i386: Properly pop restore token in signal frame

Linux CET kernel places a restore token on shadow stack for signal
handler to enhance security.  The restore token is 8 byte and aligned
to 8 bytes.  It is usually transparent to user programs since kernel
will pop the restore token when signal handler returns.  But when an
exception is thrown from a signal handler, now we need to pop the
restore token from shadow stack.  For x86-64, we just need to treat
the signal frame as normal frame.  For i386, we need to search for
the restore token to check if the original shadow stack is 8 byte
aligned.  If the original shadow stack is 8 byte aligned, we just
need to pop 2 slots, one restore token, from shadow stack.  Otherwise,
we need to pop 3 slots, one restore token + 4 byte padding, from
shadow stack.

This patch also includes 2 tests, one has a restore token with 4 byte
padding and one without.

Tested on Linux/x86-64 CET machine with and without -m32.

libgcc/

	PR libgcc/85334
	* config/i386/shadow-stack-unwind.h (_Unwind_Frames_Increment):
	New.

gcc/testsuite/

	PR libgcc/85334
	* g++.target/i386/pr85334-1.C: New test.
	* g++.target/i386/pr85334-2.C: Likewise.

4 files changed, 115 insertions, 0 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 60bd347..e416516 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2020-02-10  H.J. Lu  <hongjiu.lu@intel.com>
+
+	PR libgcc/85334
+	* config/i386/shadow-stack-unwind.h (_Unwind_Frames_Increment):
+	New.
+
 2020-02-10  Richard Earnshaw  <rearnsha@arm.com>
 
 	PR target/91913
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index b3966ec..64835ff 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2020-02-10  H.J. Lu  <hongjiu.lu@intel.com>
+
+	PR libgcc/85334
+	* g++.target/i386/pr85334-1.C: New test.
+	* g++.target/i386/pr85334-2.C: Likewise.
+
 2020-02-10  Jakub Jelinek  <jakub@redhat.com>
 
 	PR other/93641
diff --git a/gcc/testsuite/g++.target/i386/pr85334-1.C b/gcc/testsuite/g++.target/i386/pr85334-1.C
new file mode 100644
index 0000000..3c5ccad
--- /dev/null
+++ b/gcc/testsuite/g++.target/i386/pr85334-1.C
@@ -0,0 +1,55 @@
+// { dg-do run }
+// { dg-require-effective-target cet }
+// { dg-additional-options "-fexceptions -fnon-call-exceptions -fcf-protection" }
+
+// Delta between numbers of call stacks of pr85334-1.C and pr85334-2.C is 1.
+
+#include <signal.h>
+#include <stdlib.h>
+
+void sighandler (int signo, siginfo_t * si, void * uc)
+{
+  throw (5);
+}
+
+char *
+__attribute ((noinline, noclone))
+dosegv ()
+{    
+  * ((volatile int *)0) = 12;
+  return 0;
+}
+
+int
+__attribute ((noinline, noclone))
+func2 ()
+{
+  try {
+    dosegv ();
+  }
+  catch (int x) {
+    return (x != 5);
+  }
+  return 1;
+}
+
+int
+__attribute ((noinline, noclone))
+func1 ()
+{
+  return func2 ();
+}
+
+int main ()
+{
+  struct sigaction sa;
+  int status;
+
+  sa.sa_sigaction = sighandler;
+  sa.sa_flags = SA_SIGINFO;
+    
+  status = sigaction (SIGSEGV, & sa, NULL);
+  status = sigaction (SIGBUS, & sa, NULL);
+
+  return func1 ();
+}
diff --git a/gcc/testsuite/g++.target/i386/pr85334-2.C b/gcc/testsuite/g++.target/i386/pr85334-2.C
new file mode 100644
index 0000000..e2b5afe
--- /dev/null
+++ b/gcc/testsuite/g++.target/i386/pr85334-2.C
@@ -0,0 +1,48 @@
+// { dg-do run }
+// { dg-require-effective-target cet }
+// { dg-additional-options "-fexceptions -fnon-call-exceptions -fcf-protection" }
+
+// Delta between numbers of call stacks of pr85334-1.C and pr85334-2.C is 1.
+
+#include <signal.h>
+#include <stdlib.h>
+
+void sighandler (int signo, siginfo_t * si, void * uc)
+{
+  throw (5);
+}
+
+char *
+__attribute ((noinline, noclone))
+dosegv ()
+{    
+  * ((volatile int *)0) = 12;
+  return 0;
+}
+
+int
+__attribute ((noinline, noclone))
+func1 ()
+{
+  try {
+    dosegv ();
+  }
+  catch (int x) {
+    return (x != 5);
+  }
+  return 1;
+}
+
+int main ()
+{
+  struct sigaction sa;
+  int status;
+
+  sa.sa_sigaction = sighandler;
+  sa.sa_flags = SA_SIGINFO;
+    
+  status = sigaction (SIGSEGV, & sa, NULL);
+  status = sigaction (SIGBUS, & sa, NULL);
+
+  return func1 ();
+}