Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness

From: Greg Kroah-Hartman
Date: Mon Oct 24 2022 - 04:45:40 EST


On Mon, Oct 24, 2022 at 10:43:52AM +0200, Greg Kroah-Hartman wrote:
> On Mon, Oct 24, 2022 at 11:26:10AM +0300, Sakari Ailus wrote:
> > container_of() casts the original type to another which leads to the loss
> > of the const qualifier if it is not specified in the caller-provided type.
> > This easily leads to container_of() returning a non-const pointer to a
> > const struct which the C compiler does not warn about.
> >
> > Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> > ---
> > include/linux/container_of.h | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> > index 2f4944b791b81..c7c21d0f41a87 100644
> > --- a/include/linux/container_of.h
> > +++ b/include/linux/container_of.h
> > @@ -13,6 +13,10 @@
> > * @type: the type of the container struct this is embedded in.
> > * @member: the name of the member within the struct.
> > *
> > + * WARNING: as container_of() casts the given struct to another, also the
>
> No need for "also" here (sorry for the grammar nit.)
>
> > + * possible const qualifier of @ptr is lost unless it is also specified in
> > + * @type. This is not a problem if the containing object is not const. Use with
> > + * care.
>
> I do not think these last two sentences you added here are needed
> either.
>
>
> > */
> > #define container_of(ptr, type, member) ({ \
> > void *__mptr = (void *)(ptr); \
> > @@ -27,6 +31,11 @@
> > * @type: the type of the container struct this is embedded in.
> > * @member: the name of the member within the struct.
> > *
> > + * WARNING: as container_of() casts the given struct to another, also the

Wrong function name here.

> > + * possible const qualifier of @ptr is lost unless it is also specified in
> > + * @type. This is not a problem if the containing object is not const. Use with
> > + * care.
>
> Same comments here.

Wait, no one uses this macro, so why not just remove it entirely?

thanks,

greg k-h