[PATCH][next] container_of: add container_first() macro

[PATCH][next] container_of: add container_first() macro

[Gustavo A. R. Silva]

This is like container_of() but it has an assert to ensure that it's
using the first struct member.

Co-developed-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 include/linux/container_of.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index 713890c867be..928c5865c1b4 100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -22,6 +22,24 @@
 		      "pointer type mismatch in container_of()");	\
 	((type *)(__mptr - offsetof(type, member))); })
 
+/**
+ * container_first - cast first member of a structure out to the containing
+ *		     structure
+ * @ptr:	     the pointer to the member.
+ * @type:	     the type of the container struct this is embedded in.
+ * @member:	     the name of the member within the struct.
+ *
+ * WARNING: any const qualifier of @ptr is lost.
+ */
+#define container_first(ptr, type, member) ({				\
+	void *__mptr = (void *)(ptr);                                   \
+	static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
+		      __same_type(*(ptr), void),                        \
+		      "pointer type mismatch in container_of()");       \
+	static_assert(offsetof(type, member) == 0, "not first member");	\
+	((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
-- 
2.34.1

Re: [PATCH][next] container_of: add container_first() macro

[Dan Carpenter]

On Fri, Sep 20, 2024 at 02:50:02PM +0200, Gustavo A. R. Silva wrote:
> This is like container_of() but it has an assert to ensure that it's
> using the first struct member.
> 

I just remembered that Greg wanted this based on container_of_const().

Really container_of_const() should be renamed to container_of() and
container_of() should be renamed to container_of_helper() and we
would add a #define container_of_const container_of for the transition.

> Co-developed-by: Dan Carpenter <dan.carpenter@linaro.org>
> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  include/linux/container_of.h | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> index 713890c867be..928c5865c1b4 100644
> --- a/include/linux/container_of.h
> +++ b/include/linux/container_of.h
> @@ -22,6 +22,24 @@
>  		      "pointer type mismatch in container_of()");	\
>  	((type *)(__mptr - offsetof(type, member))); })
>  
> +/**
> + * container_first - cast first member of a structure out to the containing
> + *		     structure
> + * @ptr:	     the pointer to the member.
> + * @type:	     the type of the container struct this is embedded in.
> + * @member:	     the name of the member within the struct.
> + *
> + * WARNING: any const qualifier of @ptr is lost.
> + */
> +#define container_first(ptr, type, member) ({				\
> +	void *__mptr = (void *)(ptr);                                   \
> +	static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
> +		      __same_type(*(ptr), void),                        \
> +		      "pointer type mismatch in container_of()");       \
> +	static_assert(offsetof(type, member) == 0, "not first member");	\
> +	((type *)(__mptr - offsetof(type, member))); })
> +
> +

I added too many blank lines.

regards,
dan carpenter

Re: [PATCH][next] container_of: add container_first() macro

[Greg KH]

On Sat, Sep 21, 2024 at 08:45:28AM +0300, Dan Carpenter wrote:
> On Fri, Sep 20, 2024 at 02:50:02PM +0200, Gustavo A. R. Silva wrote:
> > This is like container_of() but it has an assert to ensure that it's
> > using the first struct member.
> > 
> 
> I just remembered that Greg wanted this based on container_of_const().

Yes, that would be best.

> Really container_of_const() should be renamed to container_of() and
> container_of() should be renamed to container_of_helper() and we
> would add a #define container_of_const container_of for the transition.

I agree, but I wonder if it's safe to just do that already and how many
build warnings would happen.  Last I checked it was a lot.

thanks,

greg k-h

Re: [PATCH][next] container_of: add container_first() macro

[Gustavo A. R. Silva]

On 21/09/24 07:52, Greg KH wrote:
> On Sat, Sep 21, 2024 at 08:45:28AM +0300, Dan Carpenter wrote:
>> On Fri, Sep 20, 2024 at 02:50:02PM +0200, Gustavo A. R. Silva wrote:
>>> This is like container_of() but it has an assert to ensure that it's
>>> using the first struct member.
>>>
>>
>> I just remembered that Greg wanted this based on container_of_const().
> 
> Yes, that would be best.

Okay, I'll change that.

--
Gustavo

> 
>> Really container_of_const() should be renamed to container_of() and
>> container_of() should be renamed to container_of_helper() and we
>> would add a #define container_of_const container_of for the transition.
> 
> I agree, but I wonder if it's safe to just do that already and how many
> build warnings would happen.  Last I checked it was a lot.
> 
> thanks,
> 
> greg k-h
>