Re: [Xen-devel] [PATCH v2 09/16] x86/32on64: use generic instruction decoding for call gate emulation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/09/16 09:13, Jan Beulich wrote:
> @@ -3204,179 +3285,59 @@ static void emulate_gate_op(struct cpu_u
>          return;
>      }
>  
> -    op_bytes = op_default = ar & _SEGMENT_DB ? 4 : 2;
> -    ad_default = ad_bytes = op_default;
> -    opnd_sel = opnd_off = 0;
> -    jump = -1;
> -    for ( eip = regs->eip; eip - regs->_eip < 10; )
> +    ctxt.ctxt.addr_size = ar & _SEGMENT_DB ? 32 : 16;
> +    /* Leave zero in ctxt.ctxt.sp_size, as it's not needed for decoding. */

Are you sure this is safe?  What if the instruction is substituted under
our feet?

Currently, the only issues I can spot would be a load of "& 0" in
truncate_word() and friends, but my gut feeling is that this is not a
safe or sensible thing to rely on.

Everything else looks fine though.

~Andrew

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