Re: [PATCH v10 4/5] io_uring: add fsetxattr and setxattr support

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

On Wed, Dec 29, 2021 at 12:30:01PM -0800, Stefan Roesch wrote:

> +static int __io_setxattr_prep(struct io_kiocb *req,
> +			const struct io_uring_sqe *sqe)
> +{
> +	struct io_xattr *ix = &req->xattr;
> +	const char __user *name;
> +	int ret;
> +
> +	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
> +		return -EINVAL;
> +	if (unlikely(sqe->ioprio))
> +		return -EINVAL;
> +	if (unlikely(req->flags & REQ_F_FIXED_FILE))
> +		return -EBADF;
> +
> +	ix->filename = NULL;
> +	name = u64_to_user_ptr(READ_ONCE(sqe->addr));
> +	ix->ctx.value = u64_to_user_ptr(READ_ONCE(sqe->addr2));
> +	ix->ctx.kvalue = NULL;
> +	ix->ctx.size = READ_ONCE(sqe->len);
> +	ix->ctx.flags = READ_ONCE(sqe->xattr_flags);
> +
> +	ix->ctx.kname = kmalloc(sizeof(*ix->ctx.kname), GFP_KERNEL);
> +	if (!ix->ctx.kname)
> +		return -ENOMEM;
> +
> +	ret = setxattr_copy(name, &ix->ctx);
> +	if (ret) {
> +		kfree(ix->ctx.kname);
> +		return ret;
> +	}
> +
> +	req->flags |= REQ_F_NEED_CLEANUP;
> +	return 0;
> +}

OK, so you
	* allocate a buffer for xattr name
	* have setxattr_copy() copy the name in *and* memdup the contents
	* on failure, you have the buffer for xattr name freed and return
an error.  memdup'ed stuff is left for cleanup, presumably.

> +static int io_setxattr_prep(struct io_kiocb *req,
> +			const struct io_uring_sqe *sqe)
> +{
> +	struct io_xattr *ix = &req->xattr;
> +	const char __user *path;
> +	int ret;
> +
> +	ret = __io_setxattr_prep(req, sqe);
> +	if (ret)
> +		return ret;
> +
> +	path = u64_to_user_ptr(READ_ONCE(sqe->addr3));
> +
> +	ix->filename = getname_flags(path, LOOKUP_FOLLOW, NULL);
> +	if (IS_ERR(ix->filename)) {
> +		ret = PTR_ERR(ix->filename);
> +		ix->filename = NULL;
> +	}
> +
> +	return ret;
> +}

... and here you use it and bring the pathname in.  Should the latter
step fail, you restore ->filename to NULL and return an error.

Could you explain what kind of magic could allow the caller to tell
whether ix->ctx.kname needs to be freed on error?  I don't see any way
that could possibly work...