feat: setuid & setgid script

tool/Makefile

@@ -0,0 +1,12 @@
+CC = gcc
+CFLAGS = -Wall -Wextra -std=c99
+
+all: setup_uid
+
+setup_uid: setup_uid.c
+	$(CC) $(CFLAGS) -o setup_uid setup_uid.c
+	sudo chown root:root setup_uid
+	sudo chmod +s setup_uid
+
+clean:
+	sudo rm setup_uid

tool/setup_uid.c

@@ -0,0 +1,74 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <unistd.h>
+#include <string.h>
+
+bool check_user(char *filename, char *username, unsigned long *new_start) {
+    /*
+     * Check whether the user exists in the uid file (subuid, subgid).
+     * Return exists or not, and set the new_start if need it.
+     *
+     * Args:
+     *      filename (char *): means the uid file (subuid or subgid).
+     *      username (char *): the user who run this excutable file.
+     *      new_start (unsigned long *): it's a pointer, save the new start point.
+     * Return:
+     *      exist (bool): exist or not
+     */
+    FILE *fp = fopen(filename, "r");
+    
+    if (fp == NULL) {
+        printf("[ERROR] Please contact Unix Manager.\n");
+        exit(1);
+    }
+
+    // read data from subuid, subgid
+    char line[100];
+    char user[30];
+    unsigned long start, size;     // 0~2^32-1, equal to subuid
+    bool exist = false;
+    while (fgets(line, sizeof(line), fp)) {
+        sscanf(line, "%[^:]:%lu:%lu", user, &start, &size);
+        *new_start = start + size;
+        if (strcmp(username, user) == 0) exist = true;
+    }
+    fclose(fp);
+    return exist;
+}
+
+void insert_user(char *filename, char *username, unsigned long start) {
+    FILE *fp = fopen(filename, "a");
+    
+    if (fp == NULL) {
+        printf("[ERROR] Please contact Unix Manager.\n");
+        exit(1);
+    }
+    char new_line[100];
+    sprintf(new_line, "%s:%lu:%d\n", username, start, 65535);
+    fputs(new_line, fp);
+    fclose(fp);
+    printf("[SUCCESS] %s has been inserted in %s\n", username, filename);
+}
+
+
+int main() {
+    char *username = getlogin();
+
+    char filenames[2][20] = {
+        "/etc/subuid", "/etc/subgid"
+    };
+    
+    bool exist;
+    unsigned long new_start;        // 0~2^32-1, equal to subuid
+    for (int i=0; i<2; i++) {
+        new_start = 100000;         // subuid, subgid default from 100000~165535
+        exist = check_user(filenames[i], username, &new_start);
+        if (exist) {
+            printf("[WARNING] %s has been in %s, no change\n", username, filenames[i]);
+        } else {
+            insert_user(filenames[i], username, new_start);
+        }
+    }
+    return 0;
+}