Re: [PATCH] [iov_iter] use memmove() when copying to/from user page

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, May 16, 2017 at 01:10:56PM -0700, Dmitry Vyukov wrote:

> >         * behaviour of sendfile() in such a case.  And there I've no problem
> > with saying "contents after operation is undefined".  If you wish to change
> > that, by all means start with documenting the semantics you want to promise
> > to userland.
> 
> I would say it's already documented.
> sendfile says that it "copies data". memmove says that it "copies
> data". memcpy says that it "copies data, but data must not overlap".
> sendfile does not say that "data must not overlap".

In that case your patch does not suffice.  Overlapping move _forwards_ still
yields unexpected results, doesn't it?  I'm all for documenting that
resulting contents is undefined in case of overlap.  The same goes for write()
from mmap'ed area, BTW.  You suggest changing that undefined behaviour *and*
either pretending that it's not undefined anymore (obviously false) or
failing to describe the new cases when it is not undefined anymore.

It's not the cost of extra branch; it's ill-defined rules that would need to
be followed to be able to rely upon the "improvement".