posix_acl_update_mode() determines what's the equivalent mode and if SGID
needs to be cleared or not. I need to make use of this code in fuse
as well. Fuse will send this information to virtiofs file server and
file server will take care of clearing SGID if it needs to be done.

Hence move this code in a separate helper so that more than one place
can call into it.

Cc: Jan Kara <jack@suse.cz>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 fs/posix_acl.c            |  3 +--
 include/linux/posix_acl.h | 11 +++++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index f3309a7edb49..2d62494c4a5b 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c

@@ -684,8 +684,7 @@ int posix_acl_update_mode(struct user_namespace *mnt_userns,
                return error;
        if (error == 0)
                *acl = NULL;
-       if (!in_group_p(i_gid_into_mnt(mnt_userns, inode)) &&
-           !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+       if (posix_acl_mode_clear_sgid(mnt_userns, inode))
                mode &= ~S_ISGID;
        *mode_p = mode;
        return 0;

diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
index 307094ebb88c..073c5e546de3 100644
--- a/include/linux/posix_acl.h
+++ b/include/linux/posix_acl.h

@@ -59,6 +59,17 @@ posix_acl_release(struct posix_acl *acl)
 }


+static inline bool
+posix_acl_mode_clear_sgid(struct user_namespace *mnt_userns,
+                         struct inode *inode)
+{
+       if (!in_group_p(i_gid_into_mnt(mnt_userns, inode)) &&
+           !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+               return true;
+
+       return false;