[PATCH] UML - arch_prctl should set thread fs

[Jeff Dike <jdike@addtoit.com>]

[ This missed getting into -stable the first time I sent it ]

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: Jeff Dike <jdike@linux.intel.com>
--
 arch/um/sys-x86_64/syscalls.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux-2.6.17/arch/um/sys-x86_64/syscalls.c
===================================================================
--- linux-2.6.17.orig/arch/um/sys-x86_64/syscalls.c	2007-03-07 12:18:38.000000000 -0500
+++ linux-2.6.17/arch/um/sys-x86_64/syscalls.c	2007-03-16 12:04:40.000000000 -0400
@@ -103,6 +103,9 @@ long arch_prctl_skas(struct task_struct 
 
         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
 
 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);
 }
-