[PATCH v3 10/13] aarch64: configure check for pac-ret code generation

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

Return address signing requires unwinder support, which is
present in libgcc since >=gcc-7, however due to bugs the
support may be broken in <gcc-10 (and similarly there may
be issues in custom unwinders), so pac-ret is not always
safe to use. So in assembly code glibc should only use
pac-ret if the compiler uses it too. Unfortunately there
is no predefined feature macro for it set by the compiler
so pac-ret is inferred from the code generation.
---
 config.h.in                  |  3 +++
 sysdeps/aarch64/configure    | 39 ++++++++++++++++++++++++++++++++++++
 sysdeps/aarch64/configure.ac | 21 +++++++++++++++++++
 3 files changed, 63 insertions(+)

diff --git a/config.h.in b/config.h.in
index 506b0c416c..f441470385 100644
--- a/config.h.in
+++ b/config.h.in
@@ -112,6 +112,9 @@
 /* AArch64 BTI support enabled.  */
 #undef HAVE_AARCH64_BTI
 
+/* AArch64 PAC-RET code generation is enabled.  */
+#undef HAVE_AARCH64_PAC_RET
+
 /* C-SKY ABI version.  */
 #undef CSKYABI
 
diff --git a/sysdeps/aarch64/configure b/sysdeps/aarch64/configure
index 70477a7fa5..5f1cdf5d9b 100644
--- a/sysdeps/aarch64/configure
+++ b/sysdeps/aarch64/configure
@@ -214,3 +214,42 @@ if test $libc_cv_aarch64_bti = yes; then
   $as_echo "#define HAVE_AARCH64_BTI 1" >>confdefs.h
 
 fi
+
+# Check if glibc is built with return address signing, i.e.
+# if -mbranch-protection=pac-ret is on. We need this because
+# pac-ret relies on unwinder support so it's not safe to use
+# it in assembly code unconditionally, but there is no
+# feature test macro for it in gcc.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if pac-ret is enabled" >&5
+$as_echo_n "checking if pac-ret is enabled... " >&6; }
+if ${libc_cv_aarch64_pac_ret+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+    cat > conftest.c <<EOF
+int bar (void);
+int foo (void) { return bar () + 1; }
+EOF
+  libc_cv_aarch64_pac_ret=no
+  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -S -o conftest.s conftest.c'
+  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+  (eval $ac_try) 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; } \
+     && { ac_try='grep -q -E '\''(hint( |	)+25|paciasp)'\'' conftest.s'
+  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+  (eval $ac_try) 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; }
+  then
+    libc_cv_aarch64_pac_ret=yes
+  fi
+  rm -rf conftest.*
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_aarch64_pac_ret" >&5
+$as_echo "$libc_cv_aarch64_pac_ret" >&6; }
+if test $libc_cv_aarch64_pac_ret = yes; then
+  $as_echo "#define HAVE_AARCH64_PAC_RET 1" >>confdefs.h
+
+fi
diff --git a/sysdeps/aarch64/configure.ac b/sysdeps/aarch64/configure.ac
index 798f494740..8248ecf2ed 100644
--- a/sysdeps/aarch64/configure.ac
+++ b/sysdeps/aarch64/configure.ac
@@ -39,3 +39,24 @@ EOF
 if test $libc_cv_aarch64_bti = yes; then
   AC_DEFINE(HAVE_AARCH64_BTI)
 fi
+
+# Check if glibc is built with return address signing, i.e.
+# if -mbranch-protection=pac-ret is on. We need this because
+# pac-ret relies on unwinder support so it's not safe to use
+# it in assembly code unconditionally, but there is no
+# feature test macro for it in gcc.
+AC_CACHE_CHECK([if pac-ret is enabled], [libc_cv_aarch64_pac_ret], [dnl
+  cat > conftest.c <<EOF
+int bar (void);
+int foo (void) { return bar () + 1; }
+EOF
+  libc_cv_aarch64_pac_ret=no
+  if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -S -o conftest.s conftest.c]) \
+     && AC_TRY_COMMAND([grep -q -E '\''(hint( |	)+25|paciasp)'\'' conftest.s])
+  then
+    libc_cv_aarch64_pac_ret=yes
+  fi
+  rm -rf conftest.*])
+if test $libc_cv_aarch64_pac_ret = yes; then
+  AC_DEFINE(HAVE_AARCH64_PAC_RET)
+fi
-- 
2.17.1