# 3d: Dependent Data Descriptions for Verified Validation¶

Note

The documentation from this page covers the EverParse/3d releases v2021.03.05 or later. Documentations for earlier versions may be found in the Git history.

EverParse/3d lets you write a data format description and automatically produces efficient data validation code formally verified for memory-safety (absence of buffer overruns), functional correctness (validation is correct wrt. your data format) and security (absence of double memory fetches.) A gentle overview of formal verification is available in the documentation of EverCrypt; EverParse provides similar formal guarantees except secret-independence.

## Summary¶

1. Write your data format description in the 3d language, as MyFile1.3d, MyFile2.3d.
2. Give MyFile1.3d and MyFile2.3d to EverParse:
• with the Windows binary package: everparse.cmd MyFile1.3d MyFile2.3d
• with the Linux binary package: everparse.sh MyFile1.3d MyFile2.3d
• with the source tree: bin/3d.exe --batch MyFile1.3d MyFile2.3d

Then, EverParse/3d produces:

• MyFile1.fsti, MyFile1.fst: the verified F* specification and Low* implementation of parsers and validators for your data format from MyFile1.3d, and similarly for MyFile2.3d
• MyFile1.c, MyFile1.h: the verified C code generated from the verified F* and Low* files, and similarly for MyFile2.3d
• MyFile1Wrapper.c, MyFile1Wrapper.h: an entrypoint API to call into the verified C code, and similarly for MyFile2.3d
• EverParse.h, EverParseEndianness.h: various EverParse helpers for the implementation of verified validators, common to MyFile1.3d and MyFile2.3d
1. Insert #include "MyFile1Wrapper.h" into any of your C compilation units that need to call a validator from MyFile1.3d, and similarly for MyFile2.3d.

### Formal guarantees¶

For each message type in the source data formats, the generated C code provides a _validator_, a procedure to determine if a given input array of bytes contains a valid representation of a message, with the following properties:

• Memory safety: The validator is memory safe, meaning it never accesses memory out of bounds, accesses memory at an incompatible type, uses memory after it has been freed, etc.
• Arithmetic safety: The validator has no integer overflows or underflows.
• Double-fetch freedom: The validator reads every byte in the input array at most once, defending against certain kinds of time-of-check/time-of-use bugs.
• Write footprint: The validator does not modify the input array, and only modifies memory as described by user-controlled imperative actions that are explicit in the specification.
• No heap allocation: The validator does not allocate any memory on the heap.
• Functionally correct: The validator accepts all input arrays that contain valid representations of the message according to its specification, rejecting all others.

## The 3d Dependent Data Description language¶

We provide binary (x86_64 only) packages for Windows and Linux.

1. Download the binary package corresponding to your platform, from the latest EverParse/3d release.
2. Extract it (Windows: unzip; Linux: tar xzf) anywhere you like. No need to configure it anyhow.

Then, everything is contained in a everparse directory, whose actual entrypoint executable is everparse.cmd (for Windows) or everparse.sh (for Linux.)

Binary packages already contain all dependencies required by EverParse, such as F*, KaRaMeL and z3, so no need to install anything beforehand.

The Windows binary package contains a binary snapshot of clang-format to pretty-print the produced .c and .h files. On Linux, you may want to install clang-format (more info at the LLVM download page), but this is optional.

The Windows binary package is fully native, in the sense that it does not require any POSIX emulation layer such as Cygwin or WSL.

## Run¶

On Windows: everparse.cmd [options] input_files

On Linux: everparse.sh [options] input_files

From the source tree: bin/3d.exe. Caveat: in that case, no options are toggled by default (not even --batch or --clang_format) and you need to provide all options by hand.

### Default mode¶

Options are treated from left to right:

• --odir output_directory_name

Tell EverParse to write all files to the directory output_directory_name. This directory must already exist (EverParse will not create it.) The default is . (current directory)

• --batch

Verify the produced F* .fst and .fsti files and generate C .c and .h files. This is toggled by default.

• --no_batch

Do not verify the produced F* .fst and .fsti files or generate C .c and .h files.

• --clang_format

Call clang-format on extracted .c/.h files. This option automatically toggles --batch.

This is toggled by default on Windows, since the Windows binary package contains a copy of clang-format. On Linux, this is toggled by default only if clang-format is reachable through the PATH environment variable.

• --no_clang_format

Do not call clang-format on extracted .c/.h files.

• --clang_format_executable

Provide the path to clang-format if not reachable through PATH (or to override the one provided in the Windows binary package.) This option automatically toggles --clang_format and --batch.

• --cleanup

With --batch, remove all produced intermediate files other than .c and .h files (thus: .fst, .fsti, .checked, .krml)

• --no_cleanup

Keep all produced intermediate files. This is toggled by default.

Options are treated from left to right, with higher priority on right-most options. Thus, any --no_clang_format will override any occurrence of --clang_format or --clang_format_executable to its left, and vice-versa.

You can provide one or more input_files to EverParse. Then, EverParse.h and EverParseEndianness.h will be shared across .c and .h files produced for all the .3d files that you provided.

