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BUILDING.md

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Building AWS-LC

Build Prerequisites

The standalone CMake build is primarily intended for developers. If embedding AWS-LC into another project with a pre-existing build system, see INCORPORATING.md.

If in doubt, use the most recent stable version of each build tool.

  • CMake 3.0 or later is required.

  • A recent version of Perl is required. On Windows, Active State Perl has been reported to work, as has MSYS Perl. Strawberry Perl also works but it adds GCC to PATH, which can confuse some build tools when identifying the compiler (removing C:\Strawberry\c\bin from PATH should resolve any problems). If Perl is not found by CMake, it may be configured explicitly by setting PERL_EXECUTABLE.

  • Go 1.17.13 or later is required. If not found by CMake, the go executable may be configured explicitly by setting GO_EXECUTABLE.

  • Building with Ninja instead of Make is recommended, because it makes builds faster. On Windows, CMake's Visual Studio generator may also work, but it not tested regularly and requires recent versions of CMake for assembly support.

  • On Windows only, NASM is required. If not found by CMake, it may be configured explicitly by setting CMAKE_ASM_NASM_COMPILER.

  • C and C++ compilers with C++11 support are required. On Windows, MSVC 14 (Visual Studio 2015) or later with Platform SDK 8.1 or later are supported, but newer versions are recommended. We will drop support for Visual Studio 2015 in March 2022, five years after the release of Visual Studio 2017. Recent versions of GCC (4.1.3+) and Clang should work on non-Windows platforms, and maybe on Windows too.

  • On x86_64 Linux, the tests have an optional libunwind dependency to test the assembly more thoroughly.

Building

We use CMake to manage the build process. Note that the executable name for CMake version 3.0 and later differs depending on the OS. For example, on Amazon Linux 2 the executable name is cmake3 while on Ubuntu 20.04 the executable name is cmake. Modify command snippets below accordingly.

Using Ninja (note the 'N' is capitalized in the cmake invocation):

cmake -GNinja -B build
ninja -C build

Using Make (does not work on Windows):

cmake -B build
make -C build

This produces a debug build by default. Optimisation isn't enabled, and debug assertions are included. Pass -DCMAKE_BUILD_TYPE=Release to cmake to configure a release build:

cmake -GNinja -B build -DCMAKE_BUILD_TYPE=Release
ninja -C build

If you want to cross-compile then there is an example toolchain file for 32-bit Intel in util/. Wipe out the build directory, run cmake like this:

cmake -B build -DCMAKE_TOOLCHAIN_FILE=../util/32-bit-toolchain.cmake -GNinja

If you want to build as a shared library, pass -DBUILD_SHARED_LIBS=1. On Windows, where functions need to be tagged with dllimport when coming from a shared library, define BORINGSSL_SHARED_LIBRARY in any code which #includes the BoringSSL headers.

In order to serve environments where code-size is important as well as those where performance is the overriding concern, OPENSSL_SMALL can be defined to remove some code that is especially large.

See CMake's documentation for other variables which may be used to configure the build.

You usually don't need to run cmake again after changing CMakeLists.txt files because the build scripts will detect changes to them and rebuild themselves automatically.

Building for Android

It's possible to build BoringSSL with the Android NDK using CMake. Recent versions of the NDK include a CMake toolchain file which works with CMake 3.6.0 or later. This has been tested with version r16b of the NDK.

Unpack the Android NDK somewhere and export ANDROID_NDK to point to the directory. Then run CMake like this:

cmake -DANDROID_ABI=armeabi-v7a \
      -DANDROID_PLATFORM=android-19 \
      -DCMAKE_TOOLCHAIN_FILE=${ANDROID_NDK}/build/cmake/android.toolchain.cmake \
      -GNinja -B build

Once you've run that, Ninja should produce Android-compatible binaries. You can replace armeabi-v7a in the above with arm64-v8a and use API level 21 or higher to build aarch64 binaries.

For other options, see the documentation in the toolchain file.

To debug the resulting binaries on an Android device with gdb, run the commands below. Replace ARCH with the architecture of the target device, e.g. arm or arm64.

adb push ${ANDROID_NDK}/prebuilt/android-ARCH/gdbserver/gdbserver \
    /data/local/tmp
adb forward tcp:5039 tcp:5039
adb shell /data/local/tmp/gdbserver :5039 /path/on/device/to/binary

Then run the following in a separate shell. Replace HOST with the OS and architecture of the host machine, e.g. linux-x86_64.

${ANDROID_NDK}/prebuilt/HOST/bin/gdb
target remote :5039  # in gdb

Building for iOS

To build for iOS, pass -DCMAKE_OSX_SYSROOT=iphoneos and -DCMAKE_OSX_ARCHITECTURES=ARCH to CMake, where ARCH is the desired architecture, matching values used in the -arch flag in Apple's toolchain.

Passing multiple architectures for a multiple-architecture build is not supported.

Building with Prefixed Symbols

BoringSSL's build system has experimental support for adding a custom prefix to all symbols. This can be useful when linking multiple versions of BoringSSL in the same project to avoid symbol conflicts.

