The kernel does it!

gistfile1.c

// fs/open.c
static int chown_common(struct path *path, uid_t user, gid_t group)
{
        struct inode *inode = path->dentry->d_inode;
  [...]
        if (!S_ISDIR(inode->i_mode))
                newattrs.ia_valid |=
                        ATTR_KILL_SUID | ATTR_KILL_SGID | ATTR_KILL_PRIV;
  [...]
}

// fs/attr.c
int notify_change(struct dentry * dentry, struct iattr * attr)
{
  [...]
        unsigned int ia_valid = attr->ia_valid;
  [...]
        if (ia_valid & ATTR_KILL_SUID) {
                if (mode & S_ISUID) {
                        ia_valid = attr->ia_valid |= ATTR_MODE;
                        attr->ia_mode = (inode->i_mode & ~S_ISUID);
                }
        }
        if (ia_valid & ATTR_KILL_SGID) {
                if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
                        if (!(ia_valid & ATTR_MODE)) {
                                ia_valid = attr->ia_valid |= ATTR_MODE;
                                attr->ia_mode = inode->i_mode;
                        }
                        attr->ia_mode &= ~S_ISGID;
                }
        }
  [...]
}