WASI tutorial

Let's start with a simple C program which performs a file copy, which will show to compile and run programs, as well as perform simple sandbox configuration. The C code here uses standard POSIX APIs, and doesn't have any knowledge of WASI, WebAssembly, or sandboxing.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

int
main(int argc, char **argv) {
    int n, m;
    char buf[BUFSIZ];

    if (argc != 3) {
        fprintf(stderr, "usage: %s <from> <to>\n", argv[0]);
        exit(1);
    }

    int in = open(argv[1], O_RDONLY);
    if (in < 0) {
        fprintf(stderr, "error opening input %s: %s\n", argv[1], strerror(errno));
        exit(1);
    }

    int out = open(argv[2], O_WRONLY | O_CREAT, 0660);
    if (out < 0) {
        fprintf(stderr, "error opening output %s: %s\n", argv[2], strerror(errno));
        exit(1);
    }

    while ((n = read(in, buf, BUFSIZ)) > 0) {
        while (n > 0) {
            m = write(out, buf, n);
            if (m < 0) {
                fprintf(stderr, "write error: %s\n", strerror(errno));
                exit(1);
            }
            n -= m;
        }
    }

    if (n < 0) {
        fprintf(stderr, "read error: %s\n", strerror(errno));
        exit(1);
    }

    return EXIT_SUCCESS;
}

We'll put this source in a file called demo.c.

The wasi-sdk provides a clang which is configured to target WASI and use the WASI sysroot by default, so we can compile our program like so:

$ clang demo.c

A few things to note here. First, this is just regular clang, configured to use a WebAssembly target and sysroot. The name a.out is the traditional default output name that C compilers use, and can be overridden with the "-o" flag in the usual way. And, the output of clang here is a standard WebAssembly module:

$ file a.out
a.out: WebAssembly (wasm) binary module version 0x1 (MVP)

It's a single file containing a self-contained wasm module, that doesn't require any supporting JS code.

We can execute it with wasmtime directly, like so:

$ wasmtime a.out
usage: a.out <from> <to>

Ok, this program needs some command-line arguments. So let's give it some:

$ echo hello world > test.txt
$ wasmtime a.out test.txt /tmp/somewhere.txt
error opening input test.txt: Capabilities insufficient

Aha, now we're seeing the sandboxing in action. This program is attempting to access a file by the name of test.txt, however it hasn't been given the capability to do so.

So let's give it capabilities to access files in the requisite directories:

$ wasmtime --dir=. --dir=/tmp a.out test.txt /tmp/somewhere.txt
$ cat /tmp/somewhere.txt
hello world

Now our program runs as expected!

What's going on under the covers? The --dir= option instructs Wasmtime to preopen a directory, and make it available to the program as a capability which can be used to open files inside that directory. Now when the program calls the C open function, passing it either an absolute or relative path, the WASI libc transparently translates that path into a path that's relative to one of the given preopened directories, if possible (using a technique based on libpreopen). This way, we can have a simple capability-oriented model at the system call level, while portable application code doesn't have to do anything special.

As a brief aside, note that we used the path . above to grant the program access to the current directory. This is needed because the mapping from paths to associated capabilities is performed by libc, so it's part of the WebAssembly program, and we don't expose the actual current working directory to the WebAssembly program. So providing a full path doesn't work:

$ wasmtime --dir=$PWD --dir=/tmp a.out test.txt /tmp/somewhere.txt
$ cat /tmp/somewhere.txt
error opening input test.txt: Capabilities insufficient

So, we always have to use . to refer to the current directory.

Speaking of ., what about ..? Does that give programs a way to break out of the sandbox? Let's see:

$ wasmtime --dir=. --dir=/tmp a.out test.txt /tmp/../etc/passwd
error opening output /tmp/../etc/passwd: Capabilities insufficient

The sandbox says no. And note that this is the capabilities system saying no here ("Capabilities insufficient"), rather than Unix access controls ("Permission denied"). Even if the user running wasmtime had write access to /etc/passwd, WASI programs don't have the capability to access files outside of the directories they've been granted. This is true when resolving symbolic links as well.

Wasmtime also has the ability to remap directories, with the --mapdir command-line option:

$ wasmtime --dir=. --mapdir=/tmp:/var/tmp a.out test.txt /tmp/somewhere.txt
$ cat /var/tmp/somewhere.txt
hello world

This maps the name /tmp within the WebAssembly program to /var/tmp in the host filesystem. So the WebAssembly program itself never sees the /var/tmp path, but that's where the output file goes.

See here for more information on the capability-based security model.

The capability model is very powerful, and what's shown here is just the beginning. In the future, we'll be exposing much more functionality, including finer-grained capabilities, capabilities for network ports, and the ability for applications to explicitly request capabilities.

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WASI tutorial

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