Re: [PATCH] container_of: add type safety

Michael S. Tsirkin
2023-03-08T20:59:30Z

On Wed, Mar 08, 2023 at 06:57:13PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Mar 08, 2023 at 12:35:03PM -0500, Michael S. Tsirkin wrote:
> > Using a wrong member in container_of will result in an error.
> > No so for container_of_const - it is just a cast so will
> > happily give you a wrong pointer.
> >
> > Use logic from container_of to add safety.
> >
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: Jason Gunthorpe <jgg@nvidia.com>
> > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
> > Cc: Rafael J. Wysocki <rafael@kernel.org>
> > Link: https://lore.kernel.org/r/20221205121206.166576-1-gregkh@linuxfoundation.org
>
> That's the wrong link, that's not this patch, that was an old patch.
>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>
> I did not sign off on this. NEVER do that, you just made a legal
> statement in my name, why?
>
> Why did you not sign off on it?
>
> totally confused...

Ooops. I started with people who signed off on the original one
and replaced S.O.B with a CC. Forgot to replace it for yours
and then the script checking patches saw a signoff and was happy
so I didn't notice I forgot to sign it myself ...

However ...

> > ---
> > include/linux/container_of.h | 7 +++++--
> > 1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> > index 1d898f9158b4..5d87faf72e0a 100644
> > --- a/include/linux/container_of.h
> > +++ b/include/linux/container_of.h
> > @@ -29,10 +29,13 @@
> > * @type: the type of the container struct this is embedded in.
> > * @member: the name of the member within the struct.
> > */
> > -#define container_of_const(ptr, type, member) \
> > +#define container_of_const(ptr, type, member) ({ \
> > + static_assert(__same_type(*(ptr), ((type *)0)->member) || \
> > + __same_type(*(ptr), void), \
> > + "pointer type mismatch in container_of()"); \
>
> Why is this needed because:
>
>
> > _Generic(ptr, \
> > const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
> > default: ((type *)container_of(ptr, type, member)) \
>
> container_of() is used here, so shouldn't the assert trigger there if
> you get things wrong?
>
> So why is this change needed at all?
>
> thanks,
>
> greg k-h

Hmm. I think I was confused. Error did not seem to trigger but
I tried again and it does. Ignore this please.

Sorry about the noise.

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MST