From: Jens Axboe > Sent: 22 August 2021 00:14 > > On 8/21/21 4:25 PM, Al Viro wrote: > > On Sat, Aug 21, 2021 at 03:24:28PM +0100, Pavel Begunkov wrote: > >> On 8/12/21 9:40 PM, Pavel Begunkov wrote: > >>> For the bug description see 2/2. As mentioned there the current problems > >>> is because of generic_write_checks(), but there was also a similar case > >>> fixed in 5.12, which should have been triggerable by normal > >>> write(2)/read(2) and others. > >>> > >>> It may be better to enforce reexpands as a long term solution, but for > >>> now this patchset is quickier and easier to backport. > >> > >> We need to do something with this, hopefully soon. > > > > I still don't like that approach ;-/ If anything, I would rather do > > something like this, and to hell with one extra word on stack in > > several functions; at least that way the semantics is easy to describe. > > Pavel suggested this very approach initially as well when we discussed > it, and if you're fine with the extra size_t, it is by far the best way > to get this done and not have a wonky/fragile API. All (well maybe almost all) the users of iov_iter have the short iov[] cache and the pointer to the big iov[] to kfree() allocated together with the iov_iter structure itself. These are almost always on stack. Putting the whole lot together in a single structure would make the call sequences a lot less complex and wouldn't use any more stack/data is almost all the cases. It would also mean that the 'iter' code could always have a pointer to the base of the original iov[] list. The lack of which is probably makes the 'revert' code hard? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
