Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()

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

On Fri, Jun 18, 2021 at 02:40:51PM -0700, Linus Torvalds wrote:
> On Fri, Jun 18, 2021 at 2:32 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > Huh?  All corner cases are already taken care of by copy_from_iter{,_full}().
> > What I'm proposing is to have the size as a field in 'encoded' and
> > do this
> 
> Hmm. Making it part of the structure does make it easier (also for the
> sending userspace side, that doesn't now have to create yet another
> iov or copy the structure or whatever).
> 
> Except your code doesn't actually handle the "smaller than expected"
> case correctly, since by the time it even checks for that, it will
> possibly already have failed. So you actually had a bug there - you
> can't use the "xyz_full()" version and get it right.

Right you are - should be something along the lines of

#define MIN_ENCODED_SIZE minimal size, e.g. offsetof of the next field after .size

	size = copy_from_iter(&encoded, sizeof(encoded), &i);
	if (unlikely(size < sizeof(encoded))) {
		// the total length is less than expected
		// must be at least encoded.size, though, and it would better
		// cover the .size field itself.
	    	if (size < MIN_ENCODED_SIZE || size < encoded.size)
			sod off
	}
	if (sizeof(encoded) < encoded.size) {
		// newer than expected
		same as in previous variant
	} else if (size > encoded.size) {
		// older than expected
		iov_iter_revert(size - encoded.size);
		memset(....) as in previous variant
	}