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

From: Matthew Wilcox
Date: Tue Dec 06 2022 - 15:18:57 EST


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.