linux-kernel - [PATCH] kfifo: DECLARE_KIFO_PTR(fifo, u64) does not work on arm 32 bit

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Date:   Thu,  8 Jul 2021 10:23:41 -0500
From:   Matthew Weber <matthew.weber@...lins.com>
To:     sashal@...nel.org
Cc:     gregkh@...uxfoundation.org, Sean Young <sean@...s.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Matthew Weber <matthew.weber@...lins.com>,
        Stefani Seibold <stefani@...bold.net>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] kfifo: DECLARE_KIFO_PTR(fifo, u64) does not work on arm 32 bit

From: Sean Young <sean@...s.org>

If you try to store u64 in a kfifo (or a struct with u64 members),
then the buf member of __STRUCT_KFIFO_PTR will cause 4 bytes
padding due to alignment (note that struct __kfifo is 20 bytes
on 32 bit).

That in turn causes the __is_kfifo_ptr() to fail, which is caught
by kfifo_alloc(), which now returns EINVAL.

So, ensure that __is_kfifo_ptr() compares to the right structure.

Signed-off-by: Sean Young <sean@...s.org>
Acked-by: Stefani Seibold <stefani@...bold.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@...pensource.com>
Signed-off-by: Matthew Weber <matthew.weber@...lins.com>
---

This patch originally made it into 4.16 as 
8a866fee3909c49738e1c4429a8d2b9bf27e015d but is a bug on at least 4.14
for any 32bit system(PPC/ARM/...) using kfifo with u64 datatypes.

Please add to linux-4.14.y for the next LTS tag.
(Below patch is a cherry-pick of the commit onto linux-4.14.y and was
 verified on 32bit hardware.)

---
 include/linux/kfifo.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/kfifo.h b/include/linux/kfifo.h
index 41eb6fdf87a8..86b5fb08e96c 100644
--- a/include/linux/kfifo.h
+++ b/include/linux/kfifo.h
@@ -113,7 +113,8 @@ struct kfifo_rec_ptr_2 __STRUCT_KFIFO_PTR(unsigned char, 2, void);
  * array is a part of the structure and the fifo type where the array is
  * outside of the fifo structure.
  */
-#define	__is_kfifo_ptr(fifo)	(sizeof(*fifo) == sizeof(struct __kfifo))
+#define	__is_kfifo_ptr(fifo) \
+	(sizeof(*fifo) == sizeof(STRUCT_KFIFO_PTR(typeof(*(fifo)->type))))

 /**
  * DECLARE_KFIFO_PTR - macro to declare a fifo pointer object
-- 
2.17.1