[PATCH] Re: [SPARC32] NULL pointer derefference

[Mark Fortescue <mark@mtfhpc.demon.co.uk>]

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Hi David,

I have formulated a patch that prevents the update_mmu_cache from doing 
enything if there is no context available. This apears to have no 
immediate, undesirable side effects.

This worked better than the alternative of setting up a context to work with.

Can you for see any issues in doing this?

If not, can you check+apply the attached (un-mangled) patch.

diff -ruNpd linux-2.6/arch/sparc/mm/sun4c.c linux-test/arch/sparc/mm/sun4c.c
--- linux-2.6/arch/sparc/mm/sun4c.c	2007-07-30 03:19:15.000000000 +0100
+++ linux-test/arch/sparc/mm/sun4c.c	2007-07-31 08:28:13.000000000 +0100
@@ -1999,6 +2029,9 @@ void sun4c_update_mmu_cache(struct vm_ar
  	unsigned long flags;
  	int pseg;

+	if (vma->vm_mm->context == NO_CONTEXT)
+		return;
+
  	local_irq_save(flags);
  	address &= PAGE_MASK;
  	if ((pseg = sun4c_get_segmap(address)) == invalid_segment) {

Regards
 	Mark Fortescue.

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From: Mark Fortescue <mark@mtfhpc.demon.co.uk>

This deals with a sun4c issue caused by commit b6a2fea39318e43fee84fa7b0b90d68bed92d2ba:
mm: variable length argument support.
The new way the code works means that sun4c_update_mmu_cache gets called before a context
has been selected, which results in invalid operation of the underling mm code.

Simply ignoring update requests when there is no valid context solves the problem.

Signed-off-by Mark Fortescue <mark@mtfhpc.demon.co.uk>
---
This worked better than the alternative of setting up a context to work with.
I definatly need to spend some time writting up the sun4c MMU and how Linux code uses it.
diff -ruNpd -x '.[a-z]*' linux-2.6/arch/sparc/mm/sun4c.c linux-test/arch/sparc/mm/sun4c.c
--- linux-2.6/arch/sparc/mm/sun4c.c	2007-07-30 03:19:15.000000000 +0100
+++ linux-test/arch/sparc/mm/sun4c.c	2007-07-31 08:28:13.000000000 +0100
@@ -1999,6 +2029,9 @@ void sun4c_update_mmu_cache(struct vm_ar
 	unsigned long flags;
 	int pseg;
 
+	if (vma->vm_mm->context == NO_CONTEXT)
+		return;
+
 	local_irq_save(flags);
 	address &= PAGE_MASK;
 	if ((pseg = sun4c_get_segmap(address)) == invalid_segment) {