mips: Force RWX stack for hard-float builds that can run on pre-4.8 kernels

Linux/Mips kernels prior to 4.8 could potentially crash the user
process when doing FPU emulation while running on non-executable
user stack.

Currently, gcc doesn't emit .note.GNU-stack for mips, but that will
change in the future. To ensure that glibc can be used with such
future gcc, without silently resulting in binaries that might crash
in runtime, this patch forces RWX stack for all built objects if
configured to run against minimum kernel version less than 4.8.

	* sysdeps/unix/sysv/linux/mips/Makefile
	(test-xfail-check-execstack):
	Move under mips-has-gnustack != yes.
	(CFLAGS-.o*, ASFLAGS-.o*): New rules.
	Apply -Wa,-execstack if mips-force-execstack == yes.
	* sysdeps/unix/sysv/linux/mips/configure: Regenerated.
	* sysdeps/unix/sysv/linux/mips/configure.ac
	(mips-force-execstack): New var.
	Set to yes for hard-float builds with minimum_kernel < 4.8.0
	or minimum_kernel not set at all.
	(mips-has-gnustack): New var.
	Use value of libc_cv_as_noexecstack
	if mips-force-execstack != yes, otherwise set to no.

3 files changed, 89 insertions, 5 deletions

diff --git a/sysdeps/unix/sysv/linux/mips/Makefile b/sysdeps/unix/sysv/linux/mips/Makefile
index 8217f42..03044e7 100644
--- a/sysdeps/unix/sysv/linux/mips/Makefile
+++ b/sysdeps/unix/sysv/linux/mips/Makefile
@@ -63,14 +63,25 @@ sysdep-dl-routines += dl-static
 
 sysdep_routines += dl-vdso
 endif
-
-# Supporting non-executable stacks on MIPS requires changes to both
-# the Linux kernel and glibc.  See
-# <https://sourceware.org/ml/libc-alpha/2016-01/msg00567.html> and
-# <https://sourceware.org/ml/libc-alpha/2016-01/msg00719.html>.
+# If the compiler doesn't use GNU.stack note,
+# this test is expected to fail.
+ifneq ($(mips-has-gnustack),yes)
 test-xfail-check-execstack = yes
 endif
+endif
 
 ifeq ($(subdir),stdlib)
 gen-as-const-headers += ucontext_i.sym
 endif
+
+ifeq ($(mips-force-execstack),yes)
+CFLAGS-.o += -Wa,-execstack
+CFLAGS-.os += -Wa,-execstack
+CFLAGS-.op += -Wa,-execstack
+CFLAGS-.oS += -Wa,-execstack
+
+ASFLAGS-.o += -Wa,-execstack
+ASFLAGS-.os += -Wa,-execstack
+ASFLAGS-.op += -Wa,-execstack
+ASFLAGS-.oS += -Wa,-execstack
+endif
diff --git a/sysdeps/unix/sysv/linux/mips/configure b/sysdeps/unix/sysv/linux/mips/configure
index 1ee7f41..25f98e0 100644
--- a/sysdeps/unix/sysv/linux/mips/configure
+++ b/sysdeps/unix/sysv/linux/mips/configure
@@ -475,3 +475,44 @@ if test -z "$arch_minimum_kernel"; then
     arch_minimum_kernel=4.5.0
   fi
 fi
+
+# Check if we are supposed to run on kernels older than 4.8.0. If so,
+# force executable stack to avoid potential runtime problems with fpu
+# emulation.
+# NOTE: The check below assumes that in absence of user-provided minumum_kernel
+# we will default to arch_minimum_kernel which is currently less than 4.8.0 for
+# all known configurations. If this changes, the check must be updated.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler must use executable stack" >&5
+$as_echo_n "checking whether the compiler must use executable stack... " >&6; }
+if ${libc_cv_mips_force_execstack+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  libc_cv_mips_force_execstack=no
+  if test $libc_mips_float = hard; then
+    if test -n "$minimum_kernel"; then
+
+       min_version=$((`echo "$minimum_kernel.0.0.0" | sed 's/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\).*/\1 \* 65536 + \2 \* 256 + \3/'`))
+
+       if test $min_version -lt 264192; then
+         libc_cv_mips_force_execstack=yes
+       fi
+    else
+      libc_cv_mips_force_execstack=yes
+    fi
+  fi
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_mips_force_execstack" >&5
+$as_echo "$libc_cv_mips_force_execstack" >&6; }
+
+libc_mips_has_gnustack=$libc_cv_as_noexecstack
+
+if test $libc_cv_mips_force_execstack = yes; then
+  libc_mips_has_gnustack=no
+  { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: forcing executable stack for pre-4.8.0 Linux kernels" >&5
+$as_echo "$as_me: WARNING: forcing executable stack for pre-4.8.0 Linux kernels" >&2;}
+fi
+
+config_vars="$config_vars
+mips-force-execstack = ${libc_cv_mips_force_execstack}"
+config_vars="$config_vars
+mips-has-gnustack = ${libc_mips_has_gnustack}"
diff --git a/sysdeps/unix/sysv/linux/mips/configure.ac b/sysdeps/unix/sysv/linux/mips/configure.ac
index 9147aa4..3db1b32 100644
--- a/sysdeps/unix/sysv/linux/mips/configure.ac
+++ b/sysdeps/unix/sysv/linux/mips/configure.ac
@@ -134,3 +134,35 @@ if test -z "$arch_minimum_kernel"; then
     arch_minimum_kernel=4.5.0
   fi
 fi
+
+# Check if we are supposed to run on kernels older than 4.8.0. If so,
+# force executable stack to avoid potential runtime problems with fpu
+# emulation.
+# NOTE: The check below assumes that in absence of user-provided minumum_kernel
+# we will default to arch_minimum_kernel which is currently less than 4.8.0 for
+# all known configurations. If this changes, the check must be updated.
+AC_CACHE_CHECK([whether the compiler must use executable stack],
+        libc_cv_mips_force_execstack, [dnl
+libc_cv_mips_force_execstack=no
+  if test $libc_mips_float = hard; then
+    if test -n "$minimum_kernel"; then
+       changequote(,)
+       min_version=$((`echo "$minimum_kernel.0.0.0" | sed 's/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\).*/\1 \* 65536 + \2 \* 256 + \3/'`))
+       changequote([,])
+       if test $min_version -lt 264192; then
+         libc_cv_mips_force_execstack=yes
+       fi
+    else
+      libc_cv_mips_force_execstack=yes
+    fi
+  fi])
+
+libc_mips_has_gnustack=$libc_cv_as_noexecstack
+
+if test $libc_cv_mips_force_execstack = yes; then
+  libc_mips_has_gnustack=no
+  AC_MSG_WARN([forcing executable stack for pre-4.8.0 Linux kernels])
+fi
+
+LIBC_CONFIG_VAR([mips-force-execstack],[${libc_cv_mips_force_execstack}])
+LIBC_CONFIG_VAR([mips-has-gnustack],[${libc_mips_has_gnustack}])