On Fri, Jun 18, 2021 at 10:32:54PM +0000, Al Viro wrote:

On Fri, Jun 18, 2021 at 03:10:03PM -0700, Omar Sandoval wrote:

quoted

Or do the same reverting thing that Al did, but with copy_from_iter()
instead of copy_from_iter_full() and being careful with the copied count
(which I'm not 100% sure I got correct here):

	size_t copied = copy_from_iter(&encoded, sizeof(encoded), &i);
	if (copied < offsetofend(struct encoded_iov, size))
		return -EFAULT;
	if (encoded.size > PAGE_SIZE)
		return -E2BIG;
	if (encoded.size < ENCODED_IOV_SIZE_VER0)
		return -EINVAL;
	if (encoded.size > sizeof(encoded)) {
		if (copied < sizeof(encoded)
			return -EFAULT;
		if (!iov_iter_check_zeroes(&i, encoded.size - sizeof(encoded))
			return -EINVAL;
	} else if (encoded.size < sizeof(encoded)) {
		// older than what we expect
		if (copied < encoded.size)
			return -EFAULT;
		iov_iter_revert(&i, copied - encoded.size);
		memset((void *)&encoded + encoded.size, 0, sizeof(encoded) - encoded.size);
	}    

simpler than that, actually -

	copied = copy_from_iter(&encoded, sizeof(encoded), &i);
	if (unlikely(copied < sizeof(encoded))) {
		if (copied < offsetofend(struct encoded_iov, size) ||
		    copied < encoded.size)
			return iov_iter_count(i) ? -EFAULT : -EINVAL;
	}
	if (encoded.size > sizeof(encoded)) {
		if (!iov_iter_check_zeroes(&i, encoded.size - sizeof(encoded))
			return -EINVAL;
	} else if (encoded.size < sizeof(encoded)) {
		// copied can't be less than encoded.size here - otherwise
		// we'd have copied < sizeof(encoded) and the check above
		// would've buggered off
		iov_iter_revert(&i, copied - encoded.size);
		memset((void *)&encoded + encoded.size, 0, sizeof(encoded) - encoded.size);
	}

should do it.

Thanks, Al, I'll send an updated version with this approach next week.