Messages in this thread | | | Date | Fri, 2 Dec 2022 12:48:57 +0000 | From | Sakari Ailus <> | Subject | Re: [PATCH 1/4] container_of: add container_of_const() that preserves const-ness of the pointer |
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Hi Greg,
On Thu, Dec 01, 2022 at 08:30:54PM +0100, Greg Kroah-Hartman wrote: > container_of does not preserve the const-ness of a pointer that is > passed into it, which can cause C code that passes in a const pointer to > get a pointer back that is not const and then scribble all over the data > in it. To prevent this, container_of_const() will preserve the const > status of the pointer passed into it using the newly available _Generic() > method. > > Co-developed-by: Jason Gunthorpe <jgg@ziepe.ca> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Sakari Ailus <sakari.ailus@linux.intel.com> > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> > Cc: "Rafael J. Wysocki" <rafael@kernel.org> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > --- > include/linux/container_of.h | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) > > diff --git a/include/linux/container_of.h b/include/linux/container_of.h > index 2008e9f4058c..3c290e865151 100644 > --- a/include/linux/container_of.h > +++ b/include/linux/container_of.h > @@ -22,4 +22,18 @@ > "pointer type mismatch in container_of()"); \ > ((type *)(__mptr - offsetof(type, member))); }) > > +/** > + * container_of_const - cast a member of a structure out to the containing > + * structure and preserve the const-ness of the pointer > + * @ptr_type: the type of the pointer @ptr > + * @ptr: the pointer to the member > + * @member_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, ptr, member_type, member) \
I missed earlier you had four arguments for the macro instead of three.
With default: this can be done with just three:
#define container_of_const(ptr, member_type, member) \ _Generic(ptr, \ const typeof(*(ptr)) *: ((const member_type *)container_of(ptr, member_type, member)),\ default: ((member_type *)container_of(ptr, member_type, member)) \ )
The const typeof(*(ptr)) * will match if ptr is const, otherwise default matches. But you can't have typeof(*(ptr)) * instead of default as the two types are still the same, hence default.
I've tested this on GCC 10.2.1 and clang 11.0.1.
This should also make it a bit easier for existing users to switch to the new macro and hopefully eventual rename back to container_of() once all users have been converted.
> + _Generic(ptr, \ > + const ptr_type *: ((const member_type *)container_of(ptr, member_type, member)),\ > + ptr_type *: ((member_type *)container_of(ptr, member_type, member)) \ > + ) > + > #endif /* _LINUX_CONTAINER_OF_H */
-- Kind regards,
Sakari Ailus
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