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

From: Coly Li
Date: Mon Sep 27 2021 - 11:22:26 EST

On 9/27/21 8:43 PM, Arnd Bergmann wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>

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]
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.

Hi Arnd,

Yes, the variable part is necessary for bcache-tools to understand the on-disk format. If other program wants to understand the bcache on-disk format, IMHO such variable part might be necessary too.

BKEY_PADDED() is a special usage of the variable size array. In this case it indicates uuid_bucket is 8 x u64, and 6 x u64 space for ptr[]. And the bcache code make sure no more than 6 pointers are used for uuid_bucket.

I know BKEY_PADDED() works, but I don't know how to simply remove the -Wgnu-variable-sized-type-not-at-end warning.

Maybe Kent may offer some hint ?

Coly Li

Fixes: 81ab4190ac17 ("bcache: Pull on disk data structures out into a separate header")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
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[];
+#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);
/* Enough for a key with 6 pointers */
#define BKEY_PAD 8