README for gcompat

Authors:	A. Wilcox, initial concept, maintainer, lead developer Adélie Software and its contributors, development
Status:	Production
Copyright:	© 2020 Adélie Software in the Public Benefit, Inc. and its contributors. NCSA open source license.

Introduction

This distribution contains the implementation of gcompat, a library which provides glibc-compatible APIs for use on musl libc systems.

This library is designed to be used for binaries that are already compiled against glibc. It does not contain any headers, and cannot be used to build software that requires glibc. It is instead recommended that any software that requires glibc APIs be modified to become more portable.

This library can optionally be compiled with other libraries to make a single, unfied solution. This can include fts, libucontext, obstack, and others.

gcompat additionally provides a loader stub. This is a small library that has the same name as the glibc dynamic linker on glibc platforms; when a binary is run that uses glibc as its dynamic linker, the stub will run, redirecting it to use musl and automatically preloading the gcompat library.

License

This library is provided under the NCSA open source license.

Changes

Any changes to this repository must be reviewed before being pushed to the current branch. There are no exceptions to this rule. For security-sensitive updates, contact the Security Team at [email protected].

Background

Many applications distributed for Linux are built against glibc. Unfortunately, many of glibc's internals are leaked out in to the resultant binary which makes it difficult or impossible to run these binaries on systems using the musl libc.

While the goal of Adélie Linux is to support only true Libre Software that can be built from source code, we recognise that some users may require the ability to run pre-compiled binaries on their computers. This may include older software that does not compile with modern compilers, or non-libre software such as firmware or device management software.

Supported Environments

The following architectures are supported:

• x86 (32-bit)

  This is a Tier 1 architecture in the Adélie Linux system, as pmmx.

• x86_64

  This is a Tier 1 architecture in the Adélie Linux system.

• armv6+ (as arm)

  This is a Tier 2 architecture in the Adélie Linux system, as armv6 and armv7. As such, it may not receive thorough testing.

• aarch64

  This is a Tier 1 architecture in the Adélie Linux system.

• ppc (32-bit)

  This is a Tier 1 architecture in the Adélie Linux system.

• ppc64

  This is a Tier 1 architecture in the Adélie Linux system. Only the ELFv2 ABI is supported, and only Big Endian modes of operation are tested by the release team. Most glibc installations use ELFv1; only Void Linux currently utilises ELFv2 and glibc. For that reason, it is difficult to test gcompat on ppc64.

Building

In order to build correctly, the gcompat build system requires a few details about the host environment, regarding both the musl host and glibc target. To provide this information to the build system, you will need to set the following variables or pass them to make.

• LINKER_PATH

  The location of the musl ELF interpreter. On PowerPC, this is /lib/ld-musl-powerpc64.so.1.

• LOADER_NAME

  The location of the glibc ELF interpreter. On PowerPC, this is ld.so.2.

You may additionally specify any of the following optional variables for further customisation:

• DESTDIR

  The directory in which to install files. The default value is /.

• LIBGCOMPAT_PATH

  The location in which to install libgcompat.so, relative to DESTDIR. The default value is /lib/libgcompat.so.0.

• LOADER_PATH

  The location in which to install the glibc interpreter stub. The default value is /lib/$LOADER_NAME.

• WITH_LIBUCONTEXT

  Determines whether to build with libucontext.

For example, on an x86_64 musl host and x86_64 glibc target, you may run:

$ make LINKER_PATH=/lib/ld-musl-x86_64.so.1 LOADER_NAME=ld-linux-x86-64.so.2

$ make LINKER_PATH=/lib/ld-musl-x86_64.so.1 LOADER_NAME=ld-linux-x86-64.so.2 install

Please note that the gcompat build system does not attempt to guess the correct values for you; specifying at least LINKER_PATH and LOADER_NAME are required.

Usage

Running binaries linked against glibc

If the values specified during build were correct, programs built for glibc should run normally on your musl host. If they are not working, check the program's ELF interpreter entry, using scanelf(8) like so:

$ scanelf -i ~/glibcfile
 TYPE   INTERP FILE
ET_EXEC /lib64/ld-linux-x86-64.so.2 /home/kaniini/glibcfile

If the interpreter path is different than expected, use PatchELF or install a symlink from LOADER_PATH to the specified interpreter.

Using glibc binaries via dlopen(3) from a musl program

There are two ways to accomplish this:

• You can add libgcompat.so to the DT_NEEDED entries in the ELF that you want to have the gcompat symbols available in. Use a tool such as PatchELF for this.
• Alternatively, you can link the musl binary against libgcompat.

Environment variables

• GLIBC_FAKE_VERSION

  The value of GLIBC_FAKE_VERSION will be returned as the result of gnu_get_libc_version. The current default is "2.8".

• GLIBC_FAKE_DEBUG

  Setting this variable will cause dlmopen and dlvsym to display the overrides they are performing, and may cause additional output to stderr with other functions in the future.

Reporting Issues

gcompat is released by the Adélie Linux project in the hopes that it is useful to the community. Current issues may be found at our BTS; you may also submit an issue there.

For general discussion, questions, or to submit a patch, please use the gcompat mailing list.