Re: [PATCH bpf-next] bpf: Support default .validate() and .update() behavior for struct_ops links

[Yonghong Song <yonghong.song@linux.dev>]

On 8/11/23 8:09 AM, David Vernet wrote:
> On Thu, Aug 10, 2023 at 11:43:26PM -0700, Yonghong Song wrote:
>>
>>
>> On 8/10/23 3:04 PM, David Vernet wrote:
>>> Currently, if a struct_ops map is loaded with BPF_F_LINK, it must also
>>> define the .validate() and .update() callbacks in its corresponding
>>> struct bpf_struct_ops in the kernel. Enabling struct_ops link is useful
>>> in its own right to ensure that the map is unloaded if an application
>>> crashes. For example, with sched_ext, we want to automatically unload
>>> the host-wide scheduler if the application crashes. We would likely
>>> never support updating elements of a sched_ext struct_ops map, so we'd
>>> have to implement these callbacks showing that they _can't_ support
>>> element updates just to benefit from the basic lifetime management of
>>> struct_ops links.
>>>
>>> Let's enable struct_ops maps to work with BPF_F_LINK even if they
>>> haven't defined these callbacks, by assuming that a struct_ops map
>>> element cannot be updated by default.
>>
>> Maybe you want to add one map_flag to indicate validate/update callbacks
>> are optional for a struct_ops link? In this case, some struct_ops maps
>> can still require validate() and update(), but others can skip them?
> 
> Are you proposing that a map flag be added that a user space caller can
> specify to say that they're OK with a struct_ops implementation not
> supporting .validate() and .update(), but still want to use a link to
> manage registration and unregistration?  Assuming I'm understanding your
> suggestion correctly, I don't think it's what we want. Updating a
> struct_ops map value is arguably orthogonal to the bpf link handling
> registration and unregistration, so it seems confusing to require a user
> to specify that it's the behavior they want as there's no reason they
> shouldn't want it. If they mistakenly thought that update element is
> supposed for that struct_ops variant, they'll just get an -EOPNOTSUPP
> error at runtime, which seems reasonable. If a struct_ops implementation
> should have implemented .validate() and/or .update() and neglects to,
> that would just be a bug in the struct_ops implementation.
> 
> Apologies if I've misunderstood your proposal, and please feel free to
> clarify if I have.

You understanding with my proposal is correct.
Okay, after further thought, I agree with your above point.
Lacking implementation of 'validate' and 'update' itself is
equivalent to a flag. So flag itself is not really needed.

> 
> Thanks,
> David
> 
>>
>>>
>>> Signed-off-by: David Vernet <void@manifault.com>
>>> ---
>>>    kernel/bpf/bpf_struct_ops.c | 17 +++++++++++------
>>>    1 file changed, 11 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
>>> index eaff04eefb31..3d2fb85186a9 100644
>>> --- a/kernel/bpf/bpf_struct_ops.c
>>> +++ b/kernel/bpf/bpf_struct_ops.c
>>> @@ -509,9 +509,12 @@ static long bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
>>>    	}
>>>    	if (st_map->map.map_flags & BPF_F_LINK) {
>>> -		err = st_ops->validate(kdata);
>>> -		if (err)
>>> -			goto reset_unlock;
>>> +		err = 0;
>>> +		if (st_ops->validate) {
>>> +			err = st_ops->validate(kdata);
>>> +			if (err)
>>> +				goto reset_unlock;
>>> +		}
>>>    		set_memory_rox((long)st_map->image, 1);
>>>    		/* Let bpf_link handle registration & unregistration.
>>>    		 *
>>> @@ -663,9 +666,6 @@ static struct bpf_map *bpf_struct_ops_map_alloc(union bpf_attr *attr)
>>>    	if (attr->value_size != vt->size)
>>>    		return ERR_PTR(-EINVAL);
>>> -	if (attr->map_flags & BPF_F_LINK && (!st_ops->validate || !st_ops->update))
>>> -		return ERR_PTR(-EOPNOTSUPP);
>>> -
>>>    	t = st_ops->type;
>>>    	st_map_size = sizeof(*st_map) +
>>> @@ -838,6 +838,11 @@ static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map
>>>    		goto err_out;
>>>    	}
>>> +	if (!st_map->st_ops->update) {
>>> +		err = -EOPNOTSUPP;
>>> +		goto err_out;
>>> +	}
>>> +
>>>    	err = st_map->st_ops->update(st_map->kvalue.data, old_st_map->kvalue.data);
>>>    	if (err)
>>>    		goto err_out;
>