always save and restore %fs, %gs to ensure old segment entries are never

accessible to user from the hidden CPU segment registers.

trapasm.S

@@ -8,6 +8,8 @@ alltraps:
   # Build trap frame.
   pushl %ds
   pushl %es
+  pushl %fs
+  pushl %gs
   pushal
   
   # Set up data segments.
@@ -24,6 +26,8 @@ alltraps:
 .globl trapret
 trapret:
   popal
+  popl %gs
+  popl %fs
   popl %es
   popl %ds
   addl $0x8, %esp  # trapno and errcode

x86.h

@@ -135,21 +135,25 @@ struct trapframe {
   uint eax;
 
   // rest of trap frame
-  ushort es;
+  ushort gs;
   ushort padding1;
-  ushort ds;
+  ushort fs;
   ushort padding2;
+  ushort es;
+  ushort padding3;
+  ushort ds;
+  ushort padding4;
   uint trapno;
 
   // below here defined by x86 hardware
   uint err;
   uint eip;
   ushort cs;
-  ushort padding3;
+  ushort padding5;
   uint eflags;
 
   // below here only when crossing rings, such as from user to kernel
   uint esp;
   ushort ss;
-  ushort padding4;
+  ushort padding6;
 };