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
