Messages in this thread Patch in this message | | | Date | Fri, 2 Dec 2022 17:24:42 +0100 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH 1/4] container_of: add container_of_const() that preserves const-ness of the pointer |
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On Fri, Dec 02, 2022 at 12:48:57PM +0000, Sakari Ailus wrote: > 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.
I had tried to use typeof before, your way is easier, thanks, I couldn't get it to work myself...
> But you can't have typeof(*(ptr)) * instead of default as the two > types are still the same, hence default.
Ah. Ok, I really didn't want to use default, and that's probably why it wasn't working for me.
> 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.
True. Ok, I'll try this later tomorrow and send out a new version if all goes well, thanks!
greg k-h
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