Re: [PATCH v1 3/4] common: ubsan: ignore shifting one into sign bit

[Rouven Czerwinski <r.czerwinski@pengutronix.de>]

Hi Ahmad,

On Fri, 2019-09-20 at 09:58 +0200, Ahmad Fatoum wrote:
> The __ubsan_handle_shift_out_of_bounds handler would be called for
> code
> shifting a one into the sign bit like (1 << 31), which is all too
> common
> in barebox.  It's technically UB, but it's so prevalent that it's
> highly
> unlikely to be treated by a compiler as anything else than the
> standard-compliant (1U << 31).
> 
> Check for this case here and ignore it selectively.

Shouldn't we rather fix the the (1 << 31) to be (1U <<31)?

> Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
> ---
>  lib/ubsan.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/lib/ubsan.c b/lib/ubsan.c
> index 89ca6e580bce..41a5731dda66 100644
> --- a/lib/ubsan.c
> +++ b/lib/ubsan.c
> @@ -382,6 +382,26 @@ void __ubsan_handle_shift_out_of_bounds(struct
> shift_out_of_bounds_data *data,
>  	if (suppress_report(&data->location))
>  		return;
>  
> +	/* This handler would be called for code shifting a one into
> the
> +	 * sign bit like (1 << 31), which is all too common in barebox.
> +	 * It's technically UB, but it's so prevalent that it's highly
> +	 * unlikely to be treated by a compiler as anything else than
> the
> +	 * standard-compliant (1U << 31). Thus check for this case here
> +	 * and ignore it selectively
> +	 */
> +	if (type_is_signed(lhs_type)) {
> +		s_max lhs_int, rhs_int;
> +
> +		lhs_int = get_signed_val(lhs_type, lhs);
> +		rhs_int = get_signed_val(rhs_type, rhs);
> +
> +		if (fls(lhs_int) + rhs_int == type_bit_width(lhs_type))
> {
> +			pr_debug("signed left shift of %lld by %lld
> ignored.\n",
> +				(s64)lhs_int, (s64)rhs_int);
> +			return;
> +		}
> +	}
> +
>  	ubsan_prologue(&data->location, &flags);
>  
>  	val_to_string(rhs_str, sizeof(rhs_str), rhs_type, rhs);

- rcz


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