x86-64: Restore LD_PREFER_MAP_32BIT_EXEC support [BZ #28656]

diff --git a/elf/dl-support.c b/elf/dl-support.c
index 9714f75db0..39447d2fad 100644
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -35,6 +35,7 @@ 
 #include <dl-machine.h>
 #include <libc-lock.h>
 #include <dl-cache.h>
+#include <dl-librecon.h>
 #include <dl-procinfo.h>
 #include <unsecvars.h>
 #include <hp-timing.h>
@@ -320,6 +321,9 @@  _dl_non_dynamic_init (void)
     {
       static const char unsecure_envvars[] =
 	UNSECURE_ENVVARS
+#ifdef EXTRA_UNSECURE_ENVVARS
+	EXTRA_UNSECURE_ENVVARS
+#endif
 	;
       const char *cp = unsecure_envvars;
 
diff --git a/elf/rtld.c b/elf/rtld.c
index b8467f37cf..dd7952efe4 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -32,6 +32,7 @@ 
 #include <fpu_control.h>
 #include <hp-timing.h>
 #include <libc-lock.h>
+#include <dl-librecon.h>
 #include <unsecvars.h>
 #include <dl-cache.h>
 #include <dl-osinfo.h>
@@ -2679,6 +2680,14 @@  process_envvars (struct dl_main_state *state)
 		= _dl_strtoul (&envline[21], NULL) > 1;
 	    }
 	  break;
+
+	  /* We might have some extra environment variable to handle.  This
+	     is tricky due to the pre-processing of the length of the name
+	     in the switch statement here.  The code here assumes that added
+	     environment variables have a different length.  */
+#ifdef EXTRA_LD_ENVVARS
+	  EXTRA_LD_ENVVARS
+#endif
 	}
     }
 
