Re: [PATCH net-next] ethtool: tsconfig: reject zero-valued tx_type and rx_filter bitsets

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 12.08.2026 17:22, Jakub Kicinski wrote:
> The ffs()/fls() guard in ethnl_set_tsconfig() was meant to enforce
> that the user selects exactly one tx_type (and one rx_filter)
> at a time (off / none are explicit types with non-zero values).
> However, both ffs(0) and fls(0) return 0, so the guard passes
> a zero-valued bitset through.
> 
> The subsequent ffs(req_tx_type) - 1 would produce -1, if user selected
> no bit. net_hwtstamp_validate() catches the invalid -1 downstream,
> but returns a generic error (-ERANGE) without telling the user
> what went wrong. Return -EINVAL + extack instead.
> 
> Replace the ffs()/fls() comparison with a hweight32() == 1 check.
> 
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
> CC: andrew@lunn.ch
> CC: vadim.fedorenko@linux.dev
> ---
>   net/ethtool/tsconfig.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ethtool/tsconfig.c b/net/ethtool/tsconfig.c
> index 24b64862011f..6be3aa5d4bc1 100644
> --- a/net/ethtool/tsconfig.c
> +++ b/net/ethtool/tsconfig.c
> @@ -359,8 +359,10 @@ static int ethnl_set_tsconfig(struct ethnl_req_info *req_base,
>   		if (ret < 0)
>   			goto err_free_hwprov;
>   
> -		/* Select only one tx type at a time */
> -		if (ffs(req_tx_type) != fls(req_tx_type)) {
> +		/* Select exactly one tx type at a time */
> +		if (hweight32(req_tx_type) != 1) {
> +			NL_SET_BAD_ATTR(info->extack,
> +					tb[ETHTOOL_A_TSCONFIG_TX_TYPES]);
>   			ret = -EINVAL;
>   			goto err_free_hwprov;
>   		}
> @@ -380,8 +382,10 @@ static int ethnl_set_tsconfig(struct ethnl_req_info *req_base,
>   		if (ret < 0)
>   			goto err_free_hwprov;
>   
> -		/* Select only one rx filter at a time */
> -		if (ffs(req_rx_filter) != fls(req_rx_filter)) {
> +		/* Select exactly one rx filter at a time */
> +		if (hweight32(req_rx_filter) != 1) {
> +			NL_SET_BAD_ATTR(info->extack,
> +					tb[ETHTOOL_A_TSCONFIG_RX_FILTERS]);
>   			ret = -EINVAL;
>   			goto err_free_hwprov;
>   		}
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>