crosvm: add kernel command line builder

The kernel command line is dynamically built up based on what devices
are added and what extra parameters are passed in my the user. The
module this CL adds ensures the kernel command line that is built is a
valid one.

TEST=None
BUG=None

Change-Id: Ia2b7b9fae7eb29140ae25ab34119d050984f8ef2
Reviewed-on: https://chromium-review.googlesource.com/514689
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

src/kernel_cmdline.rs

@@ -0,0 +1,228 @@
+// Copyright 2017 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+//! Helper for creating valid kernel command line strings.
+
+use std::result;
+use std::fmt;
+
+/// The error type for command line building operations.
+#[derive(PartialEq, Debug)]
+pub enum Error {
+    /// Operation would have resulted in a non-printable ASCII character.
+    InvalidAscii,
+    /// Key/Value Operation would have had a space in it.
+    HasSpace,
+    /// Key/Value Operation would have had an equals sign in it.
+    HasEquals,
+    /// Operation would have made the command line too large.
+    TooLarge,
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f,
+               "{}",
+               match *self {
+                   Error::InvalidAscii => "string contains non-printable ASCII character",
+                   Error::HasSpace => "string contains a space",
+                   Error::HasEquals => "string contains an equals sign",
+                   Error::TooLarge => "inserting string would make command line too long",
+               })
+    }
+}
+
+/// Specialized Result type for command line operations.
+pub type Result<T> = result::Result<T, Error>;
+
+fn valid_char(c: char) -> bool {
+    match c {
+        ' '...'~' => true,
+        _ => false,
+    }
+}
+
+fn valid_str(s: &str) -> Result<()> {
+    if s.chars().all(valid_char) {
+        Ok(())
+    } else {
+        Err(Error::InvalidAscii)
+    }
+}
+
+fn valid_element(s: &str) -> Result<()> {
+    if !s.chars().all(valid_char) {
+        Err(Error::InvalidAscii)
+    } else if s.contains(' ') {
+        Err(Error::HasSpace)
+    } else if s.contains('=') {
+        Err(Error::HasEquals)
+    } else {
+        Ok(())
+    }
+}
+
+/// A builder for a kernel command line string that validates the string as its being built. A
+/// `CString` can be constructed from this directly using `CString::new`.
+pub struct Cmdline {
+    line: String,
+    capacity: usize,
+}
+
+impl Cmdline {
+    /// Constructs an empty Cmdline with the given capacity, which includes the nul terminator.
+    /// Capacity must be greater than 0.
+    pub fn new(capacity: usize) -> Cmdline {
+        assert_ne!(capacity, 0);
+        Cmdline {
+            line: String::new(),
+            capacity: capacity,
+        }
+    }
+
+    fn has_capacity(&self, more: usize) -> Result<()> {
+        let needs_space = if self.line.is_empty() { 0 } else { 1 };
+        if self.line.len() + more + needs_space < self.capacity {
+            Ok(())
+        } else {
+            Err(Error::TooLarge)
+        }
+    }
+
+    fn start_push(&mut self) {
+        if !self.line.is_empty() {
+            self.line.push(' ');
+        }
+    }
+
+    fn end_push(&mut self) {
+        // This assert is always true because of the `has_capacity` check that each insert method
+        // uses.
+        assert!(self.line.len() < self.capacity);
+    }
+
+    /// Validates and inserts a key value pair into this command line
+    pub fn insert<T: AsRef<str>>(&mut self, key: T, val: T) -> Result<()> {
+        let k = key.as_ref();
+        let v = val.as_ref();
+
+        valid_element(k)?;
+        valid_element(v)?;
+        self.has_capacity(k.len() + v.len() + 1)?;
