Re: ARM32 UBSAN failure in credit2

[Jürgen Groß <jgross@xxxxxxxx>]

On 15.02.25 00:36, Andrew Cooper wrote:
> This is nasty.
>
> https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/9137008215
>
> When preprocessed, we get:
>
> diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
> index 0a83f237259f..6b8d3660240a 100644
> --- a/xen/common/sched/credit2.c
> +++ b/xen/common/sched/credit2.c
> @@ -958,7 +958,28 @@ cpu_add_to_runqueue(const struct scheduler *ops,
> unsigned int cpu)
>       write_lock_irqsave(&prv->lock, flags);
>   
>       rqd_ins = &prv->rql;
> +
> +#if 0
>       list_for_each_entry ( rqd, &prv->rql, rql )
> +#else
> +    for ( (rqd) = ({
> +                typeof(((typeof(*(rqd)) *)((void*)0))->rql) *__mptr =
> +                    ((&prv->rql)->next);
> +                (typeof(*(rqd)) *)
> +                    ((char *)__mptr -
> +                     __builtin_offsetof(typeof(*(rqd)),rql) );
> +            });
> +          &(rqd)->rql != // <-- problem expression
> +              (&prv->rql);
> +          (rqd) = ({
> +                  typeof(((typeof(*(rqd)) *)((void*)0))->rql) *__mptr =
> +                      ((rqd)->rql.next);
> +                  (typeof(*(rqd)) *)
> +                      ((char *)__mptr -
> +                       __builtin_offsetof(typeof(*(rqd)),rql) );
> +              })
> +        )
> +#endif
>       {
>           /* Remember first unused queue index. */
>           if ( !rqi_unused && rqd->id > rqi )
>
>
> The alignment of csched2_runqueue_data is 8, while csched2_private is 4.
>
> priv's list_head for rql is at +28 (+0x1c), and list_for_each_entry()
> performs a buggily-typed container_of(), treating a csched2_private as
> if it were csched2_runqueue_data.
No, I don't think so. It just compares the addresses of 2 struct list_head.
1 in csched2_runqueue_data and 1 in csched2_private.

> It functions because it's only an address equality check, but it's also
> why UBSAN objects.
struct list_head should require only 4 byte alignment, so I don't see why
this would trigger UBSAN. Could it be that UBSAN has a bug here?


Juergen

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