On 21/08/2024 2:30 pm, Jan Beulich wrote:
> Delivering #UD for an internal shortcoming of the emulator isn't quite
> right. Similarly BUG() is bigger a hammer than needed.
>
> Switch to using EXPECT() instead.
>
> Suggested-by: Andrew Cooper <[email protected]>
> Signed-off-by: Jan Beulich <[email protected]>
To confirm, this is ASSERT_UNREACHABLE() (which fuzzing will now notice
as an error), and unhandleable in release builds (which ultimately ends
up in #UD)?
I think it would be helpful to at least note the fuzzing aspect in the
commit message.
>
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -8114,13 +8114,13 @@ x86_emulate(
> }
> else if ( state->simd_size != simd_none )
> {
> - generate_exception_if(!op_bytes, X86_EXC_UD);
> generate_exception_if((vex.opcx && (d & TwoOp) &&
> (vex.reg != 0xf || (evex_encoded() &&
> !evex.RX))),
> X86_EXC_UD);
>
> - if ( !opc )
> - BUG();
> + EXPECT(op_bytes);
> + EXPECT(opc);
This is the only BUG() in x86_emulate.c, and it's right to get rid of it
IMO.
Therefore, we should have a hunk removing it from
tools/tests/x86_emulator/x86-emulate.h too, which will prevent
reintroduction.
Maybe even undef BUG somewhere in x86_emulate/private.h?
~Andrew
