Re: [PATCH 1/1] x86: fix text_poke

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, 25 Apr 2008, Ingo Molnar wrote:
> > 
> > ah, on 64-bit. That we better make consistent anyway, via the patch 
> > below. set_pte_phys() needs to become non-init as well.
> 
> Make it return the "pte_t *", and now you don't have to walk the page 
> tables twice to just clear it immediately afterwards. At that point I 
> think my patch will be happy and useful, but I also worry a bit 
> whether it was worth the changes..

performance i dont think we should be too worried about at this moment - 
this code is so rarely used that it should be driven by robustness i 
think.

one theoretical worry i have is that we've got the pending immediate 
values changes from Mathieu. Those end up removing the original 
BUG_ON(len > sizeof(long)) restriction (and the alignment check) and 
uses a carefully crafted (but scary as hell) sequence of text_poke() 
sequences to turn a marker into a single-instruction NOP when the marker 
is inactive.

Single-instruction NOP markers is a rather ... tempting goal and it can 
(and must be able to) patch instructions across page boundaries as well.

i think with the PageReserved WARN_ON() we should be sufficiently 
protected against stray scribbles so Mathieu's fix might be usable as 
well - see it below.

Note that the BUG_ON()s at the end of the text_poke() version below 
should have caught this bug too i think - because the bug was due to 
mis-mapping the pages due to the incorrect kernel_text_address() 
condition so we'd have noticed that the expected bits did not end up in 
the right place.

	Ingo


----------------------->
Subject: Fix sched-devel text_poke
From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Thu, 24 Apr 2008 11:03:33 -0400

Use core_text_address() instead of kernel_text_address(). Deal with modules in
the same way used for the core kernel.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 arch/x86/kernel/alternative.c |   38 ++++++++++++++++++--------------------
 1 file changed, 18 insertions(+), 20 deletions(-)

Index: linux/arch/x86/kernel/alternative.c
===================================================================
--- linux.orig/arch/x86/kernel/alternative.c
+++ linux/arch/x86/kernel/alternative.c
@@ -511,31 +511,29 @@ void *__kprobes text_poke(void *addr, co
 	unsigned long flags;
 	char *vaddr;
 	int nr_pages = 2;
+	struct page *pages[2];
+	int i;
 
-	BUG_ON(len > sizeof(long));
-	BUG_ON((((long)addr + len - 1) & ~(sizeof(long) - 1))
-		- ((long)addr & ~(sizeof(long) - 1)));
-	if (kernel_text_address((unsigned long)addr)) {
-		struct page *pages[2] = { virt_to_page(addr),
-			virt_to_page(addr + PAGE_SIZE) };
-		if (!pages[1])
-			nr_pages = 1;
-		vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
-		BUG_ON(!vaddr);
-		local_irq_save(flags);
-		memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
-		local_irq_restore(flags);
-		vunmap(vaddr);
+	if (!core_kernel_text((unsigned long)addr)) {
+		pages[0] = vmalloc_to_page(addr);
+		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
 	} else {
-		/*
-		 * modules are in vmalloc'ed memory, always writable.
-		 */
-		local_irq_save(flags);
-		memcpy(addr, opcode, len);
-		local_irq_restore(flags);
+		pages[0] = virt_to_page(addr);
+		pages[1] = virt_to_page(addr + PAGE_SIZE);
 	}
+	BUG_ON(!pages[0]);
+	if (!pages[1])
+		nr_pages = 1;
+	vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
+	BUG_ON(!vaddr);
+	local_irq_save(flags);
+	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
+	local_irq_restore(flags);
+	vunmap(vaddr);
 	sync_core();
 	/* Could also do a CLFLUSH here to speed up CPU recovery; but
 	   that causes hangs on some VIA CPUs. */
+	for (i = 0; i < len; i++)
+		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
 	return addr;
 }