Emboss

Macros to embed metadata as an ELF/Mach-O section in your final binary.

Pairs quite nicely with vergen.

Quickstart

Include emboss in your Cargo.toml:

[package]
# <snip>

[dependencies]
emboss = "0.4.0"

Import the emboss macro and call it with the key and value you want to embed:

use emboss::emboss;

emboss!(key = "my-custom-value", value = "1");

Run a quick cargo build and then examine the resulting binary:

$ strings target/debug/kana | grep CARGO_PKG_VERSION
my-custom-value=1

You can also parse this yourself from the binary or use rsps to fetch it from a running process.

Detailed Usage

Emboss Many

You can emboss multiple key value pairs at once using emboss_many:

use emboss::emboss_many;

// The `pairs` property takes an array of key-value tuples
emboss_many!(pairs = [("foo", "1"), ("bar", "2")]);

Emboss From Environment Variables

If you need to pull the value of a environment variable for the embossing, there is emboss_env:

use emboss::emboss_env;

// `env_var` will be the environment variable to evaluate at compile time
// `key` will be the key in the embossing, same as it is in the normal emboss macro
// If you omit `key`, it'll reuse the env var as the `key`
emboss_env!(env_var = "FOO_VAR", key = "foo");

If the environment variable is not present at compile time, the macro will fail. You can change this behavior with the fallback property:

use emboss::emboss_env;

// The `Fail` variant is the default behavior, blowing up when the `env_var` is missing  
emboss_env!(env_var = "DOES_NOT_EXIST", fallback = Fail);

// The `Empty` variant will use an empty value when the `env_var` is missing  
emboss_env!(env_var = "DOES_NOT_EXIST", fallback = Empty);

// The `Value` variant will use a specific, user-specified value when the `env_var` is missing
emboss_env!(env_var = "DOES_NOT_EXIST", fallback = Value("1"));

Emboss Many From Environment Variables

Similar to emboss_many, but for values pulled from environment variables. You can use all the same properties present in emboss_env:

use emboss::emboss_envs;

// The `env_vars` property takes an array of environment variable spec structs
emboss_envs!(env_vars = [
    { env_var = "FOO_VAR" },
    { env_var = "BAR_VAR", key = "bar", fallback = Empty },
    { env_var = "BAZ_VAR", key = "baz", fallback = Value("3") },
]);

Extended Arguments

All emboss macros support the following properties:

• stored_in: The name of the section to store embossed data. Defaults to .emboss.meta.
• separator: The character to use as a separator between embossed keys and their associated values. Defaults to =.
• terminator: The character to use as a terminator for embossed key-value pairs. Defaults to a null byte.

On macOS, an additional segment property is exposed which allows you to customize the segment that the section is placed in. By default embossed data will appear in the __DATA segment.

Reading Embossed Data

We provide a simple helper function to retrieve embossed data from a given sequence of bytes.

Here is an example using the object crate:

fn read_binary_metadata(file: &object::File) {
  // Pull the raw data from the metadata section of the binary
  // For this example, we'll assume that the following is returned:
  //    VERGEN_RUSTC_CHANNEL=stable\0VERGEN_RUSTC_COMMIT_DATE=2021-05-09\0
  let section = file.section_by_name(emboss::DEFAULT_SECTION_NAME).expect("metadata should exist");
  let data = section.data().expect("data should be available");
  let text = String::from_utf8_lossy(data).to_string();

  let metadata = emboss::extract_metadata(&text, emboss::EmbossingOptions::default())
          .expect("should be able to parse metadata");

  let value = metadata.get("VERGEN_RUSTC_CHANNEL").expect("VERGEN_RUSTC_CHANNEL should be present");
  assert_eq!(value, "stable");

  let value = metadata.get("VERGEN_RUSTC_COMMIT_DATE").expect("VERGEN_RUSTC_COMMIT_DATE should be present");
  assert_eq!(value, "2021-05-09");
}

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.