Re: [PATCH v2 1/4] container_of: add container_of_const() that preserves const-ness of the pointer

[Rasmus Villemoes]

On 06/12/2022 21.18, Matthew Wilcox wrote:
> On Tue, Dec 06, 2022 at 07:46:47PM +0100, Greg Kroah-Hartman wrote:
>> On Tue, Dec 06, 2022 at 05:18:22PM +0000, Matthew Wilcox wrote:
>>>  static inline struct external_name *external_name(struct dentry *dentry)
>>>  {
>>> -	return container_of(dentry->d_name.name, struct external_name, name[0]);
>>> +	return container_of_not_const(dentry->d_name.name,
>>> +				      struct external_name, name[0]);
>>>  }
>>
>> Will just:
>> 	return container_of((unsigned char *)dentry->d_name.name, struct external_name, name[0]);
>> work by casting away the "const" of the name?
>>
>> Yeah it's ugly, I never considered the address of a const char * being
>> used as a base to cast back from.  The vfs is fun :)
> 
> Yes, that also works.  This isn't particularly common in the VFS, it's
> just the dcache.  And I understand why it's done like this; you don't
> want rando filesystems modifying dentry names without also updating
> the hash.
> 
> I feel like all the options here are kind of ugly.  Seeing casts in
> the arguments to container_of should be a red flag!
> 
> Here's a bit of a weird option ...
> 
> +#define container_of_2(ptr, p_m, type, member)                         \
> +       _Generic(ptr,                                                   \
> +               const typeof(*(ptr)) *: (const type *)container_of(ptr->p_m, type, member), \
> +               default: ((type *)container_of(ptr->p_m, type, member)))
> +
> 
>  static inline struct external_name *external_name(struct dentry *dentry)
>  {
> -       return container_of(dentry->d_name.name, struct external_name, name[0]);
> +       return container_of_2(dentry, d_name.name, struct external_name,
> +                               name[0]);
>  }
> 
> so we actually split the first argument into two -- the pointer which
> isn't const, then the pointer member which might be const, but we don't
> use it for the return result of container_of_2.

Wait, what? The const-ness or not of dentry is completely irrelevant,
we're not doing any pointer arithmetic on that to obtain some other
pointer that morally should have the same const-ness. We're
dereferencing dentry to get a pointer value, and _that_ pointer value is
then subject to the pointer arithmetic.

Note that external_name might as well have had the prototype

static inline struct external_name *external_name(const struct dentry
*dentry)

and then your container_of_2 would break.

I think that if we want to move towards container_of preserving the
constness of the member pointer, the right fix here is indeed a cast,
but not inside container_of, but rather to cast away the const afterwards:

  return (struct external_name *)container_of(dentry->d_name.name,
struct external_name, name[0]);

knowing that yes, the dentry only keeps a const pointer to the name[]
member for good reasons, but the callers very much do need to modify the
rest of the struct.

Rasmus