Re: [PATCH] x86/traps: Enable UBSAN traps on x86

[Andrew Cooper]

On 29/05/2024 3:20 am, Gatlin Newhouse wrote:
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index a3ec87d198ac..e3fbed9073f8 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -13,6 +13,14 @@
>  #define INSN_UD2	0x0b0f
>  #define LEN_UD2		2
>  
> +/*
> + * In clang we have UD1s reporting UBSAN failures on X86, 64 and 32bit.
> + */
> +#define INSN_UD1	0xb90f
> +#define LEN_UD1		2
> +#define INSN_REX	0x67
> +#define LEN_REX		1

That's an address size override prefix, not a REX prefix.

What information is actually encoded in this UD1 instruction?  I can't
find anything any documentation which actually discusses how the ModRM
byte is encoded.

~Andrew