x86-64: Check if mprotect works before rewriting PLT

Systemd execution environment configuration may prohibit changing a memory
mapping to become executable:

MemoryDenyWriteExecute=
Takes a boolean argument. If set, attempts to create memory mappings
that are writable and executable at the same time, or to change existing
memory mappings to become executable, or mapping shared memory segments
as executable, are prohibited.

When it is set, systemd service stops working if PLT rewrite is enabled.
Check if mprotect works before rewriting PLT.  This fixes BZ #31230.
This also works with SELinux when deny_execmem is on.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

sysdeps/unix/sysv/linux/x86_64/dl-plt-rewrite.h

@@ -0,0 +1,43 @@
+/* PLT rewrite helper function.  Linux/x86-64 version.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   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 <stdbool.h>
+#include <sys/mman.h>
+
+static __always_inline bool
+dl_plt_rewrite_supported (void)
+{
+  /* PLT rewrite is enabled.  Check if mprotect works.  */
+  void *plt = (void *) INTERNAL_SYSCALL_CALL (mmap, NULL, 4096,
+					      PROT_READ | PROT_WRITE,
+					      MAP_PRIVATE | MAP_ANONYMOUS,
+					      -1, 0);
+  if (__glibc_unlikely (plt == MAP_FAILED))
+    return false;
+
+  /* Touch the PROT_READ | PROT_WRITE page.  */
+  *(int32_t *) plt = 1;
+
+  /* If the updated PROT_READ | PROT_WRITE page can be changed to
+     PROT_EXEC | PROT_READ, rewrite PLT.  */
+  bool status = (INTERNAL_SYSCALL_CALL (mprotect, plt, 4096,
+					PROT_EXEC | PROT_READ) == 0);
+
+  INTERNAL_SYSCALL_CALL (munmap, plt, 4096);
+
+  return status;
+}

sysdeps/x86/cpu-features.c

@@ -28,10 +28,16 @@ extern void TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *)
   attribute_hidden;
 
 #if defined SHARED && defined __x86_64__
+# include <dl-plt-rewrite.h>
+
 static void
 TUNABLE_CALLBACK (set_plt_rewrite) (tunable_val_t *valp)
 {
-  if (valp->numval != 0)
+  /* We must be careful about where we put the call to
+     dl_plt_rewrite_supported() since it may generate
+     spurious SELinux log entries.  It should only be
+     attempted if the user requested a PLT rewrite.  */
+  if (valp->numval != 0 && dl_plt_rewrite_supported ())
     {
       /* Use JMPABS only on APX processors.  */
       const struct cpu_features *cpu_features = __get_cpu_features ();

sysdeps/x86_64/dl-plt-rewrite.h

@@ -0,0 +1,25 @@
+/* PLT rewrite helper function.  x86-64 version.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   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 <stdbool.h>
+#include <sys/mman.h>
+
+static __always_inline bool
+dl_plt_rewrite_supported (void)
+{
+  return true;
+}