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

Remove the LD_PREFER_MAP_32BIT_EXEC environment variable support since
the first PT_LOAD segment is no longer executable due to defaulting to
-z separate-code.

This fixes [BZ #28656].

Reviewed-by: Florian Weimer <fweimer@redhat.com>

NEWS

@@ -9,6 +9,10 @@ Version 2.35
 
 Major new features:
 
+* The LD_PREFER_MAP_32BIT_EXEC environment variable support has been
+  removed since the first PT_LOAD segment is no longer executable due
+  to defaulting to -z separate-code.
+
 * Unicode 14.0.0 Support: Character encoding, character type info, and
   transliteration tables are all updated to Unicode 14.0.0, using
   generator scripts contributed by Mike FABIAN (Red Hat).

sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h

@@ -1,45 +0,0 @@
-/* Optional code to distinguish library flavours.  x86-64 version.
-   Copyright (C) 2015-2021 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
-
-#include <sysdeps/unix/sysv/linux/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 */

sysdeps/unix/sysv/linux/x86_64/64/mmap_internal.h

@@ -1,42 +0,0 @@
-/* Linux mmap system call.  x86-64 version.
-   Copyright (C) 2015-2021 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 pages
-   with MAP_32BIT first.  */
-#define MMAP_PREPARE(addr, len, prot, flags, fd, offset)		\
-  if ((addr) == NULL							\
-      && ((prot) & PROT_EXEC) != 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

sysdeps/x86/cpu-tunables.c

@@ -258,13 +258,6 @@ 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

sysdeps/x86/include/cpu-features-preferred_feature_index_1.def

@@ -26,7 +26,6 @@ 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)