Re: [RFC] struct filename, io_uring and audit troubles

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

On Mon, Sep 23, 2024 at 09:36:59PM +0100, Al Viro wrote:

> 	* go through the VFS side of things and make sure we have a consistent
> set of helpers that would take struct filename * - *not* the ad-hoc mix we
> have right now, when that's basically driven by io_uring borging them in
> one by one - or duplicates them without bothering to share helpers.
> E.g. mkdirat(2) does getname() and passes it to do_mkdirat(), which
> consumes the sucker; so does mknodat(2), but do_mknodat() is static.  OTOH,
> path_setxattr() does setxattr_copy(), then retry_estale loop with
> user_path_at() + mnt_want_write() + do_setxattr() + mnt_drop_write() + path_put()
> as a body, while on io_uring side we have retry_estale loop with filename_lookup() +
> (io_uring helper that does mnt_want_write() + do_setxattr() + mnt_drop_write()) +
> path_put().
> 	Sure, that user_path_at() call is getname() + filename_lookup() + putname(),
> so they are equivalent, but keeping that shite in sync is going to be trouble.

BTW, re mess around xattr:
static int __io_getxattr_prep(struct io_kiocb *req,
                              const struct io_uring_sqe *sqe)
{
...
        ix->ctx.cvalue = u64_to_user_ptr(READ_ONCE(sqe->addr2));
	ix->ctx.size = READ_ONCE(sqe->len);
...
        ret = strncpy_from_user(ix->ctx.kname->name, name,
				sizeof(ix->ctx.kname->name));

}

int io_fgetxattr(struct io_kiocb *req, unsigned int issue_flags)
{
...
        ret = do_getxattr(file_mnt_idmap(req->file),
			req->file->f_path.dentry,
			&ix->ctx);
...
}

ssize_t
do_getxattr(struct mnt_idmap *idmap, struct dentry *d,
        struct xattr_ctx *ctx)
{
...
        if (error > 0) {
		if (ctx->size && copy_to_user(ctx->value, ctx->kvalue, error))
...
}

and we have
struct xattr_ctx {
        /* Value of attribute */
	union {
		const void __user *cvalue;
		void __user *value;
	};
	...
}

Undefined behaviour aside, there's something odd going on here:
why do we bother with copy-in in ->prep() when we do copy-out in
->issue() anyway?  ->issue() does run with initiator's ->mm in use,
right?

IOW, what's the io_uring policy on what gets copied in ->prep() vs.
in ->issue()?