[PATCH] Lguest32, use guest page tables to find paddr for emulated instructions

[PATCH] Lguest32, use guest page tables to find paddr for emulated instructions

[Steven Rostedt]

[Bug that was found by my previous patch]

This patch allows things like modules, which don't have a direct
__pa(EIP) mapping to do emulated instructions.

Sure, the emulated instruction probably should be a paravirt_op, but
this patch lets you at least boot a kernel that has modules needing
emulated instructions.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Index: linux-2.6.21-rc5-mm2/drivers/lguest/core.c
===================================================================
--- linux-2.6.21-rc5-mm2.orig/drivers/lguest/core.c
+++ linux-2.6.21-rc5-mm2/drivers/lguest/core.c
@@ -160,11 +160,14 @@ static int emulate_insn(struct lguest *l
 {
 	u8 insn;
 	unsigned int insnlen = 0, in = 0, shift = 0;
-	unsigned long physaddr = guest_pa(lg, lg->regs->eip);
+	unsigned long physaddr = lguest_find_guest_paddr(lg, lg->regs->eip);
 
-	/* This only works for addresses in linear mapping... */
-	if (lg->regs->eip < lg->page_offset)
+	/* FIXME: Handle physaddr's that crosses pages (modules are in VM) */
+
+	/* did we actually find the physaddr? */
+	if (physaddr == (unsigned long)-1UL)
 		return 0;
+
 	lgread(lg, &insn, physaddr, 1);
 
 	/* Operand size prefix means it's actually for ax. */
Index: linux-2.6.21-rc5-mm2/drivers/lguest/lg.h
===================================================================
--- linux-2.6.21-rc5-mm2.orig/drivers/lguest/lg.h
+++ linux-2.6.21-rc5-mm2/drivers/lguest/lg.h
@@ -218,6 +218,7 @@ void guest_set_pte(struct lguest *lg, un
 void map_hypervisor_in_guest(struct lguest *lg, struct lguest_pages *pages);
 int demand_page(struct lguest *info, unsigned long cr2, int write);
 void pin_page(struct lguest *lg, unsigned long vaddr);
+unsigned long lguest_find_guest_paddr(struct lguest *lg, unsigned long vaddr);
 
 /* lguest_user.c: */
 int lguest_device_init(void);
Index: linux-2.6.21-rc5-mm2/drivers/lguest/page_tables.c
===================================================================
--- linux-2.6.21-rc5-mm2.orig/drivers/lguest/page_tables.c
+++ linux-2.6.21-rc5-mm2/drivers/lguest/page_tables.c
@@ -105,6 +105,25 @@ static spte_t gpte_to_spte(struct lguest
 	return spte;
 }
 
+unsigned long lguest_find_guest_paddr(struct lguest *lg, unsigned long vaddr)
+{
+	gpgd_t gpgd;
+	gpte_t gpte;
+	unsigned long gpte_ptr;
+
+	gpgd = mkgpgd(lgread_u32(lg, gpgd_addr(lg, vaddr)));
+	if (!(gpgd.flags & _PAGE_PRESENT))
+		return -1;
+
+	gpte_ptr = gpte_addr(lg, gpgd, vaddr);
+	gpte = mkgpte(lgread_u32(lg, gpte_ptr));
+
+	if (!(gpte.flags & _PAGE_PRESENT))
+		return -1;
+
+	return (gpte.pfn << PAGE_SHIFT) | (vaddr & (PAGE_SIZE-1));
+}
+
 /* FIXME: We hold reference to pages, which prevents them from being
    swapped.  It'd be nice to have a callback when Linux wants to swap out. */
 

Re: [PATCH] Lguest32, use guest page tables to find paddr for emulated instructions

[Rusty Russell]

On Wed, 2007-04-04 at 15:07 -0400, Steven Rostedt wrote:
> [Bug that was found by my previous patch]
> 
> This patch allows things like modules, which don't have a direct
> __pa(EIP) mapping to do emulated instructions.
> 
> Sure, the emulated instruction probably should be a paravirt_op, but
> this patch lets you at least boot a kernel that has modules needing
> emulated instructions.

Yeah, I haven't tried loading random modules but I can imagine this does
happen (what module was it, BTW?)

I used to have a function just like this, but managed to get rid of
it.  

Hmm, perhaps we should have an "int lgread_virt_byte(u8 *)" which does
the pgtable walk and read all in one?  It won't be efficient, but it'll
be more correct and maybe even fewer lines 8)

Thanks for the patch!
Rusty.

Re: [PATCH] Lguest32, use guest page tables to find paddr for emulated instructions

[Steven Rostedt]

On Thu, 2007-04-05 at 12:59 +1000, Rusty Russell wrote:
> On Wed, 2007-04-04 at 15:07 -0400, Steven Rostedt wrote:

> Yeah, I haven't tried loading random modules but I can imagine this does
> happen (what module was it, BTW?)

I have no idea of which module it crashed on. I didn't investigate that
too much.  I could simply send a trap to guest when 
__pa(addr) != lguest_find_guest_paddr(addr) and see which module it
crashed on.

My block device I used was basically a copy of a RHEL5 system. I only
modified the inittab and fstab to get it working.  So on startup and
doing the udev init was when it crashed.

> 
> I used to have a function just like this, but managed to get rid of
> it.  
> 
> Hmm, perhaps we should have an "int lgread_virt_byte(u8 *)" which does
> the pgtable walk and read all in one?  It won't be efficient, but it'll
> be more correct and maybe even fewer lines 8)

I forgot that you have a goal to keep lguest small :)

Perhaps we can fork, and have lguest and lguest-lite.

-- Steve