linux-kernel - [PATCH] x86/fpu: Properly align size in CHECK_MEMBER_AT_END

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Date:	Mon, 21 Dec 2015 15:25:30 +0100
From:	Jiri Olsa <jolsa@...nel.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dave Hansen <dave@...1.net>,
	lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/fpu: Properly align size in CHECK_MEMBER_AT_END_OF macro

The CHECK_MEMBER_AT_END_OF(TYPE, MEMBER) checks whether MEMBER
is last member of TYPE by evaluating:

  offsetof(TYPE::MEMBER) + sizeof(TYPE::MEMBER) == sizeof(TYPE)

and ensuring TYPE::MEMBER is the last member of the TYPE.

This condition breaks on structs that are padded to be
aligned. This patch ensures the TYPE alignment is taken
into account.

This bug was revealed after adding cacheline alignment into
struct sched_entity, which broke task_struct::thread check:
  CHECK_MEMBER_AT_END_OF(struct task_struct, thread);

Signed-off-by: Jiri Olsa <jolsa@...nel.org>
---
 arch/x86/kernel/fpu/init.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index be39b5fde4b9..8e839e7f5e2f 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -143,9 +143,18 @@ static void __init fpu__init_system_generic(void)
 unsigned int xstate_size;
 EXPORT_SYMBOL_GPL(xstate_size);
 
-/* Enforce that 'MEMBER' is the last field of 'TYPE': */
+/* Get alignment of the TYPE. */
+#define TYPE_ALIGN(TYPE) offsetof(struct { char x; TYPE test; }, test)
+
+/*
+ * Enforce that 'MEMBER' is the last field of 'TYPE'.
+ *
+ * Align the computed size with alignment of the TYPE,
+ * because that's how C aligns structs.
+ */
 #define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER) \
-	BUILD_BUG_ON(sizeof(TYPE) != offsetofend(TYPE, MEMBER))
+	BUILD_BUG_ON(sizeof(TYPE) != ALIGN(offsetofend(TYPE, MEMBER), \
+					   TYPE_ALIGN(TYPE)))
 
 /*
  * We append the 'struct fpu' to the task_struct:
-- 
2.4.3

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