Re: [PATCH][next] UAPI: nfsfh.h: Replace one-element array with flexible-array member

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

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@kernel.org> 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@kernel.org>
>> ---
>>  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
>>
> 
>