Re: [PATCH] overflow: Introduce struct_offset() to get offset of member

[Kees Cook]

On Wed, Nov 26, 2025 at 02:52:49PM -0500, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> 
> The trace_marker_raw file in tracefs takes a buffer from user space that
> contains an id as well as a raw data string which is usually a binary
> structure. The structure used has the following:
> 
> 	struct raw_data_entry {
> 		struct trace_entry	ent;
> 		unsigned int		id;
> 		char			buf[];
> 	};
> 
> Since the passed in "cnt" variable is both the size of buf as well as the
> size of id, the code to allocate the location on the ring buffer had:
> 
>    size = struct_size(entry, buf, cnt - sizeof(entry->id));
> 
> Which is quite ugly and hard to understand. Instead, add a helper macro
> called struct_offset() which then changes the above to a simple and easy
> to understand:
> 
>    size = struct_offset(entry, id) + cnt;
> 
> This will likely come in handy for other use cases too.
> 
> Link: https://lore.kernel.org/all/CAHk-=whYZVoEdfO1PmtbirPdBMTV9Nxt9f09CK0k6S+HJD3Zmg@xxxxxxxxxxxxxx/
> 
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

Works for me!

Reviewed-by: Kees Cook <kees@xxxxxxxxxx>

> ---
>  include/linux/overflow.h | 12 ++++++++++++
>  kernel/trace/trace.c     |  2 +-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 725f95f7e416..736f633b2d5f 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -458,6 +458,18 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
>  #define struct_size_t(type, member, count)					\
>  	struct_size((type *)NULL, member, count)
>  
> +/**
> + * struct_offset() - Calculate the offset of a member within a struct
> + * @p: Pointer to the struct
> + * @member: Name of the member to get the offset of
> + *
> + * Calculates the offset of a particular @member of the structure pointed
> + * to by @p.
> + *
> + * Return: number of bytes to the location of @member.
> + */
> +#define struct_offset(p, member) (offsetof(typeof(*(p)), member))

I wonder if the kerndoc for this and offsetof() should reference each
other? "For a type instead of a pointer, use offsetof()" etc...

-- 
Kees Cook