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From:   Allen <allen.lkml@gmail.com>
Date:   Fri, 28 Aug 2020 12:39:42 +0530
Message-ID: <CAOMdWSLJuG+EN3gDUYpoPjYU2cuRrN34NYZZ9Uhb08_egw3ZCA@mail.gmail.com>
Subject: Re: [PATCH] linux/kernel.h: add container_from()
To:     Kees Cook <keescook@chromium.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jens Axboe <axboe@kernel.dk>
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> > > You really have to pick some pretty excessive type names (or variable
> > > names) to get close to 80 characters. Again, to pick an example:
> > >
> > >         struct timer_group_priv *priv = container_of(handle,
> > >                         struct timer_group_priv, timer[handle->num]);
> > >
> > > ends up being long even if you were to split it, but that funky
> > > container_from() wouldn't have helped the real problem - the fact that
> > > the above is complex and nasty.
>
> The point about doing the assignment with the declaration certainly makes
> the "ugliness" worse, I agree. I'm still not generally convinced about
> the redundancy level pros/cons, but I concede that having a common idiom
> (rather than a succinct but subsystem-dependent idiom) is better for
> people reading the code for the first time.
>
> > > And I had to _search_ for that example. All the normal cases of
> > > split-line container-of's were due to doing it with the declaration,
> > > or beause the first argument ended up being an expression in itself
> > > and the nested expressions made it more complex.
> >
> > Speaking of searching, this kind of typeof use is, IMO, actively
> > harmful - it makes finding the places where we might get from
> > e.g. linked list to containing objects much harder.  container_of
> > (unless combined with obfuscating use of typeof()) at least gives
> > you a chance to grep - struct foo *not* followed by '*' is a pattern
> > that doesn't give too many false positives.  This one, OTOH, is
> > essentially impossible to grep for.
>
> And this observation about workflow does strike a chord with me. I do end
> up with those kind of searches too. In trying to examine my preferences
> here, I think my instincts are to avoid open-coded types (leading me to
> want to use typeof()) but I think those instincts were actually developed
> from dealing with _sizeof_ and all the way it goes terribly wrong. So,
> okay, I'm convinced. container_of() it is.

 so container_of() it is :)
Will start updating the rest of the patches.

Thanks,
- Allen

> Doing these conversions becomes a little less mechanical if assignment
> needs to be split from declaration, but hey, we've got a 100 character
> line "limit" now, so maybe it'll be less needed. :)
>
> --
> Kees Cook