Re: [Bug 10732] REGRESSION: 2.6.26-rc2-git4: X server failed startonX61s laptop

[Hugh Dickins]

On Sat, 17 May 2008, Theodore Tso wrote:
> On Sat, May 17, 2008 at 08:32:38PM +0200, Gabriel C wrote:
> > > 
> > > Which repository is commit 1c12c4cf in?
> > 
> > It is in linus-git tree , http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=1c12c4cf9411eb130b245fa8d0fbbaf989477c7b
> 
> ...post 2.6.26-rc2-git3.  For Linux kernels 2.6.26-rc2-git4 and
> newer, you need to revert this commit or the X Server shipped with
> Ubuntu Gutsy will die horribly when run on an X61s laptop with the
> Intel 965GM video chipset.
> 
> So the question is what's the right fix to keep the kernel compatible
> with the X server, besides just reverting the commit entirely?

[PATCH] x86: fix mprotect's NX handling on PAE

2.6.26-rc3 with CONFIG_X86_PAE may leave the NX bit set when PROT_EXEC
is intending to clear it, causing irqbalance and others to segfault,
and X startup to fail.

This comes from an assumption in 1c12c4cf9411eb130b245fa8d0fbbaf989477c7b
mprotect: prevent alteration of the PAT bits, that PTE_MASK is what it's
supposed to be: whereas it's been wrong forever with PAE, staying 32-bit
where 64-bit is needed.

Jeremy Fitzhardinge already has patches to fix that in Ingo's tree;
but if they're not considered 2.6.26 material, or people want a quick
two-liner to get working, here's a shorter hack to fix up pte_modify.

I apologize to those we've broken, and to Venki Pallipadi: it was I who
persuaded him to change his working patch to make that false assumption.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---

 include/asm-x86/pgtable.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- 2.6.26-rc3/include/asm-x86/pgtable.h	2008-05-19 11:19:03.000000000 +0100
+++ linux/include/asm-x86/pgtable.h	2008-05-19 21:51:08.000000000 +0100
@@ -57,8 +57,8 @@
 #define _KERNPG_TABLE	(_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED |	\
 			 _PAGE_DIRTY)
 
-#define _PAGE_CHG_MASK	(PTE_MASK | _PAGE_PCD | _PAGE_PWT |		\
-			 _PAGE_ACCESSED | _PAGE_DIRTY)
+#define _PAGE_CHG_MASK	(((pteval_t) PTE_MASK & ~_PAGE_NX) | 		\
+			 _PAGE_PCD | _PAGE_PWT | _PAGE_ACCESSED | _PAGE_DIRTY)
 
 #define _PAGE_CACHE_MASK	(_PAGE_PCD | _PAGE_PWT)
 #define _PAGE_CACHE_WB		(0)
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