sys.c 代码分析

setregid

/*
 * This is done BSD-style, with no consideration of the saved gid, except
 * that if you set the effective gid, it sets the saved gid too.  This
 * makes it possible for a setgid program to completely drop its privileges,
 * which is often a useful assertion to make when you are doing a security
 * audit over a program.
 *
 * The general idea is that a program which uses just setregid() will be
 * 100% compatible with BSD.  A program which uses just setgid() will be
 * 100% compatible with POSIX w/ Saved ID‘s.
 */
int sys_setregid(int rgid, int egid)//设置real group ID ，effective group ID
{
    if (rgid>0) {
        if ((current->gid == rgid) ||
            suser())
        //如果当前进程的gid == real group ID 或者拥有超级用户权限，就可以把当前进程的group ID更改为 real Group ID　
            current->gid = rgid;
        else //否则setregid是不允许的，返回错误值
            return(-EPERM);
    }
    if (egid>0) {
        if ((current->gid == egid) ||
        //如果当前进程的gid 或者effective gid 等于egid 或者拥有超级用户权限，则可以修改当前进程的egid和sgid
            (current->egid == egid) ||
            suser()) {
            current->egid = egid;
            current->sgid = egid;
        } else
            return(-EPERM);
    }
    return 0;
}

setgid

/*
 * setgid() is implemeneted like SysV w/ SAVED_IDS
 */
int sys_setgid(int gid) //设置当前进程的group ID
{
    if (suser()) //有超级用户权限就可以更改当前进程的gid,egid（effective gid） ,sgid(saved gid)都设置为gid
        current->gid = current->egid = current->sgid = gid;
    else if ((gid == current->gid) || (gid == current->sgid))
    //如果当前进程的sgid 或者gid(current) 等于 gid(传入参数) ,那么把当前进程的effective gid 设置为gid
        current->egid = gid;
    else
        return -EPERM;
    return 0;
}

sys_time

int sys_time(long * tloc) //设置系统时间
{
    int i;

    i = CURRENT_TIME;
    if (tloc) {
        verify_area(tloc,4);
        put_fs_long(i,(unsigned long *)tloc);
    }
    return i;
}

sys_setreuid

/*
 * Unprivileged users may change the real user id to the effective uid
 * or vice versa.  (BSD-style)
 *
 * When you set the effective uid, it sets the saved uid too.  This
 * makes it possible for a setuid program to completely drop its privileges,
 * which is often a useful assertion to make when you are doing a security
 * audit over a program.
 *
 * The general idea is that a program which uses just setreuid() will be
 * 100% compatible with BSD.  A program which uses just setuid() will be
 * 100% compatible with POSIX w/ Saved ID‘s.
 */
int sys_setreuid(int ruid, int euid) //uid == user ID 设置real 和 effective user ID
{
    int old_ruid = current->uid;
    
    if (ruid>0) {
        if ((current->euid==ruid) ||
                    (old_ruid == ruid) ||
            suser())
            current->uid = ruid;
        else
            return(-EPERM);
    }
    if (euid>0) {
        if ((old_ruid == euid) ||
                    (current->euid == euid) ||
            suser()) {
            current->euid = euid;
            current->suid = euid;
        } else {
            current->uid = old_ruid;
            return(-EPERM);
        }
    }
    return 0;
}

setuid()

/*
 * setuid() is implemeneted like SysV w/ SAVED_IDS
 *
 * Note that SAVED_ID‘s is deficient in that a setuid root program
 * like sendmail, for example, cannot set its uid to be a normal
 * user and then switch back, because if you‘re root, setuid() sets
 * the saved uid too.  If you don‘t like this, blame the bright people
 * in the POSIX commmittee and/or USG.  Note that the BSD-style setreuid()
 * will allow a root program to temporarily drop privileges and be able to
 * regain them by swapping the real and effective uid.  
 */
int sys_setuid(int uid) //设置user ID
{
    if (suser())
        current->uid = current->euid = current->suid = uid;
    else if ((uid == current->uid) || (uid == current->suid))
        current->euid = uid;
    else
        return -EPERM;
    return(0);
}

int sys_stime(long * tptr) //设置系统时间
{
    if (!suser())
        return -EPERM;
    startup_time = get_fs_long((unsigned long *)tptr) - jiffies/HZ;
    jiffies_offset = 0;
    return 0;
}

sys_times

int sys_times(struct tms * tbuf) //获取系统时间把内核数据段的数据读到tbuf里去
{
    if (tbuf) {
        verify_area(tbuf,sizeof *tbuf);
        put_fs_long(current->utime,(unsigned long *)&tbuf->tms_utime);
        put_fs_long(current->stime,(unsigned long *)&tbuf->tms_stime);
        put_fs_long(current->cutime,(unsigned long *)&tbuf->tms_cutime);
        put_fs_long(current->cstime,(unsigned long *)&tbuf->tms_cstime);
    }
    return jiffies;
}

