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

[Sakari Ailus]

Hi Greg,

Thanks for the comments.

On Mon, Oct 24, 2022 at 10:45:25AM +0200, Greg Kroah-Hartman wrote:
> 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.

I'll address this and the other two issues above in v2.

> 
> > > + * 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?

Good question. It appears to be a (relatively) common pattern to look up
something and the return its containing object if the lookup was
successful. Doing a quick

	$ git grep 'container_of.*:' drivers include

reveals more than 20 instances of the pattern. There are probably more
those that use if for testing for NULL. I guess people don't know about
this macro, apart from the developers of the staging driver it was added
for (commit 05e6557b8ed833546ee2b66ce6b58fecf09f439e).

-- 
Kind regards,

Sakari Ailus