Re: [PATCH v2 1/7] hw/cxl/cxl-host: Fix missing ERRP_GUARD() in cxl_fixed_memory_window_config()

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Fri, 23 Feb 2024 16:56:47 +0800
Zhao Liu <zhao1.liu@linux.intel.com> wrote:

> From: Zhao Liu <zhao1.liu@intel.com>
> 
> As the comment in qapi/error, dereferencing @errp requires
> ERRP_GUARD():
> 
> * = Why, when and how to use ERRP_GUARD() =
> *
> * Without ERRP_GUARD(), use of the @errp parameter is restricted:
> * - It must not be dereferenced, because it may be null.
> ...
> * ERRP_GUARD() lifts these restrictions.
> *
> * To use ERRP_GUARD(), add it right at the beginning of the function.
> * @errp can then be used without worrying about the argument being
> * NULL or &error_fatal.
> *
> * Using it when it's not needed is safe, but please avoid cluttering
> * the source with useless code.
> 
> But in cxl_fixed_memory_window_config(), @errp is dereferenced in 2
> places without ERRP_GUARD():
> 
> fw->enc_int_ways = cxl_interleave_ways_enc(fw->num_targets, errp);
> if (*errp) {
>     return;
> }
> 
> and
> 
> fw->enc_int_gran =
>     cxl_interleave_granularity_enc(object->interleave_granularity,
>                                    errp);
> if (*errp) {
>     return;
> }
> 
> For the above 2 places, we check "*errp", because neither function
> returns a suitable error code. And since machine_set_cfmw() - the caller
> of cxl_fixed_memory_window_config() - doesn't get the NULL @errp
> parameter as the "set" method of object property,
> cxl_fixed_memory_window_config() hasn't triggered the bug that
> dereferencing the NULL @errp.
> 
> To follow the requirement of @errp, add missing ERRP_GUARD() in
> cxl_fixed_memory_window_config().
> 
> Suggested-by: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> ---
> Suggested by credit:
>  Markus: Referred his explanation about ERRP_GUARD().
> ---
> v2:
>  * Add the @errp dereference code in commit message to make review
>    easier. (Markus)
> ---
>  hw/cxl/cxl-host.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/cxl/cxl-host.c b/hw/cxl/cxl-host.c
> index 2aa776c79c74..c5f5fcfd64d0 100644
> --- a/hw/cxl/cxl-host.c
> +++ b/hw/cxl/cxl-host.c
> @@ -26,6 +26,7 @@ static void cxl_fixed_memory_window_config(CXLState *cxl_state,
>                                             CXLFixedMemoryWindowOptions *object,
>                                             Error **errp)
>  {
> +    ERRP_GUARD();
>      g_autofree CXLFixedWindow *fw = g_malloc0(sizeof(*fw));
>      strList *target;
>      int i;