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

[Omar Sandoval <osandov@osandov.com>]

On Wed, Jun 23, 2021 at 09:39:48PM +0000, Al Viro wrote:
> On Wed, Jun 23, 2021 at 01:46:50PM -0700, Omar Sandoval wrote:
> 
> > Suppose we add a new field representing a new type of encoding to the
> > end of encoded_iov. On the write side, the caller might want to specify
> > that the data is encoded in that new way, of course. But on the read
> > side, if the data is encoded in that new way, then the kernel will want
> > to return that. The kernel needs to know if the user's structure
> > includes the new field (otherwise when it copies the full struct out, it
> > will write into what the user thinks is the data instead).
> 
> Er...  What's the problem with simply copying that extended structure out,
> followed by the data?
> 
> IOW, why can't the caller pick the header out of the whole thing and
> deal with it in whatever way it likes?  Why should kernel need to do
> anything special here?
> 
> IDGI...  Userland had always been able to deal with that kind of stuff;
> you read e.g. gzipped data into buffer, you decode the header, you figure
> out how long it is and how far out does the payload begin, etc.
> 
> How is that different?

Ah, I was stuck on thinking about this calling convention:

	struct encoded_iov encoded_iov;
	char compressed_data[...];
	struct iovec iov[] = {
		{ &encoded_iov, sizeof(encoded_iov) },
		{ compressed_data, sizeof(compressed_data) },
	};
	preadv2(fd, iov, 2, -1, RWF_ENCODED);

But what you described would look more like:

	// Needs to be large enough for maximum returned header + data.
	char buffer[...];
	struct iovec iov[] = {
		{ buffer, sizeof(buffer) },
	};
	preadv2(fd, iov, 2, -1, RWF_ENCODED);
	// We should probably align the buffer.
	struct encoded_iov *encoded_iov = (void *)buffer;
	char *data = buffer + encoded_iov->size;

That's a little uglier, but it should work, and allows for arbitrary
extensions. So, among these three alternatives (fixed size structure
with reserved space, variable size structure like above, or ioctl),
which would you prefer?