+
+        self.start_push();
+        self.line.push_str(k);
+        self.line.push('=');
+        self.line.push_str(v);
+        self.end_push();
+
+        Ok(())
+    }
+
+    /// Validates and inserts a string to the end of the current command line
+    pub fn insert_str<T: AsRef<str>>(&mut self, slug: T) -> Result<()> {
+        let s = slug.as_ref();
+        valid_str(s)?;
+
+        self.has_capacity(s.len())?;
+
+        self.start_push();
+        self.line.push_str(s);
+        self.end_push();
+
+        Ok(())
+    }
+
+    /// Returns the cmdline in progress without nul termination
+    pub fn as_str(&self) -> &str {
+        self.line.as_str()
+    }
+}
+
+impl Into<Vec<u8>> for Cmdline {
+    fn into(self) -> Vec<u8> {
+        self.line.into_bytes()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::ffi::CString;
+
+    #[test]
+    fn insert_hello_world() {
+        let mut cl = Cmdline::new(100);
+        assert_eq!(cl.as_str(), "");
+        assert!(cl.insert("hello", "world").is_ok());
+        assert_eq!(cl.as_str(), "hello=world");
+
+        let s = CString::new(cl).expect("failed to create CString from Cmdline");
+        assert_eq!(s, CString::new("hello=world").unwrap());
+    }
+
+    #[test]
+    fn insert_multi() {
+        let mut cl = Cmdline::new(100);
+        assert!(cl.insert("hello", "world").is_ok());
+        assert!(cl.insert("foo", "bar").is_ok());
+        assert_eq!(cl.as_str(), "hello=world foo=bar");
+    }
+
+    #[test]
+    fn insert_space() {
+        let mut cl = Cmdline::new(100);
+        assert_eq!(cl.insert("a ", "b"), Err(Error::HasSpace));
+        assert_eq!(cl.insert("a", "b "), Err(Error::HasSpace));
+        assert_eq!(cl.insert("a ", "b "), Err(Error::HasSpace));
+        assert_eq!(cl.insert(" a", "b"), Err(Error::HasSpace));
+        assert_eq!(cl.as_str(), "");
+    }
+
+    #[test]
+    fn insert_equals() {
+        let mut cl = Cmdline::new(100);
+        assert_eq!(cl.insert("a=", "b"), Err(Error::HasEquals));
+        assert_eq!(cl.insert("a", "b="), Err(Error::HasEquals));
+        assert_eq!(cl.insert("a=", "b "), Err(Error::HasEquals));
+        assert_eq!(cl.insert("=a", "b"), Err(Error::HasEquals));
+        assert_eq!(cl.insert("a", "=b"), Err(Error::HasEquals));
+        assert_eq!(cl.as_str(), "");
+    }
+
+    #[test]
+    fn insert_emoji() {
+        let mut cl = Cmdline::new(100);
+        assert_eq!(cl.insert("heart", "💖"), Err(Error::InvalidAscii));
+        assert_eq!(cl.insert("💖", "love"), Err(Error::InvalidAscii));
+        assert_eq!(cl.as_str(), "");
+    }
+
+    #[test]
+    fn insert_string() {
+        let mut cl = Cmdline::new(13);
+        assert_eq!(cl.as_str(), "");
+        assert!(cl.insert_str("noapic").is_ok());
+        assert_eq!(cl.as_str(), "noapic");
+        assert!(cl.insert_str("nopci").is_ok());
+        assert_eq!(cl.as_str(), "noapic nopci");
+    }
+
+    #[test]
+    fn insert_too_large() {
+        let mut cl = Cmdline::new(4);
+        assert_eq!(cl.insert("hello", "world"), Err(Error::TooLarge));
+        assert_eq!(cl.insert("a", "world"), Err(Error::TooLarge));
+        assert_eq!(cl.insert("hello", "b"), Err(Error::TooLarge));
+        assert!(cl.insert("a", "b").is_ok());
+        assert_eq!(cl.insert("a", "b"), Err(Error::TooLarge));
+        assert_eq!(cl.insert_str("a"), Err(Error::TooLarge));
+        assert_eq!(cl.as_str(), "a=b");
+
+        let mut cl = Cmdline::new(10);
+        assert!(cl.insert("ab", "ba").is_ok()); // adds 5 length
+        assert_eq!(cl.insert("c", "da"), Err(Error::TooLarge)); // adds 5 (including space) length
+        assert!(cl.insert("c", "d").is_ok()); // adds 4 (including space) length
+    }
+}