sys_brk

int sys_brk(unsigned long end_data_seg) //brk 数据段结尾
{
    if (end_data_seg >= current->end_code &&
    //如果end_data_seg大于当前进程的代码段结尾并且小于当前进程的(堆栈-16K)，于是
        //把end_date_seg作为新的数据段结尾
        end_data_seg < current->start_stack - 16384)
        current->brk = end_data_seg;
    return current->brk;
}

sys_setpgid

/*
 * This needs some heave checking ...
 * I just haven‘t get the stomach for it. I also don‘t fully
 * understand sessions/pgrp etc. Let somebody who does explain it.
 *
 * OK, I think I have the protection semantics right.... this is really
 * only important on a multi-user system anyway, to make sure one user
 * can‘t send a signal to a process owned by another.  -TYT, 12/12/91
 */
int sys_setpgid(int pid, int pgid)  
{
    int i;

    if (!pid)
        pid = current->pid;
    if (!pgid)
        pgid = current->pid;
    if (pgid < 0)
        return -EINVAL;
    for (i=0 ; i<NR_TASKS ; i++)
        if (task[i] && (task[i]->pid == pid) &&
            ((task[i]->p_pptr == current) ||
             (task[i] == current))) {
            if (task[i]->leader)
                return -EPERM;
            if ((task[i]->session != current->session) ||
                ((pgid != pid) &&
                 (session_of_pgrp(pgid) != current->session)))
                return -EPERM;
            task[i]->pgrp = pgid;
            return 0;
        }
    return -ESRCH;
}

 getpgrp

int sys_getpgrp(void) //获得当前进程的pgrp == process group
{
    return current->pgrp;
}

setsid

int sys_setsid(void) //设置session ID
{
    if (current->leader && !suser()) //当前进程不是session leader或者拥有超级权限的话是无法更改session ID的
        return -EPERM;
    current->leader = 1; //当前进程被确认为session leader
    current->session = current->pgrp = current->pid;
    current->tty = -1;
    return current->pgrp;
}

getgroups

/*
 * Supplementary group ID‘s
 */
int sys_getgroups(int gidsetsize, gid_t *grouplist)
//这里应该有问题，一个进程不可能属于多一个进程组
//原因很简单，一个进程的group id只能是一个值！这就约束了它就只能属于一个进程组！他的group leader只能有一个！
{
    int    i;

    if (gidsetsize)
        verify_area(grouplist, sizeof(gid_t) * gidsetsize);

    for (i = 0; (i < NGROUPS) && (current->groups[i] != NOGROUP);
         i++, grouplist++) {
        if (gidsetsize) {
            if (i >= gidsetsize)
                return -EINVAL;
            put_fs_word(current->groups[i], (short *) grouplist);
        }
    }
    return(i);
}

uname

static struct utsname thisname = {
    UTS_SYSNAME, UTS_NODENAME, UTS_RELEASE, UTS_VERSION, UTS_MACHINE
};

int sys_uname(struct utsname * name) //获取系统名称信息
{
    int i;

    if (!name) return -ERROR;
    verify_area(name,sizeof *name);
    for(i=0;i<sizeof *name;i++)
        put_fs_byte(((char *) &thisname)[i],i+(char *) name);
    return 0;
}

/*
 * Only sethostname; gethostname can be implemented by calling uname()
 */
int sys_sethostname(char *name, int len) //设置系统名词信息
{
    int    i;
    
    if (!suser())
        return -EPERM;
    if (len > MAXHOSTNAMELEN)
        return -EINVAL;
    for (i=0; i < len; i++) {
        if ((thisname.nodename[i] = get_fs_byte(name+i)) == 0)
            break;
    }
    if (thisname.nodename[i]) {
        thisname.nodename[i>MAXHOSTNAMELEN ? MAXHOSTNAMELEN : i] = 0;
    }
    return 0;
}

int sys_getrlimit(int resource, struct rlimit *rlim) //获取当前进程的资源界限值
{
    if (resource >= RLIM_NLIMITS)
        return -EINVAL;
    verify_area(rlim,sizeof *rlim);
    put_fs_long(current->rlim[resource].rlim_cur,
            (unsigned long *) rlim);
    put_fs_long(current->rlim[resource].rlim_max,
            ((unsigned long *) rlim)+1);
    return 0;    
}

int sys_setrlimit(int resource, struct rlimit *rlim)
{
    struct rlimit new, *old;

    if (resource >= RLIM_NLIMITS)
        return -EINVAL;
    old = current->rlim + resource;
    new.rlim_cur = get_fs_long((unsigned long *) rlim);
    new.rlim_max = get_fs_long(((unsigned long *) rlim)+1);
    if (((new.rlim_cur > old->rlim_max) ||
         (new.rlim_max > old->rlim_max)) &&
        !suser())
        return -EPERM;
    *old = new;
    return 0;
}

int sys_umask(int mask)//当设置当前进程创建文件的属性
{
    int old = current->umask;

    current->umask = mask & 0777;
    return (old);
}