Re: [PATCH 08/11] x86_64/bug: Implement __WARN_printf()

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Jun 07, 2025 at 11:42:32AM +0200, Peter Zijlstra wrote:
> @@ -156,10 +166,21 @@ __always_inline int decode_bug(unsigned
>  	if (X86_MODRM_MOD(v) != 3 && X86_MODRM_RM(v) == 4)
>  		addr++;			/* SIB */
>  
> +	reg = X86_MODRM_REG(v) + 8*!!X86_REX_R(rex);
> +	rm  = X86_MODRM_RM(v)  + 8*!!X86_REX_B(rex);
> +
>  	/* Decode immediate, if present */
>  	switch (X86_MODRM_MOD(v)) {
>  	case 0: if (X86_MODRM_RM(v) == 5)
> -			addr += 4; /* RIP + disp32 */
> +			addr += 4;	/* RIP + disp32 */
> +
> +		if (rm == 0)		/* (%eax) */
> +			type = BUG_UD1_UBSAN;
> +
> +		if (rm == 1) {		/* (%ecx) */
> +			*imm = reg;
> +			type = BUG_UD1_WARN;
> +		}
>  		break;
>  
>  	case 1: *imm = *(s8 *)addr;
> @@ -176,12 +197,73 @@ __always_inline int decode_bug(unsigned
>  	/* record instruction length */
>  	*len = addr - start;
>  
> -	if (X86_MODRM_REG(v) == 0)	/* EAX */
> -		return BUG_UD1_UBSAN;
> +	return type;
> +}

Oh, this breaks the UBSAN case, it must also set UD1_UBSAN for mod 1 and
2. I'll fix.