In Linux, UID is used to identify a user (such as alice, bob, root)
In Android , UID is used to identify an application or system service

1. UID in Linux

1.1 Three UID values for each process

Each user has a UID.

The kernel maintains 3 UID identifications for each process. The three UID identifiers areRUID (Real UID, actual user ID),EUID (Effective UID, effective user ID),SUID (Saved Set-user-ID , saved settings user ID).

• RUID: Which user are we currently logged in as? The RUID of the process we run the program to generate is the UID of this user.
• EUID: refers towhich UID the current process actually runs as. Under normal circumstances, EUID is equal to RUID; but if theexecutable file corresponding to the process has SUID permission(that is, the s of rws), then the EUID of the process is the UID## of the owner of theexecutable file. #.
• SUID: A copy of EUID, related to SUID permissions.

1.2 SUID permissions of binary files

Take the executable file passwd as an example.

The permissions of the password file are:
-rwsr-xr-xThe owner of passwd is root, but other users also have execution permissions for passwd, and passwd itself has SUID permissions (s of rws) .
Then, when a non-root user executes the passwd executable file, the EUID
of thegenerated process is the UIDof theroot user. In other words, in this case, the process generated actually runs the binary file with the ID of the root user.

It should be noted that

SUID permissions are time-sensitive:is only valid during the execution of the file.

2. setuid and setresuid functions

##2.1 setuid functionFunction prototype: int setuid(uid_t uid);

In Linux, the execution steps of the setuid(uid) function are:

(1) If called by a Root privileged process (a process with EUID 0), set the RUID, EUID and SUID of the process. is uid, returns 0

(2) If called by a normal permission process, and uid is equal to RUID or uid is equal to SUID, set the EUID of the process to uid, and returns 0
(3) If called by a normal permission process , and uid is not equal to RUID or SUID, set errno to EPERM and return -1 (indicating execution failure)

In the above, the process of executing password, EUID is the UID of the owner of password, which is root The user's UID, which is 0. Therefore, it belongs to case (1). The

su file also has

-rwsr-xr-x

permissions. Therefore, in Android, when a normal process initiates a shell process through Runtime.getRuntime().exec("su") to execute su, it also falls into the situation (1).

2.2 setresuid function

Reference https://blog.csdn.net/damotiansheng/article/details/39674115

Function prototype: int setresuid(uid_t ruid, uid_t euid, uid_t suid);

When one of the following conditions

is met, setresuid is executed:①The euid of the current process is root (AID_ROOT)② Each of the three parameters ruid, euid, and suid is equal to a certain UID value of the current process

For example:

If the current process RUID=100, EUID=0, SUID=300

then setresuid(200,300,100) can be executed because the original euid=0
• 
  If the current process's RUID=100, EUID=300, SUID=200
Then setresuid(200,300,100) can be executed, because these three parameters are all one of the current UID;
• But setresuid(100,200,400) cannot be executed, because 400 is not equal to any of the current UID.
  
  

3. getuid and geteuid functionsgetuid returns the

RUID of the current process

geteuid returns theof the current process EUID
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