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Date:   Fri, 18 Jun 2021 22:32:54 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Omar Sandoval <osandov@osandov.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
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Subject: Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()
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On Fri, Jun 18, 2021 at 03:10:03PM -0700, Omar Sandoval wrote:

> 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.