Sorry for the reply via gmail, the original patch did not show up in my Oracle mailbox. I've been waiting for a resolution of this thread (and perhaps a Reviewed-by). But in the meantime I've committed this, provisionally, to the for-next topic branch in git://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git On Wed, Mar 24, 2021 at 4:39 AM Gustavo A. R. Silva <gustavoars@xxxxxxxxxx> wrote: > > There is a regular need in the kernel to provide a way to declare having > a dynamically sized set of trailing elements in a structure. Kernel code > should always use “flexible array members”[1] for these cases. The older > style of one-element or zero-length arrays should no longer be used[2]. > > Use an anonymous union with a couple of anonymous structs in order to > keep userspace unchanged: > > $ pahole -C nfs_fhbase_new fs/nfsd/nfsfh.o > struct nfs_fhbase_new { > union { > struct { > __u8 fb_version_aux; /* 0 1 */ > __u8 fb_auth_type_aux; /* 1 1 */ > __u8 fb_fsid_type_aux; /* 2 1 */ > __u8 fb_fileid_type_aux; /* 3 1 */ > __u32 fb_auth[1]; /* 4 4 */ > }; /* 0 8 */ > struct { > __u8 fb_version; /* 0 1 */ > __u8 fb_auth_type; /* 1 1 */ > __u8 fb_fsid_type; /* 2 1 */ > __u8 fb_fileid_type; /* 3 1 */ > __u32 fb_auth_flex[0]; /* 4 0 */ > }; /* 0 4 */ > }; /* 0 8 */ > > /* size: 8, cachelines: 1, members: 1 */ > /* last cacheline: 8 bytes */ > }; > > Also, this helps with the ongoing efforts to enable -Warray-bounds by > fixing the following warnings: > > fs/nfsd/nfsfh.c: In function ‘nfsd_set_fh_dentry’: > fs/nfsd/nfsfh.c:191:41: warning: array subscript 1 is above array bounds of ‘__u32[1]’ {aka ‘unsigned int[1]’} [-Warray-bounds] > 191 | ntohl((__force __be32)fh->fh_fsid[1]))); > | ~~~~~~~~~~~^~~ > ./include/linux/kdev_t.h:12:46: note: in definition of macro ‘MKDEV’ > 12 | #define MKDEV(ma,mi) (((ma) << MINORBITS) | (mi)) > | ^~ > ./include/uapi/linux/byteorder/little_endian.h:40:26: note: in expansion of macro ‘__swab32’ > 40 | #define __be32_to_cpu(x) __swab32((__force __u32)(__be32)(x)) > | ^~~~~~~~ > ./include/linux/byteorder/generic.h:136:21: note: in expansion of macro ‘__be32_to_cpu’ > 136 | #define ___ntohl(x) __be32_to_cpu(x) > | ^~~~~~~~~~~~~ > ./include/linux/byteorder/generic.h:140:18: note: in expansion of macro ‘___ntohl’ > 140 | #define ntohl(x) ___ntohl(x) > | ^~~~~~~~ > fs/nfsd/nfsfh.c:191:8: note: in expansion of macro ‘ntohl’ > 191 | ntohl((__force __be32)fh->fh_fsid[1]))); > | ^~~~~ > fs/nfsd/nfsfh.c:192:32: warning: array subscript 2 is above array bounds of ‘__u32[1]’ {aka ‘unsigned int[1]’} [-Warray-bounds] > 192 | fh->fh_fsid[1] = fh->fh_fsid[2]; > | ~~~~~~~~~~~^~~ > fs/nfsd/nfsfh.c:192:15: warning: array subscript 1 is above array bounds of ‘__u32[1]’ {aka ‘unsigned int[1]’} [-Warray-bounds] > 192 | fh->fh_fsid[1] = fh->fh_fsid[2]; > | ~~~~~~~~~~~^~~ > > [1] https://en.wikipedia.org/wiki/Flexible_array_member > [2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays > > Link: https://github.com/KSPP/linux/issues/79 > Link: https://github.com/KSPP/linux/issues/109 > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx> > --- > include/uapi/linux/nfsd/nfsfh.h | 27 +++++++++++++++++++-------- > 1 file changed, 19 insertions(+), 8 deletions(-) > > diff --git a/include/uapi/linux/nfsd/nfsfh.h b/include/uapi/linux/nfsd/nfsfh.h > index ff0ca88b1c8f..427294dd56a1 100644 > --- a/include/uapi/linux/nfsd/nfsfh.h > +++ b/include/uapi/linux/nfsd/nfsfh.h > @@ -64,13 +64,24 @@ struct nfs_fhbase_old { > * in include/linux/exportfs.h for currently registered values. > */ > struct nfs_fhbase_new { > - __u8 fb_version; /* == 1, even => nfs_fhbase_old */ > - __u8 fb_auth_type; > - __u8 fb_fsid_type; > - __u8 fb_fileid_type; > - __u32 fb_auth[1]; > -/* __u32 fb_fsid[0]; floating */ > -/* __u32 fb_fileid[0]; floating */ > + union { > + struct { > + __u8 fb_version_aux; /* == 1, even => nfs_fhbase_old */ > + __u8 fb_auth_type_aux; > + __u8 fb_fsid_type_aux; > + __u8 fb_fileid_type_aux; > + __u32 fb_auth[1]; > + /* __u32 fb_fsid[0]; floating */ > + /* __u32 fb_fileid[0]; floating */ > + }; > + struct { > + __u8 fb_version; /* == 1, even => nfs_fhbase_old */ > + __u8 fb_auth_type; > + __u8 fb_fsid_type; > + __u8 fb_fileid_type; > + __u32 fb_auth_flex[]; /* flexible-array member */ > + }; > + }; > }; > > struct knfsd_fh { > @@ -97,7 +108,7 @@ struct knfsd_fh { > #define fh_fsid_type fh_base.fh_new.fb_fsid_type > #define fh_auth_type fh_base.fh_new.fb_auth_type > #define fh_fileid_type fh_base.fh_new.fb_fileid_type > -#define fh_fsid fh_base.fh_new.fb_auth > +#define fh_fsid fh_base.fh_new.fb_auth_flex > > /* Do not use, provided for userspace compatiblity. */ > #define fh_auth fh_base.fh_new.fb_auth > -- > 2.27.0 > -- When the world is being engulfed by a comet, go ahead and excrete where you want to.