linux-kernel - [PATCH] [RFC] bcache: hide variable-sized types from uapi headers

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Date:   Mon, 27 Sep 2021 14:43:24 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Kent Overstreet <kmo@...erainc.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Jens Axboe <axboe@...nel.dk>,
        Coly Li <colyli@...e.de>, linux-kernel@...r.kernel.org
Subject: [PATCH] [RFC] bcache: hide variable-sized types from uapi headers

From: Arnd Bergmann <arnd@...db.de>

The headers_check helper complains about a GNU extension in
one of the exported headers:

linux/bcache.h:354:2: warning: field '' with variable sized type 'union jset::(anonymous at ./usr/include/linux/bcache.h:354:2)' not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
        BKEY_PADDED(uuid_bucket);
        ^
linux/bcache.h:134:2: note: expanded from macro 'BKEY_PADDED'
        union { struct bkey key; __u64 key ## _pad[BKEY_PAD]; }
        ^

We could either try to shut up the warning or remove those parts from
the user-visible section of this header. This does the second,
under the assumption that they are not actually used.

Fixes: 81ab4190ac17 ("bcache: Pull on disk data structures out into a separate header")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
Coli, Kent: can you check to see if the hidden parts are used anywhere
from user space, and apply the patch if not?

Any other ideas for addressing this warning?
---
 include/uapi/linux/bcache.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
index cf7399f03b71..e3e4889aa53e 100644
--- a/include/uapi/linux/bcache.h
+++ b/include/uapi/linux/bcache.h
@@ -23,9 +23,13 @@ static inline void SET_##name(type *k, __u64 v)			\
 struct bkey {
 	__u64	high;
 	__u64	low;
+#ifdef __KERNEL__
+	/* gcc extension not meant for user space */
 	__u64	ptr[];
+#endif
 };
 
+#ifdef __KERNEL__
 #define KEY_FIELD(name, field, offset, size)				\
 	BITMASK(name, struct bkey, field, offset, size)
 
@@ -127,6 +131,8 @@ static inline struct bkey *bkey_idx(const struct bkey *k, unsigned int nr_keys)
 
 	return (struct bkey *) (d + nr_keys);
 }
+#endif
+
 /* Enough for a key with 6 pointers */
 #define BKEY_PAD		8
 
-- 
2.29.2