overflow: Introduce struct_offset() to get offset of member

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@mail.gmail.com/

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://patch.msgid.link/20251126145249.05b1770a@gandalf.local.home
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 12 insertions, 0 deletions

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
@@ -459,6 +459,18 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 	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))
+
+/**
  * __DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
  * Enables caller macro to pass arbitrary trailing expressions
  *