Re: [PATCH 1/2] sfc: fix uninitialized variable use

[Edward Cree]

On 16/06/2023 10:08, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> The new efx_bind_neigh() function contains a broken code path when IPV6 is
> disabled:
> 
> drivers/net/ethernet/sfc/tc_encap_actions.c:144:7: error: variable 'n' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
>                 if (encap->type & EFX_ENCAP_FLAG_IPV6) {
>                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/net/ethernet/sfc/tc_encap_actions.c:184:8: note: uninitialized use occurs here
>                 if (!n) {
>                      ^
> drivers/net/ethernet/sfc/tc_encap_actions.c:144:3: note: remove the 'if' if its condition is always false
>                 if (encap->type & EFX_ENCAP_FLAG_IPV6) {
>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/net/ethernet/sfc/tc_encap_actions.c:141:22: note: initialize the variable 'n' to silence this warning
>                 struct neighbour *n;
>                                    ^
>                                     = NULL
> 
> Change it to use the existing error handling path here.
> 
> Fixes: 7e5e7d800011a ("sfc: neighbour lookup for TC encap action offload")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/net/ethernet/sfc/tc_encap_actions.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/ethernet/sfc/tc_encap_actions.c b/drivers/net/ethernet/sfc/tc_encap_actions.c
> index aac259528e73e..cfd76d5bbdd48 100644
> --- a/drivers/net/ethernet/sfc/tc_encap_actions.c
> +++ b/drivers/net/ethernet/sfc/tc_encap_actions.c
> @@ -163,6 +163,7 @@ static int efx_bind_neigh(struct efx_nic *efx,
>  			 * enabled how did someone create an IPv6 tunnel_key?
>  			 */
>  			rc = -EOPNOTSUPP;
> +			n = NULL;
>  			NL_SET_ERR_MSG_MOD(extack, "No IPv6 support (neigh bind)");
>  #endif
>  		} else {
> 

Nack.  There is a bug here, as you've identified, but the right fix
 is to add a 'goto out_free;' after setting the rc and extack msg.
Setting n to NULL and relying on the subsequent error path will not
 only produce the wrong rc and error message, it will also attempt
 to drop a reference on neigh->egdev that was never taken.

-ed