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-=[e-mail]/

Cc: Masami Hiramatsu <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: "Gustavo A. R. Silva" <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Suggested-by: Linus Torvalds <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

include/linux/overflow.h

···
 	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
  *

kernel/trace/trace.c

···
 	size_t size;
 
 	/* cnt includes both the entry->id and the data behind it. */
-	size = struct_size(entry, buf, cnt - sizeof(entry->id));
+	size = struct_offset(entry, id) + cnt;
 
 	buffer = tr->array_buffer.buffer;