linux-kernel - Re: [PATCH v2][next] container_of: add container

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Message-ID: <3979f87d-b7d7-48bb-b6ab-cd8165dbc3cc@stanley.mountain>
Date: Wed, 29 Jan 2025 17:06:54 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] container_of: add container_first() macro

On Wed, Jan 29, 2025 at 02:14:14PM +0100, Greg KH wrote:
> On Wed, Jan 29, 2025 at 01:39:27PM +0300, Dan Carpenter wrote:
> > On Wed, Jan 29, 2025 at 09:34:07AM +0100, Greg KH wrote:
> > > On Wed, Jan 29, 2025 at 06:35:18PM +1030, Gustavo A. R. Silva wrote:
> > > > 
> > > > 
> > > > On 29/01/25 16:24, Greg KH wrote:
> > > > > On Wed, Jan 29, 2025 at 03:56:01PM +1030, Gustavo A. R. Silva wrote:
> > > > > > This is like container_of_const() but it contains an assert to
> > > > > > ensure that it's using the first member in the structure.
> > > > > 
> > > > > But why?  If you "know" it's the first member, just do a normal cast.
> > > > > If you don't, then you probably shouldn't be caring about this anyway,
> > > > > right?
> > > > 
> > > > This is more about the cases where the member _must_ be first in the
> > > > structure. See below for an example related to -Wflex-array-member-not-at-end
> > > 
> > > That's fine, but that's a build-time issue, you should enforce that in
> > > the structure itself, why are you forcing people to remember to use this
> > > macro when you want to use the field?  There's nothing preventing anyone
> > > from using container_of() instead here, and nothing will catch that from
> > > what I can tell.
> > 
> > The new definition has a static_assert() in it so it's enforced about
> > build time.
> 
> Yes, but that forces you to "know" to do that in the .c file.  How do
> you know to use this, and if you remove it or change it to
> container_of(), it works just fine again.
> 

I guess my use case is different from Gustavo's.  For him, using
container_of() is fine.  We probably don't even need an assert because
once you see a struct_group_tagged() then you know the order is important.

For me, it's code like I mentioned which does:

	p = container_of();
	if (IS_ERR(p))
		...

And I did see you suggest that people re-write that kind of code, but no
one is going to do that.  :P  People know that container_of() is just a
cast in that case.  It works fine.  It's just a bit ugly.

regards,
dan carpenter