Re: [PATCH 1/1] container_of: Document container_of_const() is preferred

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Greg,

On Mon, Jun 17, 2024 at 12:44:55PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Jun 17, 2024 at 01:08:25PM +0300, Sakari Ailus wrote:
> > There is a warning in kerneldoc documentation of container_of() that
> > constness of @ptr is lost. While this is a suggestion container_of_const()
> > should be used instead, the vast majority of new code still uses
> > container_of():
> > 
> > $ git diff v6.8 v6.9|grep container_of\(|wc -l
> > 788
> > $ git diff v6.8 v6.9|grep container_of_const|wc -l
> > 11
> 
> That is because container_of_const is new, and you don't normally go
> back and change things unless you have to.  Which is what I am starting
> to do for some cases now in the driver core interactions, but generally
> it's rare to need this.

container_of_const() does provide a useful a useful sanity check and I
think we should encourage people to use it. I'm happy to see many macros
under include/ use container_of_const() already, but there seem to be more
than 1000 cases where the constness qualifier of a pointer is just
discarded just in the scope that got compiled with my current .config (not
allyesconfig). While the vast majority are probably benign, I wouldn't be
certain there aren't cases where the container of a const pointer ends up
being modified.

> 
> Also note that container_of_const does not work in an inline function,
> which is another reason people might not want to use it.

Does not work or is less useful (compared to a macro)? _Generic() would
need to be used if you'd like to have const and non-const variants of an
inline function but I guess in most cases macros are just fine.

> 
> > Make an explicit recommendation to use container_of_const(), unless @ptr
> > is const but its container isn't.
> > 
> > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > ---
> >  include/linux/container_of.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> > index 713890c867be..7563015ff165 100644
> > --- a/include/linux/container_of.h
> > +++ b/include/linux/container_of.h
> > @@ -13,7 +13,9 @@
> >   * @type:	the type of the container struct this is embedded in.
> >   * @member:	the name of the member within the struct.
> >   *
> > - * WARNING: any const qualifier of @ptr is lost.
> > + * WARNING: any const qualifier of @ptr is lost. container_of() should only be
> > + * used in cases where @ptr is const and its container is not and you know what
> > + * you're doing. Otherwise always use container_of_const().
> 
> I know of no cases where a @ptr would be const yet the container would
> not be, do you?  So why say that here?  That implies that it is a valid
> thing to actually do.
> 
> I don't understand the goal here, do you want to just not have new
> usages use container_of() at all?  Or are you trying to warn people of a
> common problem that they make?  Having a const @ptr is not normal in the
> kernel, so this should be ok.  If not, send patches to fix up those
> users please.

My immediate goal is to encourage people to use container_of_const() for
the added sanity check and stop adding technical debt (code that ignores
const qualifier). Currently people also do think they should be using
container_of() instead of container_of_const() because the pointer they
have is not const (at the time of writing the code at least).

Eventually (or hopefully?) adding that sanity check for container_of() may
be possible so we'd again have just one macro for the job.

-- 
Kind regards,

Sakari Ailus