Re: [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec

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

On Wed, Sep 23, 2020 at 02:38:24PM +0000, David Laight wrote:
> From: Al Viro
> > Sent: 23 September 2020 15:17
> > 
> > On Wed, Sep 23, 2020 at 08:05:41AM +0200, Christoph Hellwig wrote:
> > 
> > > +struct iovec *iovec_from_user(const struct iovec __user *uvec,
> > > +		unsigned long nr_segs, unsigned long fast_segs,
> > 
> > Hmm...  For fast_segs unsigned long had always been ridiculous
> > (4G struct iovec on caller stack frame?), but that got me wondering about
> > nr_segs and I wish I'd thought of that when introducing import_iovec().
> > 
> > The thing is, import_iovec() takes unsigned int there.  Which is fine
> > (hell, the maximal value that can be accepted in 1024), except that
> > we do pass unsigned long syscall argument to it in some places.
> 
> It will make diddly-squit difference.
> The parameters end up in registers on most calling conventions.
> Plausibly you get an extra 'REX' byte on x86 for the 64bit value.
> What you want to avoid is explicit sign/zero extension and value
> masking after arithmetic.

Don't tell me what I want; your telepathic abilities are consistently sucky.

I am *NOT* talking about microoptimization here.  I have described
the behaviour change of syscall caused by commit 5 years ago.  Which is
generally considered a problem.  Then I asked whether that behaviour
change would fall under the "if nobody noticed, it's not a userland ABI
breakage" exception.

Could you show me the point where I have expressed concerns about
the quality of amd64 code generated for that thing, before or after
the change in question?