[PATCH] uml: arch_prctl should set thread fs

In my previous x86_64 thread fix, I forgot to initialize thread.arch.fs in
arch_prctl.  A process calling arch_prctl to set %fs would lose it on the
next context switch.

It also turns out that you can switch to a process which is in the process
of exiting and which has lost its mm.  In this case, it's worse than
useless to try to call arch_prctl on the host process.
Signed-off-by: NJeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/um/sys-x86_64/syscalls.c

@@ -103,6 +103,9 @@ long arch_prctl_skas(struct task_struct *task, int code,
 
         switch(code){
 	case ARCH_SET_FS:
+		current->thread.arch.fs = (unsigned long) ptr;
+		save_registers(pid, &current->thread.regs.regs);
+		break;
 	case ARCH_SET_GS:
                 save_registers(pid, &current->thread.regs.regs);
 		break;
@@ -140,9 +143,8 @@ long sys_clone(unsigned long clone_flags, unsigned long newsp,
 
 void arch_switch_to_skas(struct task_struct *from, struct task_struct *to)
 {
-        if(to->thread.arch.fs == 0)
+        if((to->thread.arch.fs == 0) || (to->mm == NULL))
                 return;
 
         arch_prctl_skas(to, ARCH_SET_FS, (void __user *) to->thread.arch.fs);
 }
-