Pwn shellcraft is slower than needed

[heapcrash]

Currently, shellcraft does this:

    if args.shellcode not in shellcraft.templates:
        log.error("Unknown shellcraft template %r. Use --list to see available shellcodes." % args.shellcode)

Which requires a recursive walk of all of the shellcraft templates and their directories. This can be very slow if e.g. the pwntools installation is mounted as a shared Docker folder.

    @property
    def templates(self):
        if self._templates:
            return self._templates

        template_dir = os.path.join(os.path.dirname(__file__), 'templates')
        templates    = []

        for root, _, files in os.walk(template_dir, followlinks=True):
            for file in filter(lambda x: x.endswith('.asm'), files):
                value = os.path.splitext(file)[0]
                value = os.path.join(root, value)
                value = value.replace(template_dir, '')
                value = value.replace(os.path.sep, '.')
                value = value.lstrip('.')
                templates.append(value)

        templates = sorted(templates)
        self._templates = templates
        return templates

We should be able to check to see if the template exists by its exact name first, and avoid this recursive walk.

Here's an example showing the strace output of walking the entire hierarchy.

$ strace -r -e open,openat shellcraft i386.linux.sh
...
     0.011731 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.013546 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/powerpc", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.005800 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/powerpc/linux", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.008314 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/powerpc/linux/syscalls", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.661969 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/powerpc/android", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.009786 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/powerpc/android/syscalls", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.697323 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/thumb", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.020890 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/thumb/freebsd", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.004973 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/thumb/linux", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
     0.028175 openat(AT_FDCWD, "/home/pwntools/pwntools/pwnlib/shellcraft/templates/thumb/linux/syscalls", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3