Re: [Xen-devel] [PATCH] x86/32on64: misc adjustments to call gate emulation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/08/16 14:57, Jan Beulich wrote:
> - There's no 32-bit displacement in 16-bit addressing mode.
> - It is wrong to ASSERT() anything on parts of an instruction fetched
>   from guest memory.
> - The two scaling bits of a SIB byte don't affect whether there is no
>   scaled index register.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -3214,7 +3214,7 @@ static void emulate_gate_op(struct cpu_u
>                              sib = insn_fetch(u8, base, eip, limit);
>  
>                              modrm = (modrm & ~7) | (sib & 7);
> -                            if ( (sib >>= 3) != 4 )
> +                            if ( ((sib >>= 3) & 7) != 4 )
>                                  opnd_off = *(unsigned long *)
>                                      decode_register(sib & 7, regs, 0);
>                              opnd_off <<= sib >> 3;

Surely should shift sib by 6 rather than 3 here, so opnd_off doesn't
have the index included in its scaling factor?

The other two changes look fine.

~Andrew

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