Load user environment when executing process with execvp();

Question

i have a daemon running which might spawn a Process on incomming connections. This is done using execvp() and fork().

The Problem is that the process should not run as root and the process is dependend on a correct user environment. So im seeking a way to load the user environment.

Currently the process is executed as another user by setting gid and uid. What i have already tried is using as command su -l user -c command, which did not work for some reason.

Does somebody have an idea how i can load the user environment (especially the $HOME variable)?

Thanks in advance.

Answer 1

You might do it like this:

#include <cstring>
#include <iostream>
#include <vector>

#include <stdio.h>
#include <pwd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

// Version 0 or 1
#ifndef VERSION
#define VERSION 1
#endif

// Please compile twise TEST_PROCESS = 0 and TEST_PROCESS = 1
#ifndef TEST_PROCESS
#define TEST_PROCESS 0
#endif

// User for the process started.
#ifndef USER
#error Please define a user (just a name, no string).
#endif

#define MAKE_STR(x) DO_MAKE_STR(x)
#define DO_MAKE_STR(x) #x
#define USER_NAME MAKE_STR(USER)

class UserEnvironment
{
    public:
    typedef std::vector<char*> Values;

    UserEnvironment(const char* user) {
        auto cmd = std::string("su - ") + user + " -c '. ~/.profile; env'";
        FILE* fp = popen(cmd.c_str(), "r");
        if(fp == 0)  perror(0);
        else {
            char* line = 0;
            size_t len = 0;
            ssize_t n = 0;
            while(1 < (n = getline(&line, &len, fp))) {
                line[n - 1] = 0;
                m_values.push_back(line);
                line = 0;
            }
            pclose(fp);
        }
        m_values.push_back(0);
    };

    ~UserEnvironment() {
        m_values.pop_back();
        for(char* p: m_values) free(p);
    }

    const Values& values() const { return m_values; }
    Values& values() { return m_values; }
    operator const Values& () const { return m_values; }
    operator Values& () { return m_values; }
    char* const * data() const { return m_values.data(); }
    char** data() { return m_values.data(); }

    private:
    Values m_values;
};

inline const char* get_home() {
    const char* home = getenv("HOME");
    if(home == 0) home = "[Missing]";
    return home;
}

#if ! TEST_PROCESS
// Debug/Test
int main() {

    const char* user = USER_NAME;

    int pid = fork();
    if(pid < 0) return 1;
    else if(pid) {
        const char* home = get_home();
        std::cout << "Running Demon: Home = " << home
            << ", User ID = " << getuid() << std::endl;
        wait(NULL);
    }
    else {
        struct passwd* pw = getpwnam(user);
        if( ! pw) {
            std::cout << "Invalid User [" << user << ']'<< std::endl;
        }
        else {
            char* process = strdup("Debug/TestProcess");
            char *const argv[] = {
                process,
                NULL
            };
        #if VERSION == 0
            // Oberwrite environment variables.
            // You might call clearenv();
            if(setgid(pw->pw_gid) == 0 && setuid(pw->pw_uid) == 0) {
                setenv("HOME", pw->pw_dir, 1);
                execvp(process, argv);
            }
        #else
            // Use the user environt.
            UserEnvironment environment(user);
            if(setgid(pw->pw_gid) == 0 && setuid(pw->pw_uid) == 0) {
                execvpe(process, argv, environment.data());
            }
        #endif
            free(process);
        }
    }

    const char* home = get_home();
    std::cout << "Exit: Home = " << home << ", User ID = " << getuid() << std::endl;
    return 0;
}

#else

// Debug/TestProcess
int main() {
    const char* home = get_home();
    std::cout
         << "Running Process: Home = " << home
         << ", User ID = " << getuid()
         << std::endl;
    system("env");
    exit(0);
}

#endif

Note: Please compile twice to generate Debug/Test and Debug/TestProcess

Result:

Running Demon: Home = /root, User ID = 0
Running Process: Home = /home/..., User ID = 1000
... // The Environment
Exit: Home = /root, User ID = 0