From 6540dedaf835c2e947ddc8e5911d4f07197d98e8 Mon Sep 17 00:00:00 2001
From: David du Colombier <0intro@gmail.com>
Date: Tue, 16 Feb 1999 00:00:00 +0000
Subject: [PATCH] Plan 9 from Bell Labs 1999-02-16

---
 pc/mmu.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/pc/mmu.c b/pc/mmu.c
index 10faab73889bec0f552a19a600a9ac12bd5537b7..198ca9cb77ff956972a91e56e3b4c911d62d37c3 100644
--- a/pc/mmu.c
+++ b/pc/mmu.c
@@ -42,7 +42,7 @@ taskswitch(ulong pdb, ulong stack)
 void
 mmuinit(void)
 {
-	ulong x;
+	ulong x, *p;
 	ushort ptr[3];
 
 	m->tss = malloc(sizeof(Tss));
@@ -65,6 +65,14 @@ mmuinit(void)
 	ptr[2] = (x>>16) & 0xFFFF;
 	lidt(ptr);
 
+	/* make kernel text unwritable */
+	for(x = KTZERO; x < (ulong)etext; x += BY2PG){
+		p = mmuwalk(m->pdb, x, 2, 0);
+		if(p == nil)
+			panic("mmuinit");
+		*p &= ~PTEWRITE;
+	}
+
 	taskswitch(PADDR(m->pdb),  (ulong)m + BY2PG);
 	ltr(TSSSEL);
 }