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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