diff -ur o13/include/asm-x86_64/sigcontext.h linux-2.6.13.3/include/asm-x86_64/sigcontext.h
--- o13/include/asm-x86_64/sigcontext.h	2005-10-03 19:27:35.000000000 -0400
+++ linux-2.6.13.3/include/asm-x86_64/sigcontext.h	2005-10-05 12:06:59.000000000 -0400
@@ -43,7 +43,7 @@
 	unsigned short cs;
 	unsigned short gs;
 	unsigned short fs;
-	unsigned short __pad0; 
+	unsigned short ss; 
 	unsigned long err;
 	unsigned long trapno;
 	unsigned long oldmask;
diff -ur o13/arch/x86_64/kernel/signal.c linux-2.6.13.3/arch/x86_64/kernel/signal.c
--- o13/arch/x86_64/kernel/signal.c	2005-10-03 19:27:35.000000000 -0400
+++ linux-2.6.13.3/arch/x86_64/kernel/signal.c	2005-10-05 12:13:22.000000000 -0400
@@ -110,6 +110,15 @@
 	COPY(r14);
 	COPY(r15);
 
+	/* Kernel saves and restores only CS and DS segments on signals,
+	 * which are the bare essentials needed to allow mixed 32/64-bit code.
+	 * App's signal handler can save/restore other segments if needed. */
+	unsigned short cs, ss;
+	err |= __get_user(cs, &sc->cs);
+	err |= __get_user(ss, &sc->ss);
+	regs->cs = cs | 3;	/* Force into user mode */
+	regs->ss = ss | 3;
+
 	{
 		unsigned int tmpflags;
 		err |= __get_user(tmpflags, &sc->eflags);
@@ -187,6 +196,8 @@
 {
 	int err = 0;
 
+	err |= __put_user(regs->cs, &sc->cs);
+	err |= __put_user(regs->ss, &sc->ss);
 	err |= __put_user(0, &sc->gs);
 	err |= __put_user(0, &sc->fs);
 
@@ -318,7 +329,15 @@
 
 	regs->rsp = (unsigned long)frame;
 
+	/* Set up segment registers to run signal handlers in 64-bit mode,
+	   even if the handler happens to be interrupting 32-bit code. */
+	regs->cs = __USER_CS;
+	regs->ss = __USER_DS;
+
+	/* This, by contrast, has nothing to do with segment registers -
+	   see include/asm-x86_64/uaccess.h for details. */
 	set_fs(USER_DS);
+
 	regs->eflags &= ~TF_MASK;
 	if (test_thread_flag(TIF_SINGLESTEP))
 		ptrace_notify(SIGTRAP);