Re: [PATCH] KVM: emulator: Handle wraparound in (cs_base + offset)when fetching.

From: Avi Kivity
Date: Wed Apr 13 2011 - 12:06:48 EST

On 04/13/2011 06:44 PM, Nelson Elhage wrote:
Currently, setting a large (i.e. negative) base address for %cs does not work on
a 64-bit host. The "JOS" teaching operating system, used by MIT and other
universities, relies on such segments while bootstrapping its way to full
virtual memory management.

Signed-off-by: Nelson Elhage<nelhage@xxxxxxxxxxx>
arch/x86/kvm/emulate.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 0ad47b8..54e84b2 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -505,9 +505,12 @@ static int do_fetch_insn_byte(struct x86_emulate_ctxt *ctxt,
int size, cur_size;

if (eip == fc->end) {
+ unsigned long linear = eip + ctxt->cs_base;
+ if (ctxt->mode != X86EMUL_MODE_PROT64)
+ linear&= (u32)-1;
cur_size = fc->end - fc->start;
size = min(15UL - cur_size, PAGE_SIZE - offset_in_page(eip));
- rc = ops->fetch(ctxt->cs_base + eip, fc->data + cur_size,
+ rc = ops->fetch(linear, fc->data + cur_size,
size, ctxt->vcpu,&ctxt->exception);
if (rc != X86EMUL_CONTINUE)
return rc;

A better fix would be to call linearize() here, which does the necessary truncation as well as segment checks.

However, this patch is a lot more backportable, so I think it should be applied, and a conversion to linearize() performed afterwards.

error compiling committee.c: too many arguments to function

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at