Messages in this thread | | | Date | Mon, 24 Oct 2022 09:11:53 +0000 | From | Sakari Ailus <> | Subject | Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness |
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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@linux.intel.com> > > > --- > > > 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
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