[PATCH] x86/x86_64: mark rodata section read only: x86 parts

x86 specific parts to make the .rodata section read only

Signed-off-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/Kconfig.debug

@@ -42,6 +42,16 @@ config DEBUG_PAGEALLOC
 	  This results in a large slowdown, but helps to find certain types
 	  of memory corruptions.
 
+config DEBUG_RODATA
+	bool "Write protect kernel read-only data structures"
+	depends on DEBUG_KERNEL
+	help
+	  Mark the kernel read-only data as write-protected in the pagetables,
+	  in order to catch accidental (and incorrect) writes to such const
+	  data. This option may have a slight performance impact because a
+	  portion of the kernel code won't be covered by a 2MB TLB anymore.
+	  If in doubt, say "N".
+
 config 4KSTACKS
 	bool "Use 4Kb for kernel stacks instead of 8Kb"
 	depends on DEBUG_KERNEL

arch/i386/mm/init.c

@@ -735,6 +735,30 @@ void free_initmem(void)
 	printk (KERN_INFO "Freeing unused kernel memory: %dk freed\n", (__init_end - __init_begin) >> 10);
 }
 
+#ifdef CONFIG_DEBUG_RODATA
+
+extern char __start_rodata, __end_rodata;
+void mark_rodata_ro(void)
+{
+	unsigned long addr = (unsigned long)&__start_rodata;
+
+	for (; addr < (unsigned long)&__end_rodata; addr += PAGE_SIZE)
+		change_page_attr(virt_to_page(addr), 1, PAGE_KERNEL_RO);
+
+	printk ("Write protecting the kernel read-only data: %luk\n",
+			(unsigned long)(&__end_rodata - &__start_rodata) >> 10);
+
+	/*
+	 * change_page_attr() requires a global_flush_tlb() call after it.
+	 * We do this after the printk so that if something went wrong in the
+	 * change, the printk gets out at least to give a better debug hint
+	 * of who is the culprit.
+	 */
+	global_flush_tlb();
+}
+#endif
+
+
 #ifdef CONFIG_BLK_DEV_INITRD
 void free_initrd_mem(unsigned long start, unsigned long end)
 {

include/asm-i386/cacheflush.h

@@ -31,4 +31,8 @@ int change_page_attr(struct page *page, int numpages, pgprot_t prot);
 void kernel_map_pages(struct page *page, int numpages, int enable);
 #endif
 
+#ifdef CONFIG_DEBUG_RODATA
+void mark_rodata_ro(void);
+#endif
+
 #endif /* _I386_CACHEFLUSH_H */