Re: [PATCH][next] UAPI: nfsfh.h: Replace one-element array with flexible-array member
From: Chuck Lever
Date: Mon Mar 29 2021 - 10:58:59 EST
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.