Note

EverParse input files must all bear the .3d file name extension, and their names must start with a capital letter.

### Alternate mode: generating a Makefile¶

By default, with --batch, given a .3d specification, EverParse produces .c and .h files by first producing .fsti and .fst files files containing verified F* specifications and implementations of validators, and then calling F* and KaRaMeL on each such file to verify and extract them to C. EverParse will issue all such calls to F* and KaRaMeL sequentially.

Alternatively, with everparse.cmd --makefile gmake Test.3d (on Windows; use ./everparse.sh on Linux), instead of producing .c and .h files, EverParse will take Test.3d and all of the modules it depends on, and produce a GNU Makefile containing all recipes and rules to produce .c and .h files and all intermediate files with proper dependencies so that the user can leverage GNU Make parallelism for any rules that do not depend on one another. Other build systems are supported with variations of the --makefile option, as stated below.

The produced Makefile will make use of three variables:

• EVERPARSE_CMD: the command to run EverParse. This variable must contain the path and name of the EverParse executable, and any options (--clang_format, etc.) useful to produce .
• EVERPARSE_INPUT_DIR: the path to the input files.

Note

Currently, all .3d files must be located in the same input directory.

• EVERPARSE_OUTPUT_DIR: the path to the output files. EverParse will append the --odir option appropriately.
• CC: the path and name of the C compiler that will be used to compile the produced C files.
• CFLAGS: options to provide to the C compiler to compile the produced C files.

The produced Makefile, by default called EverParse.Makefile, is meant to be included in the main Makefile of your project. On Linux, such a typical main Makefile for use with GNU Make may look like:

all: test

EVERPARSE_CMD=/path/to/everparse.sh
EVERPARSE_OUTPUT_DIR=obj
EVERPARSE_INPUT_DIR=src

everparse_makefile=$(EVERPARSE_OUTPUT_DIR)/EverParse.Makefile source_files=$(wildcard $(EVERPARSE_INPUT_DIR)/*.3d)$(EVERPARSE_OUTPUT_DIR):
mkdir -p $@$(everparse_makefile): $(source_files)$(EVERPARSE_OUTPUT_DIR)
$(EVERPARSE_CMD) --makefile gmake --makefile_name$@ $(source_files) include$(everparse_makefile)

test: $(EVERPARSE_ALL_O_FILES) clean: rm -rf$(EVERPARSE_OUTPUT_DIR)

.PHONY: all test clean


A fully commented version of that main Makefile is available in the EverParse repository.

EverParse supports the following command-line options relevant to this mode:

• --makefile gmake: produce a Makefile for use with GNU Make.
• --makefile nmake: produce a Makefile for use with Microsoft NMAKE.
• --makefile_name my_file_name: the name of the produced Makefile shall be my_file_name instead of the default EverParse.Makefile.
• --skip_o_rules: do not output rules that would produce .o files by compiling .c files.

### Alternate mode: hash checking¶

To speed up some scenarios where the user maintains a snapshot of the generated .c and .h files to skip unnecessary verification time when .3d files are not changed, EverParse provides an optional mechanism to record the hash of .3d, .c and .h files and to compare them to skip reverification and regeneration. This mechanism is disabled by default, and has to be explicitly enabled using the --check_hashes option as described below. WARNING: this mode will weaken verification guarantees to the probability of hash collisions!

If enabled, then EverParse checks the hashes of the .3d and maybe .c and .h files (depending on the variant, as described below) and fails if hashes do not match; no verification or regeneration is performed at all.

Hashes are computed and checked using the EverCrypt verified cryptographic library, which is included in EverParse binary packages.

Options relevant to this mode:

• --save_hashes: save the hashes of the contents of source .3d, .c and .h files into a .json file. This option can be added in default mode when generating .c and .h files.
• --check_hashes strong : checks the hashes of the contents of source .3d, .c and .h files, by reading the recorded hash from the .json file created by --save_hashes.
• --check_hashes weak : checks the hashes of the contents of source .3d files only, by reading the recorded hash from the .json file created by --save_hashes, leaving the option for the user to manually change the generated .c and .h files (WARNING: this will void all verification guarantees!)
• --check_hashes inplace: same as --check_hashes weak, but uses the hash recorded in the copyright header of the generated .c and .h files instead of the .json file. The hash is introduced by using the EVERPARSEHASHES keyword in the corresponding .copyright.txt used to prepend generated .c and .h files with copyright headers; see Copyright headers for more details.
• --check_inplace_hash MyFile.3d=MyFile.h: same as --check_hashes inplace, but instead manually specifies the pair of the source .3d file and the already-generated .h (or .c) file in which to check the hash. This option can be specified several times to provide several such pairs to check.

### Alternate mode: help and version¶

Some options make EverParse do nothing other than show a help or version message:

• --version

Print the version of EverParse/3d and exit (do not process any further options to the right, nor any files at all.)

• --help`

Show the usage message and the status of some options given so far, and exit (do not process any further options to the right, nor any files at all.)