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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <06aa1194-a7aa-4a5a-adb8-f6cd447d35d9@stanley.mountain>
Date: Wed, 29 Jan 2025 13:39:27 +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 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.

+#define container_first(ptr, type, member) ({                           \
+       static_assert(offsetof(type, member) == 0, "not first member"); \
+       container_of_const(ptr, type, member); })

That was the discussion at plumbers, Gustavo just wanted to use
container_of() but I told him I was tired of code which assumes that
container_of() is just a cast.  If we're going to write code with that
assumption then lets create a different macro for it and let's make the
build break if someone changes it.

regards,
dan carpenter