[PATCH 05/14] fs: don't allow kernel reads and writes without iter ops

[Linus Torvalds]

On Thu, Oct 1, 2020 at 3:41 PM Al Viro <viro at zeniv.linux.org.uk> wrote:
>
> Better
>         loff_t dummy = 0;
> ...
>                 wr = __kernel_write(file, data, bytes, &dummy);

No, just fix __kernel_write() to work correctly.

The fact is, NULL _is_ the right pointer for ppos these days.

That commit by Christoph is buggy: it replaces new_sync_write() with a
buggy open-coded version.

Notice how new_sync_write does

        kiocb.ki_pos = (ppos ? *ppos : 0);
,,,
        if (ret > 0 && ppos)
                *ppos = kiocb.ki_pos;

but the open-coded version doesn't.

So just fix that in linux-next. The *last* thing we want is to have
different semantics for the "same" kernel functions.

               Linus