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;
}
sethostname
/*
* 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;
}
getrlimit
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;
}
setrlimit
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;
}
umask
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);
}