linux-hardening - Re: [PATCH v2 1/2] sh: Added kernel.h to word-at-a-time

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Date: Mon, 18 Dec 2023 19:23:23 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Tanzir Hasan <tanzirh@...gle.com>, Kees Cook <keescook@...omium.org>,
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
	Nick DeSaulniers <nnn@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, llvm@...ts.linux.dev,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 1/2] sh: Added kernel.h to word-at-a-time

On Mon, Dec 18, 2023 at 08:57:59AM -0800, Nick Desaulniers wrote:
> On Fri, Dec 15, 2023 at 11:09 AM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> > On Fri, Dec 15, 2023 at 8:31 PM Tanzir Hasan <tanzirh@...gle.com> wrote:
> > > On Fri, Dec 15, 2023 at 8:04 AM Andy Shevchenko <andy@...nel.org> wrote:
> > >> On Thu, Dec 14, 2023 at 09:06:12PM +0000, tanzirh@...gle.com wrote:

...

> > >> > +#include <linux/kernel.h>
> > >>
> > >> I highly discourage from doing that. Instead, split what is needed to
> > >> the separate (new) header and include that one.
> > >
> > >
> > > I think it would make the most sense to do this in a separate patch.
> > > What word-at-a-time.h needs from kernel.h is REPEAT_BYTE and to my knowledge,
> > > almost every other version of word-at-a-time.h includes kernel.h gets this by
> > > including kernel.h. A future change could be removing REPEAT_BYTE
> > > out of kernel.h
> >
> > Just create a patch that either moves that macro (along with upper_*()
> > and lower_*() APIs) to a more distinguishable header
> > (maybe bytes.h or words.h or wordpart.h, etc) and use it in your case
> > and fix others.
> 
> Andy,
> These are good suggestions and we should do them...
> 
> ...and Tanzir only has 3 weeks left of his internship.  I don't want
> him to get bogged down chasing build regressions from modifying the
> headers themselves.  I think what's best for him from here through the
> remainder of his internship is to stay focused on applying suggestions
> from IWYU to just modify the #include list of .c files, and not start
> splitting .h files.  Splitting the .h files will be the next step, and
> is made easier by having the codebase not have so many indirect
> includes (via IWYU), but we need time to soak header changes, and time
> Tanzir does not have.  Please can we keep the suggestions focused on
> whether the modifications to the header includes (and the tangential
> cleanups) are correct?

Understood. Can we add a comment like

/* FIXME: replace with a proper header to avoid dependency hell */
#include <linux/kernel.h>

?

> While REPEAT_BYTE has a manageable number of users, upper_* and
> lower_* have significantly more; I worry about moving those causing
> regressions.

If you look at how I did similar in the past, I back included new header
into kernel.h. Not the pretty solution, but allows to split in the new code.

> We can move them, but such changes would need significantly more soak time

s/significantly//

But I got your point, see above.

> than this series IMO. Tanzir is also working on statistical analysis; I
> suspect if he analyzes include/linux/kernel.h, he can comment on whether the
> usage of REPEAT_BYTE is correlated with the usage of upper_* and lower_* in
> order to inform whether they should be grouped together or not.

-- 
With Best Regards,
Andy Shevchenko