Messages in this thread | | | Subject | Re: [PATCH] [RFC] bcache: hide variable-sized types from uapi headers | From | Coly Li <> | Date | Mon, 27 Sep 2021 23:22:16 +0800 |
| |
On 9/27/21 8:43 PM, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@arndb.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.
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@arndb.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 >
| |