On Fri, Oct 02, 2020 at 09:27:09AM -0700, Linus Torvalds wrote:

On Thu, Oct 1, 2020 at 3:41 PM Al Viro [off-list ref] wrote:

quoted

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.

It's a bit unintuitive that ppos=NULL means "use pos 0", not "use file->f_pos".

Anyway, it works.  The important thing is, this is still broken in linux-next...

- Eric