lguest: Fix in/out emulation

We were blatting too much of the register.  Linux didn't care, but in
theory it might.

Reported-by: Jonas Maebe <jonas.maebe@elis.ugent.be>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c
index 3b9b810cbf2801fc2725430f2376f7d7329abb10..65af42f2d593cbc352b5d65318ed90ff0cc6ac92 100644 (file)
--- a/drivers/lguest/x86/core.c
+++ b/drivers/lguest/x86/core.c
@@ -269,7 +269,7 @@ void lguest_arch_run_guest(struct lg_cpu *cpu)
 static int emulate_insn(struct lg_cpu *cpu)
 {
        u8 insn;
-       unsigned int insnlen = 0, in = 0, shift = 0;
+       unsigned int insnlen = 0, in = 0, small_operand = 0;
        /*
         * The eip contains the *virtual* address of the Guest's instruction:
         * walk the Guest's page tables to find the "physical" address.
@@ -300,11 +300,10 @@ static int emulate_insn(struct lg_cpu *cpu)
        }
 
        /*
-        * 0x66 is an "operand prefix".  It means it's using the upper 16 bits
-        * of the eax register.
+        * 0x66 is an "operand prefix".  It means a 16, not 32 bit in/out.
         */
        if (insn == 0x66) {
-               shift = 16;
+               small_operand = 1;
                /* The instruction is 1 byte so far, read the next byte. */
                insnlen = 1;
                insn = lgread(cpu, physaddr + insnlen, u8);
@@ -340,11 +339,14 @@ static int emulate_insn(struct lg_cpu *cpu)
         * traditionally means "there's nothing there".
         */
        if (in) {
-               /* Lower bit tells is whether it's a 16 or 32 bit access */
-               if (insn & 0x1)
-                       cpu->regs->eax = 0xFFFFFFFF;
-               else
-                       cpu->regs->eax |= (0xFFFF << shift);
+               /* Lower bit tells means it's a 32/16 bit access */
+               if (insn & 0x1) {
+                       if (small_operand)
+                               cpu->regs->eax |= 0xFFFF;
+                       else
+                               cpu->regs->eax = 0xFFFFFFFF;
+               } else
+                       cpu->regs->eax |= 0xFF;
        }
        /* Finally, we've "done" the instruction, so move past it. */
        cpu->regs->eip += insnlen;