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

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On 10/19/23 10:07 PM, Kui-Feng Lee wrote:


On 10/19/23 09:29, Kui-Feng Lee wrote:


On 10/18/23 17:36, Martin KaFai Lau wrote:
On 10/17/23 9:23 AM, thinker.li@xxxxxxxxx wrote:


  }
  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))

While replying and looking at it again, I don't think the btf_try_get_module(st_ops->btf) is safe. The module's owned st_ops itself could have been gone with the module. The same goes with the "st_ops->owner" test, so btf_is_module(btf) should be used instead.

I am risking to act like a broken clock to repeat this question, does it really need to store btf back into the st_ops which may accidentally get into the above btf_try_get_module(st_ops->btf) usage?


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.

Just found that we relies type information in st_ops to update element and clean up maps.
We can not move get/put modules to reg/unreg except keeping a redundant copy in
st_map or somewhere. It make the code much more complicated by
introducing get/put module here and there.

I prefer to keep as it is now. WDYT?

Yeah, sure. I was asking after seeing a longer wait time for the module to go away in patch 11 selftest and requires an explicit waiting for the tasks_trace period. Releasing the module refcnt earlier will help.

Regardless of the module refcnt hold/free location, I think storing the type* and value* in the module's owned st_ops does not look correct now. It was fine and convenient to piggy back them into bpf_struct_ops when everything was built-in the kernel and no lifetime concern. It makes sense now to separate them out from the module's owned st_ops. Something like:

struct btf_struct_ops_desc {
	struct bpf_struct_ops *ops;
        const struct btf_type *type;
        const struct btf_type *value_type;
        u32 type_id;
        u32 value_id;
};

struct btf_struct_ops_tab {
        u32 cnt;
	u32 capacity;
	struct btf_struct_ops_desc *st_ops_desc[];
};

wdyt?




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