In order to build with prefixed symbols, the BORINGSSL_PREFIX CMake variable should specify the prefix to add to all symbols, and the BORINGSSL_PREFIX_SYMBOLS CMake variable should specify the path to a file which contains a list of symbols which should be prefixed (one per line; comments are supported with #). In other words, cmake -B build -DBORINGSSL_PREFIX=MY_CUSTOM_PREFIX -DBORINGSSL_PREFIX_SYMBOLS=/path/to/symbols.txt will configure the build to add the prefix MY_CUSTOM_PREFIX to all of the symbols listed in /path/to/symbols.txt.

It is currently the caller's responsibility to create and maintain the list of symbols to be prefixed. Alternatively, util/read_symbols.go reads the list of exported symbols from a .a file, and can be used in a build script to generate the symbol list on the fly (by building without prefixing, using read_symbols.go to construct a symbol list, and then building again with prefixing).

This mechanism is under development and may change over time. Please contact the BoringSSL maintainers if making use of it.

Known Limitations on Windows

  • CMake can generate Visual Studio projects, but the generated project files don't have steps for assembling the assembly language source files, so they currently cannot be used to build BoringSSL.

ARM CPU Capabilities

ARM, unlike Intel, does not have a userspace instruction that allows applications to discover the capabilities of the processor. Instead, the capability information has to be provided by a combination of compile-time information and the operating system.

BoringSSL determines capabilities at compile-time based on __ARM_NEON, __ARM_FEATURE_AES, and other preprocessor symbols defined in Arm C Language Extensions (ACLE). These values are usually controlled by the -march flag. You can also define any of the following to enable the corresponding ARM feature, but using the ACLE symbols via -march is recommended.

  • OPENSSL_STATIC_ARMCAP_NEON
  • OPENSSL_STATIC_ARMCAP_AES
  • OPENSSL_STATIC_ARMCAP_SHA1
  • OPENSSL_STATIC_ARMCAP_SHA256
  • OPENSSL_STATIC_ARMCAP_PMULL

The resulting binary will assume all such features are always present. This can reduce code size, by allowing the compiler to omit fallbacks. However, if the feature is not actually supported at runtime, BoringSSL will likely crash.

BoringSSL will additionally query the operating system at runtime for additional features, e.g. with getauxval on Linux. This allows a single binary to use newer instructions when present, but still function on CPUs without them. But some environments don't support runtime queries. If building for those, define OPENSSL_STATIC_ARMCAP to limit BoringSSL to compile-time capabilities. If not defined, the target operating system must be known to BoringSSL.

Binary Size

The implementations of some algorithms require a trade-off between binary size and performance. For instance, BoringSSL's fastest P-256 implementation uses a 148 KiB pre-computed table. To optimize instead for binary size, pass -DOPENSSL_SMALL=1 to CMake or define the OPENSSL_SMALL preprocessor symbol.

Running Tests

There are two sets of tests: the C/C++ tests and the blackbox tests. For former are built by Ninja and can be run from the top-level directory with go run util/all_tests.go. The latter have to be run separately by running go test from within ssl/test/runner.

Both sets of tests may also be run with ninja -C build run_tests, but CMake 3.2 or later is required to avoid Ninja's output buffering.

Using Pre-Generated Build Files

If your project is unable to take on a Go or Perl dependency, the AWS-LC repository provides generated build files. These can be used in place of the files that would normally be generated by these dependencies.

It is still recommended to have both Go and Perl installed to be able to run the full range of unit tests, as well as running valgrind and SDE tests. Building without Go now produces a new target, run_minimal_tests in place of run_tests.

More information on this can be found in INCORPORATING.md.

Snapsafe Detection

AWS-LC supports Snapsafe-type uniqueness breaking event detection on Linux using SysGenID (https://lkml.org/lkml/2021/3/8/677). This mechanism is used for security hardening. If a SysGenID interface is not found, then the mechanism is ignored.

Snapsafe Prerequisites

Snapshots taken on active hosts can potentially be unsafe to use. See "Snapshot Safety Prerequisites" here: https://lkml.org/lkml/2021/3/8/677

Data Independent Timing on AArch64

The Data Independent Timing (DIT) flag on Arm64 processors, when enabled, ensures the following as per Arm A-profile Architecture Registers Document:

  • The timing of every load and store instruction is insensitive to the value of the data being loaded or stored.
  • For certain data processing instructions, the instruction takes a time which is independent of the data in the registers and the NZCV flags.

It is also expected to disable the Data Memory-dependent Prefetcher (DMP) feature of Apple M-series CPUs starting at M3 as per this article.

Building with the option -DENABLE_DATA_INDEPENDENT_TIMING_AARCH64=ON will enable the macro SET_DIT_AUTO_DISABLE. This macro is present at the entry of functions that process/load/store secret data to enable the DIT flag and then set it to its original value on entry. With this build option, there is an effect on performance that varies by function and by processor architecture. The effect is mostly due to enabling and disabling the DIT flag. If it remains enabled over many calls, the effect can be largely mitigated. Hence, the macro can be inserted in the caller's application at the beginning of the code scope that makes repeated calls to AWS-LC cryptographic functions. Alternatively, the functions armv8_enable_dit and armv8_restore_dit can be placed at the beginning and the end of the code section, respectively. An example of that usage is present in the benchmarking function Speed() in tool/speed.cc when the -dit option is used

./tool/bssl speed -dit