Re: [PATCH][RFC] add a string-to-qstr constructor

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Jan 31, 2025 at 11:29:05AM -0800, Linus Torvalds wrote:
> On Fri, 31 Jan 2025 at 00:32, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > This commit lifts definition(s) of QSTR() into linux/dcache.h,
> > converts it to compound literal (all bcachefs users are fine
> > with that) and converts assorted open-coded instances to using
> > that.
> 
> Looks fine to me. I do wonder if the cast to 'struct qstr' should be
> part of QSTR_INIT, so that you can use that the same way when you
> already know the length.
> 
> We have code like this in fs/overlayfs/namei.c:
> 
>         *name = (struct qstr) QSTR_INIT(n, s - n);
> 
> in bcachefs has
> 
>         return (struct qstr) QSTR_INIT(d.v->d_name, bch2_dirent_name_bytes(d));
> 
> So both of them would seem to want to have that cast as part of the
> QSTR_INIT() thing.
> 
> Or maybe we could just make QSTR() itself more powerful, and do
> something like this:
> 
>     #define QSTR_INIT(n, l, ...) { { { .len = l } }, .name = n }
>     #define QSTR(a, ...) (struct qstr) QSTR_INIT(a , ## __VA_ARGS__, strlen(a))
> 
> which allows you to write either "QSTR(str)" or "QSTR(str, len)", and
> defaults to using 'strlen()' when no length is given.
> 
> Because if we have heper macros, let's make them _helpful_.
>
> Side note: you missed a few places in the core VFS code that could
> also use this new cleanup:
> 
>         struct qstr this = QSTR_INIT("pts", 3);
>         ...
>         child = d_hash_and_lookup(parent, &this);
> 
> can now be just
> 
>         child = d_hash_and_lookup(parent, &QSTR("pts"));
> 
> but sadly a few more are static initializers and can't use 'strlen()'.

There's also this in fs/fuse/inode.c
                const struct qstr name = QSTR_INIT(".", 1);
and
        struct qstr null_name = QSTR_INIT(NULL_FILE_NAME,
						  sizeof(NULL_FILE_NAME)-1);
in selinuxfs - I wasn't trying to get them all, just the infrastructure
and enough examples to demonstrate the usefulness.  Once the definition
is merged, those could be switched at leisure.