Messages in this thread | | | Date | Tue, 25 Oct 2022 09:47:39 +0200 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness |
| |
On Mon, Oct 24, 2022 at 07:51:11PM +0200, Rafael J. Wysocki wrote: > On Mon, Oct 24, 2022 at 7:39 PM Kees Cook <keescook@chromium.org> wrote: > > > > On Mon, Oct 24, 2022 at 12:00:16PM +0300, Andy Shevchenko wrote: > > > + Kees > > > > > > 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. > > > > > > ... > > > > > > > > > * @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? > > > > > > Kees, do you know why and what for we have container_of_safe()? > > > > It looks like it was designed to handle the cases where the pointer was > > ERR_OR_NULL: > > > > IS_ERR_OR_NULL(__mptr) ? ERR_CAST(__mptr) : \ > > ((type *)(__mptr - offsetof(type, member))); }) > > > > i.e. just pass through the NULL/ERR instead of attempting the cast, > > which would fail spectacularly. :) > > > > It seems like this version should actually be used everywhere instead of > > nowhere... (i.e. just drop container_of() and rename container_of_safe() > > to container_of()) > > As a rule, though, users of container_of() don't check the pointer > returned by it against NULL, so I'm not sure how much of an > improvement that would be.
Nor should they. This is just tiny pointer math, that always assumes a valid pointer is passed in. It should never be used in any code path where a valid pointer is NOT passed into it.
thanks,
greg k-h
| |