Re: [RFC] situation with csum_and_copy_... API

On Tue, Nov 18, 2014 at 12:49:13PM -0800, Linus Torvalds wrote:

On Tue, Nov 18, 2014 at 12:47 AM, Al Viro [off-list ref] wrote:

quoted

The minimal implementations would be

__wsum csum_and_copy_from_user(const void __user *src, void *dst, int len,
                               __wsum sum, int *err_ptr)
{
        if (unlikely(copy_from_user(dst, src, len) < 0)) {

No. That "< 0" should be "!= 0". The user copy functions return a
positive value of how many bytes they *failed* to copy.

D'oh...  Yes, indeed - sorry about the braino.

quoted

IMO the calling conventions are atrocious.

Yeah, not pretty. At the same time, the pain of changing what seems to
work might not be worth it.

And quite frankly, I *detest* your patch 3/5.

"access_ok()" isn't that expensive, and removing them as unnecessary
is fraught with errors. We've had several cases of "oops, we used
__get_user() in a loop, because it generates much better code, but
we'd forgotten to do access_ok(), so now people can read kernel data".

OK...  If netdev folks can live with that for now, I've no problem with
dropping 3/5.  However, I really think we need a variant of csum-and-copy
that would _not_ bother with access_ok() longer term.  That can wait, though...