linux-kernel - [PATCH] overflow: Introduce struct_offset() to get offset of member

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Message-ID: <20251126145249.05b1770a@gandalf.local.home>
Date: Wed, 26 Nov 2025 14:52:49 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, linux-hardening@...r.kernel.org
Cc: Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>, Masami
 Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Linus Torvalds
 <torvalds@...ux-foundation.org>, Kees Cook <kees@...nel.org>, "Gustavo A.
 R. Silva" <gustavoars@...nel.org>
Subject: [PATCH] overflow: Introduce struct_offset() to get offset of member

From: Steven Rostedt <rostedt@...dmis.org>

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/

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 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))
+
 /**
  * __DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
  * Enables caller macro to pass arbitrary trailing expressions
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f59645ab5140..1f734e289368 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7649,7 +7649,7 @@ static ssize_t write_raw_marker_to_buffer(struct trace_array *tr,
 	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;

-- 
2.51.0