[PATCH mm] fix i386 get_gate_vma oops

[Hugh Dickins <hugh@veritas.com>]

Fix oops from i386-make-compat_vdso-runtime-selectable.patch.

Even mingetty at system startup finds it easy to trigger an oops
while reading /proc/PID/maps: though it has a good hold on the mm
itself, that cannot stop exit_mm() from resetting tsk->mm to NULL.

(It is usually show_map()'s call to get_gate_vma() which oopses,
and I expect we could change that to check priv->tail_vma instead;
but no matter, even m_start()'s call just after get_task_mm() is racy.)

Signed-off-by: Hugh Dickins <hugh@veritas.com>

--- 2.6.21-rc6-mm1/arch/i386/kernel/sysenter.c	2007-04-10 11:37:23.000000000 +0100
+++ linux/arch/i386/kernel/sysenter.c	2007-04-13 16:43:45.000000000 +0100
@@ -326,8 +326,10 @@ const char *arch_vma_name(struct vm_area
 
 struct vm_area_struct *get_gate_vma(struct task_struct *tsk)
 {
+	struct mm_struct *mm = tsk->mm;
+
 	/* Check to see if this task was created in compat vdso mode */
-	if (tsk->mm->context.vdso == (void *)VDSO_HIGH_BASE)
+	if (mm && mm->context.vdso == (void *)VDSO_HIGH_BASE)
 		return &gate_vma;
 	return NULL;
 }