Pack an executable inside a shared library?

Question

Is it possible to pack an executable into a shared library and upon calling a function inside the said library:

• unpack the executable
• use the executable through fork

The reason I am asking is because I was recently faced with a situation where my shared library was being loaded in a "sandbox" environment (maybe chroot based) and I would have really like the possibility of spawning a separate process for an executable (loose coupling).

Answer 1

As long as you have permission to write to a directory on a filesystem that isn't mounted noexec, then you could just store the executable in a large array of unsigned char and write it out with fwrite, then use fork/exec to run it.

Really though, the best solution is just to use fork() without exec - just have the child side call into a different function after the fork() (and then exit with _exit() when that function is done).

Answer 2

Completely plausible.

static const char program[] = {
    0x7f, 0x45, 0x4c, 0x46, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x43, 0x05, 0x02, 0x00, 0x03, 0x00, 0x1a, 0x00, 0x43, 0x05,
    0x1a, 0x00, 0x43, 0x05, 0x04, 0x00, 0x00, 0x00, 0xb9, 0x31, 0x00, 0x43,
    0x05, 0xb2, 0x0d, 0xcd, 0x80, 0x25, 0x20, 0x00, 0x01, 0x00, 0x93, 0xcd,
    0x80, 0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x2c, 0x20, 0x77, 0x6f, 0x72, 0x6c,
    0x64, 0x0a
};

void hello(void) {
    int fd;
    pid_t child;
    char name[1024];
    char *tmp = getenv("TEMP") ?: getenv("TMP") ?: "/tmp";
    if (strlen(tmp) > sizeof(name) - 8) return;
    sprintf(name, "%s/XXXXXX", tmp);
    fd = mkstemp(name);
    if (fd == -1) return;
    if (write(fd, program, sizeof(program)) < sizeof(program)) {
        close(fd);
        unlink(name);
        return;
    }
    fchmod(fd, 0700);
    close(fd);
    (child = fork()) ? waitpid(child, 0, 0) : execl(name, name);
    unlink(name);
}

When run on Linux x86 or compatible, this function will print "hello, world" to the screen.

However, I would definitely not recommend this. If you want a separate binary, just ship a separate binary, and require that it be installed in the sandbox along with your library.