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

[Matthew Wilcox <willy@infradead.org>]

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.