Re: [PATCH][next] UAPI: nfsfh.h: Replace one-element array with flexible-array member
From: Gustavo A. R. Silva
Date: Mon Mar 29 2021 - 12:12:46 EST
On 3/29/21 09:57, Chuck Lever wrote:
> 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
Awesome. :)
Thanks, Chuck.
--
Gustavo
> 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
>>
>
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