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.

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

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

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 ();
+}

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 ();
+}

libgcc/config/i386/shadow-stack-unwind.h

@@ -49,3 +49,46 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 	}					\
     }						\
     while (0)
+
+/* 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.  */
+#ifndef __x86_64__
+#undef _Unwind_Frames_Increment
+#define _Unwind_Frames_Increment(context, frames)	\
+  if (_Unwind_IsSignalFrame (context))			\
+    do							\
+      {							\
+	_Unwind_Word ssp, prev_ssp, token;		\
+	ssp = _get_ssp ();				\
+	if (ssp != 0)					\
+	  {						\
+	    /* Align shadow stack pointer to the next	\
+	       8 byte aligned boundary.  */		\
+	    ssp = (ssp + 4) & ~7;			\
+	    do						\
+	      {						\
+		/* Look for a restore token.  */	\
+		token = (*(_Unwind_Word *) (ssp - 8));	\
+		prev_ssp = token & ~7;			\
+		if (prev_ssp == ssp)			\
+		  break;				\
+		ssp += 8;				\
+	      }						\
+	    while (1);					\
+	    frames += (token & 0x4) ? 3 : 2;		\
+	  }						\
+      }							\
+    while (0);						\
+  else							\
+    frames++;
+#endif