Re: [PATCH] container_of.h: make container_of const-aware

[Andrew Morton]

On Sat, 12 Feb 2022 01:10:09 +0300 Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> wrote:

> On Sat, 12 Feb 2022 at 01:05, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, 10 Feb 2022 20:04:34 +0300 Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> wrote:
> >
> > > container_of() macro has one major drawback. It does not check whether
> > > the passed ptr has a const pointer, the result will always be a
> > > non-const pointer. Use a _Generic() construct (supported since gcc 4.9
> > > and Clang 3.0) to teach container_of that if converting a const pointer,
> > > the returned pointer should also have the const modifier.
> > >
> >
> > Nice idea, but my x86_64 allnoconfig build explodes with zillions of
> > warnings.
> >
> > In file included from ./include/linux/list.h:5,
> >                  from ./include/linux/module.h:12,
> >                  from init/do_mounts.c:2:
> > ./include/net/sock.h: In function 'sk_entry':
> > ./include/linux/container_of.h:17:42: warning: return discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
> 
> Are these warnings legit? In other words, is the code converting a
> const pointer into a modifiable one?

It is indeed doing that.

static inline struct sock *sk_entry(const struct hlist_node *node)
{
	return hlist_entry(node, struct sock, sk_node);
}

Which is a pretty rude thing to do.  What was the point in that const arg?

> I've stumbled upon this in drm
> drivers.
> How do we proceed with this patch?

They all need to be fixed up :(

You could take a shot, if you have the time.  If not, bug the relevant
code's owners about it.  But we can't add a patch like this, blow up
everyone's builds and wait for various people to fix up the fallout!