@@ -2686,7 +2695,14 @@  process_envvars (struct dl_main_state *state)
      variables.  */
   if (__glibc_unlikely (__libc_enable_secure))
     {
-      const char *nextp = UNSECURE_ENVVARS;
+      static const char unsecure_envvars[] =
+#ifdef EXTRA_UNSECURE_ENVVARS
+	EXTRA_UNSECURE_ENVVARS
+#endif
+	UNSECURE_ENVVARS;
+      const char *nextp;
+
+      nextp = unsecure_envvars;
       do
 	{
 	  unsetenv (nextp);
diff --git a/sysdeps/generic/dl-librecon.h b/sysdeps/generic/dl-librecon.h
new file mode 100644
index 0000000000..823b200cb8
--- /dev/null
+++ b/sysdeps/generic/dl-librecon.h
@@ -0,0 +1,24 @@ 
+/* Optional code to distinguish library flavours.
+   Copyright (C) 1998-2023 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 _DL_LIBRECON_H
+#define _DL_LIBRECON_H	1
+
+/* In the general case we don't do anything.  */
+
+#endif /* dl-librecon.h */
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/Makefile b/sysdeps/unix/sysv/linux/x86_64/64/Makefile
index a7b6dc5a53..53a401e35e 100644
--- a/sysdeps/unix/sysv/linux/x86_64/64/Makefile
+++ b/sysdeps/unix/sysv/linux/x86_64/64/Makefile
@@ -1,2 +1,21 @@ 
 # The default ABI is 64.
 default-abi := 64
+
+ifeq ($(subdir),elf)
+
+tests-map-32bit = \
+  tst-map-32bit \
+# tests-map-32bit
+tst-map-32bit-no-pie = yes
+tests += $(tests-map-32bit)
+
+modules-map-32bit = \
+  tst-map-32bit-mod \
+# modules-map-32bit
+modules-names += $(modules-map-32bit)
+
+tst-map-32bit-ENV = LD_PREFER_MAP_32BIT_EXEC=1
+$(objpfx)tst-map-32bit-mod.so: $(libsupport)
+$(objpfx)tst-map-32bit: $(objpfx)tst-map-32bit-mod.so
+
+endif
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h b/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
new file mode 100644
index 0000000000..8e6ca0d999
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
@@ -0,0 +1,43 @@ 
+/* Optional code to distinguish library flavours.  x86-64 version.
+   Copyright (C) 2015-2023 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 _DL_LIBRECON_H
+
+/* Recognizing extra environment variables.  For 64-bit applications,
+   branch prediction performance may be negatively impacted when the
+   target of a branch is more than 4GB away from the branch.  Add the
+   Prefer_MAP_32BIT_EXEC bit so that mmap will try to map executable
+   pages with MAP_32BIT first.  NB: MAP_32BIT will map to lower 2GB,
+   not lower 4GB, address.  Prefer_MAP_32BIT_EXEC reduces bits available
+   for address space layout randomization (ASLR).  Prefer_MAP_32BIT_EXEC
+   is always disabled for SUID programs and can be enabled by setting
+   environment variable, LD_PREFER_MAP_32BIT_EXEC.  */
+#define EXTRA_LD_ENVVARS \
+  case 21:								  \
+    if (!__libc_enable_secure						  \
+	&& memcmp (envline, "PREFER_MAP_32BIT_EXEC", 21) == 0)		  \
+      GLRO(dl_x86_cpu_features).preferred[index_arch_Prefer_MAP_32BIT_EXEC] \
+	|= bit_arch_Prefer_MAP_32BIT_EXEC;				  \
+    break;
+
+/* Extra unsecure variables.  The names are all stuffed in a single
+   string which means they have to be terminated with a '\0' explicitly.  */
+#define EXTRA_UNSECURE_ENVVARS \
+  "LD_PREFER_MAP_32BIT_EXEC\0"
+
+#endif /* dl-librecon.h */
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/mmap_internal.h b/sysdeps/unix/sysv/linux/x86_64/64/mmap_internal.h
new file mode 100644
index 0000000000..33dec3f805
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86_64/64/mmap_internal.h
@@ -0,0 +1,43 @@ 
+/* Linux mmap system call.  x86-64 version.
+   Copyright (C) 2015-2023 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 MMAP_X86_64_INTERNAL_H
+#define MMAP_X86_64_INTERNAL_H
+
+#include <ldsodefs.h>
+
+/* If the Prefer_MAP_32BIT_EXEC bit is set, try to map executable or
+    denywrite pages with MAP_32BIT first.  */
+#define MMAP_PREPARE(addr, len, prot, flags, fd, offset)		\
+  if ((addr) == NULL							\
+      && (((prot) & PROT_EXEC) != 0					\
+	  || ((flags) & MAP_DENYWRITE) != 0)				\
+      && HAS_ARCH_FEATURE (Prefer_MAP_32BIT_EXEC))			\
+    {									\
+      void *ret = (void*) INLINE_SYSCALL_CALL (mmap, (addr), (len),	\
+					      (prot),			\
+					      (flags) | MAP_32BIT,	\
+					      (fd), (offset));		\
+      if (ret != MAP_FAILED)						\
+	return ret;							\
+    }
+
+#include_next <mmap_internal.h>
+
+#endif
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/tst-map-32bit-mod.c b/sysdeps/unix/sysv/linux/x86_64/64/tst-map-32bit-mod.c
new file mode 100644
index 0000000000..78d4b6133c
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86_64/64/tst-map-32bit-mod.c
@@ -0,0 +1,33 @@ 
+/* Check that LD_PREFER_MAP_32BIT_EXEC works in shared library.
+   Copyright (C) 2023 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 <stdio.h>
+#include <stdint.h>
+#include <support/check.h>
+
+static void
+dso_do_test (void)
+{
+}
+
+void
+dso_check_map_32bit (void)
+{
+  printf ("dso_do_test: %p\n", dso_do_test);
+  TEST_VERIFY ((uintptr_t) dso_do_test < 0xffffffffUL);
+}
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/tst-map-32bit.c b/sysdeps/unix/sysv/linux/x86_64/64/tst-map-32bit.c
new file mode 100644
index 0000000000..abc396589e
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86_64/64/tst-map-32bit.c
@@ -0,0 +1,34 @@ 
+/* Check that LD_PREFER_MAP_32BIT_EXEC works in PDE and shared library.
+   Copyright (C) 2023 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 <stdio.h>
+#include <stdint.h>
+#include <support/check.h>
+
+extern void dso_check_map_32bit (void);
+
+static int
+do_test (void)
+{
+  printf ("do_test: %p\n", do_test);
+  TEST_VERIFY ((uintptr_t) do_test < 0xffffffffUL);
+  dso_check_map_32bit ();
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/x86/cpu-tunables.c b/sysdeps/x86/cpu-tunables.c
index d3e1367bda..a381b5249c 100644
--- a/sysdeps/x86/cpu-tunables.c
+++ b/sysdeps/x86/cpu-tunables.c
@@ -259,6 +259,13 @@  TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 		 20);
 	    }
 	  break;
+	case 21:
+	    {
+	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features,
+						Prefer_MAP_32BIT_EXEC,
+						disable, 21);
+	    }
+	  break;
 	case 23:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH
diff --git a/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def b/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
index e45f9cb159..d20c5b3196 100644
--- a/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
+++ b/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
@@ -26,6 +26,7 @@  BIT (I586)
 BIT (I686)
 BIT (Slow_SSE4_2)
 BIT (AVX_Fast_Unaligned_Load)
+BIT (Prefer_MAP_32BIT_EXEC)
 BIT (Prefer_No_VZEROUPPER)
 BIT (Prefer_ERMS)
 BIT (Prefer_No_AVX512)