From: Christoph Hellwig

Sent: 21 September 2020 15:22

So looking at the various callers I'm not sure this API is the
best.  If we want to do something fancy I'd hide the struct iovec
instances entirely with something like:

struct iov_storage {
	struct iovec stack[UIO_FASTIOV], *vec;
}

int iov_iter_import_iovec(struct iov_iter *iter, struct iov_storage *s,
		const struct iovec __user *vec, unsigned long nr_segs,
		int type);

and then add a new helper to free the thing if needed:

void iov_iter_release_iovec(struct iov_storage *s)
{
	if (s->vec != s->stack)
		kfree(s->vec);
}

I've been looking at this code again now most of the pending changes
are in Linus's tree.

I was actually looking at going one stage further.
The 'iov_iter' is always allocated with the 'iov_storage' *above).
Usually both are on the callers stack - possibly in different functions.

So add:
struct iovec_iter {
	struct iov_iter iter;
	struct iovec to_free;
	struct iovec stack[UIO_FASTIOV];
};

int __iovec_import(struct iovec_iter *, const struct iovec __user *vec,
	unsigned long nr_segs, int type, bool compat);

And a 'clean' function to do kfree(iovec->to_free);

This reduces the complexity of most of the callers.

I started doing the changes, but got in a mess in io_uring.c (as usual).
I think I've got a patch pending (in my brain) to simplify the io_uring code.

The plan is to add:
	if (iter->iov != xxx->to_free)
		iter->iov = xxx->stack;
Prior to every use of the iter.
This fixes up anything that got broken by a memcpy() of the fields.
The tidyup code is then always kfree(xxx->to_free).

	David

	

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