Re: [PATCH bpf-next v5 3/9] bpf: hold module for bpf_struct_ops_map.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 10/18/23 17:36, Martin KaFai Lau wrote:
> On 10/17/23 9:23 AM, thinker.li@gmail.com wrote:
>> From: Kui-Feng Lee <thinker.li@gmail.com>
>>
>> To ensure that a module remains accessible whenever a struct_ops 
>> object of
>> a struct_ops type provided by the module is still in use.
>>
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
>>   include/linux/bpf.h         |  1 +
>>   kernel/bpf/bpf_struct_ops.c | 21 ++++++++++++++++++---
>>   2 files changed, 19 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index e6a648af2daa..1e1647c8b0ce 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1627,6 +1627,7 @@ struct bpf_struct_ops {
>>       int (*update)(void *kdata, void *old_kdata);
>>       int (*validate)(void *kdata);
>>       struct btf *btf;
>> +    struct module *owner;
>>       const struct btf_type *type;
>>       const struct btf_type *value_type;
>>       const char *name;
>> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
>> index 7758f66ad734..b561245fe235 100644
>> --- a/kernel/bpf/bpf_struct_ops.c
>> +++ b/kernel/bpf/bpf_struct_ops.c
>> @@ -112,6 +112,7 @@ static const struct btf_type *module_type;
>>   static void bpf_struct_ops_init_one(struct bpf_struct_ops *st_ops,
>>                       struct btf *btf,
>> +                    struct module *owner,
>>                       struct bpf_verifier_log *log)
>>   {
>>       const struct btf_member *member;
>> @@ -186,6 +187,7 @@ static void bpf_struct_ops_init_one(struct 
>> bpf_struct_ops *st_ops,
>>                   st_ops->name);
>>           } else {
>>               st_ops->btf = btf;
>> +            st_ops->owner = owner;
> 
> I suspect it will turn out to be just "st_ops->owner = st_ops->owner;" 
> in a latter patch. st_ops->owner should have already been initialized 
> (with THIS_MODULE?).


Yes, you are correct.  It ends up st_ops->owner passing from the caller.
I will remove this line and the argument.

> 
>>               st_ops->type_id = type_id;
>>               st_ops->type = t;
>>               st_ops->value_id = value_id;
>> @@ -193,6 +195,7 @@ static void bpf_struct_ops_init_one(struct 
>> bpf_struct_ops *st_ops,
>>                                   value_id);
>>           }
>>       }
>> +
> 
> nit. extra newline.

got it!

> 
>>   }
>>   void bpf_struct_ops_init(struct btf *btf, struct bpf_verifier_log *log)
>> @@ -215,7 +218,7 @@ void bpf_struct_ops_init(struct btf *btf, struct 
>> bpf_verifier_log *log)
>>       for (i = 0; i < ARRAY_SIZE(bpf_struct_ops); i++) {
>>           st_ops = bpf_struct_ops[i];
>> -        bpf_struct_ops_init_one(st_ops, btf, log);
>> +        bpf_struct_ops_init_one(st_ops, btf, NULL, log);
>>       }
>>   }
>> @@ -630,6 +633,7 @@ static void __bpf_struct_ops_map_free(struct 
>> bpf_map *map)
>>           bpf_jit_uncharge_modmem(PAGE_SIZE);
>>       }
>>       bpf_map_area_free(st_map->uvalue);
>> +    module_put(st_map->st_ops->owner);
>>       bpf_map_area_free(st_map);
>>   }
>> @@ -676,9 +680,18 @@ static struct bpf_map 
>> *bpf_struct_ops_map_alloc(union bpf_attr *attr)
>>       if (!st_ops)
>>           return ERR_PTR(-ENOTSUPP);
>> +    /* If st_ops->owner is NULL, it means the struct_ops is
>> +     * statically defined in the kernel.  We don't need to
>> +     * take a refcount on it.
>> +     */
>> +    if (st_ops->owner && !btf_try_get_module(st_ops->btf))
> 
> This just came to my mind. Is the module refcnt needed during map 
> alloc/free or it could be done during the reg/unreg instead?


Sure, I can move it to reg/unreg.

> 
> 
>> +        return ERR_PTR(-EINVAL);
>> +
>>       vt = st_ops->value_type;
>> -    if (attr->value_size != vt->size)
>> +    if (attr->value_size != vt->size) {
>> +        module_put(st_ops->owner);
>>           return ERR_PTR(-EINVAL);
>> +    }
>>       t = st_ops->type;
>> @@ -689,8 +702,10 @@ static struct bpf_map 
>> *bpf_struct_ops_map_alloc(union bpf_attr *attr)
>>           (vt->size - sizeof(struct bpf_struct_ops_value));
>>       st_map = bpf_map_area_alloc(st_map_size, NUMA_NO_NODE);
>> -    if (!st_map)
>> +    if (!st_map) {
>> +        module_put(st_ops->owner);
>>           return ERR_PTR(-ENOMEM);
>> +    }
>>       st_map->st_ops = st_ops;
>>       map = &st_map->map;
>