execute-as-user.c

/*
 * Copyright 2015 LinkedIn Corp.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

#include <dirent.h>
#include <fcntl.h>
#include <fts.h>
#include <errno.h>
#include <grp.h>
#include <unistd.h>
#include <signal.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <sys/stat.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <pwd.h>

FILE *LOGFILE = NULL;
FILE *ERRORFILE = NULL;
int SETUID_OPER_FAILED=10;

/*
 *  Change the real and effective user and group from super user to the specified user
 *  
 *  Adopted from:
 *  ./hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-nodemanager/src/main/native/container-executor/impl/container-executor.c
 *  
 */

int change_user(uid_t user, gid_t group) {
    if (user == getuid() && user == geteuid() &&
            group == getgid() && group == getegid()) {
        return 0;
    }

    if (seteuid(0) != 0) {
        fprintf(LOGFILE, "unable to reacquire root - %s\n", strerror(errno));
        fprintf(LOGFILE, "Real: %d:%d; Effective: %d:%d\n",
                getuid(), getgid(), geteuid(), getegid());
        return SETUID_OPER_FAILED;
    }
    if (setgid(group) != 0) {
        fprintf(LOGFILE, "unable to set group to %d - %s\n", group,
                strerror(errno));
        fprintf(LOGFILE, "Real: %d:%d; Effective: %d:%d\n",
                getuid(), getgid(), geteuid(), getegid());
        return SETUID_OPER_FAILED;
    }
    if (setuid(user) != 0) {
        fprintf(LOGFILE, "unable to set user to %d - %s\n", user, strerror(errno));
        fprintf(LOGFILE, "Real: %d:%d; Effective: %d:%d\n",
                getuid(), getgid(), geteuid(), getegid());
        return SETUID_OPER_FAILED;
    }

    return 0;
}

int main(int argc, char **argv){
    if (argc < 3) {
        fprintf(ERRORFILE, "Requires at least 3 variables: ./execute-as-user uid command [args]");
    }

    if(!LOGFILE)
        LOGFILE=stdout;
    if(!ERRORFILE)
        ERRORFILE=stderr;

    char *uid = argv[1];

    // for loop to calculate the length to malloc
    int i;
    int total_len = 0;
    for(i=2;i<argc;i++){
        total_len += strlen(argv[i])+1;
    }
    fprintf(LOGFILE, "total_len: %d\n", total_len);

    // allocate memory and clear memory
    char *cmd = malloc(total_len+2);
    if(cmd == NULL){
        fprintf(LOGFILE, "unable to malloc memory in execute-as-user.c");
        return 10;
    }
    memset(cmd, 0, total_len+2);

    // change user 
    struct passwd *userInfo = getpwnam(uid);
    fprintf(LOGFILE, "Changing user: user: %s, uid: %d, gid: %d\n", uid, userInfo->pw_uid, userInfo->pw_gid);
    int retval = change_user(userInfo->pw_uid, userInfo->pw_gid);
    fprintf(LOGFILE, "change user function return value: %d\n", retval);
    if( retval != 0){
        fprintf(LOGFILE, "Error changing user to %s\n", uid);
        return SETUID_OPER_FAILED;
    }

    // create the command
    char *cur = cmd;
    int len;
    for(i=2;i<argc;i++){
        len = strlen(argv[i]);
        memcpy(cur, argv[i], len);
        cur+=len+1;
    }

    fprintf(LOGFILE, "executing as user command: %s\n", cmd);
    retval = system(cmd);
    fprintf(LOGFILE, "system call return value: %d", retval);

    // sometimes system(cmd) returns 256, which is interpreted to 0, making a failed job a successful job
    // hence this goofy piece of if statement.
    if(retval != 0)
        return 1;
    else
        return 0;

}