1
+  }
+
+  print "/* AUTOGENERATED by gen-tunables.awk.  */"
+  print "#ifndef _TUNABLES_H_"
+  print "# error \"Do not include this file directly.\""
+  print "# error \"Include tunables.h instead.\""
+  print "#endif"
+
+  # Now, the enum names
+  print "\ntypedef enum"
+  print "{"
+  for (t in types) {
+    for (n in types[t]) {
+      for (m in types[t][n]) {
+        printf ("  TUNABLE_ENUM_NAME(%s, %s, %s),\n", t, n, m);
+      }
+    }
+  }
+  print "} tunable_id_t;\n"
+
+  # Finally, the tunable list.
+  print "\n#ifdef TUNABLES_INTERNAL"
+  print "static tunable_t tunable_list[] = {"
+  for (t in types) {
+    for (n in types[t]) {
+      for (m in types[t][n]) {
+        printf ("  {TUNABLE_NAME_S(%s, %s, %s)", t, n, m)
+        printf (", {TUNABLE_TYPE_%s, %s, %s}, {.numval = 0}, NULL, %s, %s},\n",
+		types[t][n][m], minvals[t][n][m], maxvals[t][n][m],
+		is_secure[t][n][m], env_alias[t][n][m]);
+      }
+    }
+  }
+  print "};"
+  print "#endif"
+}
diff --git a/sysdeps/mach/hurd/dl-sysdep.c b/sysdeps/mach/hurd/dl-sysdep.c
index d730f82..42bccda 100644
--- a/sysdeps/mach/hurd/dl-sysdep.c
+++ b/sysdeps/mach/hurd/dl-sysdep.c
@@ -44,6 +44,8 @@
 #include <dl-machine.h>
 #include <dl-procinfo.h>
 
+#include <dl-tunables.h>
+
 extern void __mach_init (void);
 
 extern int _dl_argc;
@@ -143,6 +145,8 @@ _dl_sysdep_start (void **start_argptr,
 
       __libc_enable_secure = _dl_hurd_data->flags & EXEC_SECURE;
 
+      __tunables_init (_environ);
+
       if (_dl_hurd_data->flags & EXEC_STACK_ARGS &&
 	  _dl_hurd_data->user_entry == 0)
 	_dl_hurd_data->user_entry = (vm_address_t) ENTRY_POINT;