Hi,

I am working on updating "struct export_args" to fix/add a few things.
One of these is that "ex_flags" is an int, but the flags are defined in mount.h
as MNT_xx bits that now exceed 32bits (mnt_flag is now uint64_t).
For now, this doesn't break anything, since the flags used by ex_flags are
all defined in the low order 32bits but...it seems like this should be addressed
by a new version of "struct export_args".

I have two versions of the updated structure:
A)
struct export_args {
        uint64_t ex_flags;              /* export related flags */
        uid_t   ex_root;                /* mapping for root uid */
        struct  xucred ex_anon;         /* mapping for anonymous user */
        struct  sockaddr *ex_addr;      /* net address to which exported */
        u_char  ex_addrlen;             /* and the net address length */
        struct  sockaddr *ex_mask;      /* mask of valid bits in saddr */
        u_char  ex_masklen;             /* and the smask length */
        char    *ex_indexfile;          /* index file for WebNFS URLs */
        int     ex_numsecflavors;       /* security flavor count */
        int     ex_secflavors[MAXSECFLAVORS]; /* list of security flavors */
        int32_t ex_fsid;                /* mnt_stat.f_fsid.val[0] if */
                                        /* MNT_EXPORTFSID set in ex_flags64 */
        gid_t   *ex_suppgroups;         /* Supplemental groups if */
                                        /* ex_anon.cr_ngroups > XU_NGROUPS */
};
B)
struct export_args {
        int     ex_flags;               /* export related flags */
        uid_t   ex_root;                /* mapping for root uid */
        struct  xucred ex_anon;         /* mapping for anonymous user */
        struct  sockaddr *ex_addr;      /* net address to which exported */
        u_char  ex_addrlen;             /* and the net address length */
        struct  sockaddr *ex_mask;      /* mask of valid bits in saddr */
        u_char  ex_masklen;             /* and the smask length */
        char    *ex_indexfile;          /* index file for WebNFS URLs */
        int     ex_numsecflavors;       /* security flavor count */
        int     ex_secflavors[MAXSECFLAVORS]; /* list of security flavors */
        uint64_t ex_flagshighbits;      /* High order bits of mnt_flag */
        int32_t ex_fsid;                /* mnt_stat.f_fsid.val[0] if */
                                        /* MNT_EXPORTFSID set in ex_flags64 */
        gid_t   *ex_suppgroups;         /* Supplemental groups if */
                                        /* ex_anon.cr_ngroups > XU_NGROUPS */
};

A) does the obvious thing. Unfortunately, this changes the vfs KABI
(specifically the function vfs_oexport_conv()) such that a file system
module compiled with an unpatched mount.h could crash a patched system.
As such, I think it couldn't be MFC'd and would be stuck in head/current
until FreeBSD13 (or FreeBSD14 if 13 gets skipped over;-).

B) doesn't change any fields, but adds a second ex_flagshighbits for the high
order bit. Since it only adds fields where none of those bits are used after
the exports are processed by vfs_export() and, as such, will not break
the VFS KABI, since vfs_domount_update() differentiates which version
of export_args is being used.
As such, I believe this version can be MFC'd. However, it does seem confusing
to have the two ex_flags fields for the low and high 32bits.

So, which do you think it preferable? rick
ps: Josh Paetzel has volunteered to do the mountd.c changes and I have a kerne
      patch that I'll put up on phabricator once Josh has been able to test it.

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