rules_patchelf

patchelf is a binary that the Nix community has developed in order to modify ELF binary files. This enables workflows that Nix requires, since it’s placing shared objects in unusual places.

Installation

Since I haven’t set up github releases / CI yet ont this repo, you have to clone into your source tree / use git_repository to actually schlep this into your bazel build.

local_repository( # or git_repository
    name = "com_github_rules_patchelf",
    path = "bazel/rules/rules_patchelf",
)

load("@com_github_rules_patchelf//:patchelf_configure.bzl", "patchelf_configure")

patchelf_configure(
    version = "0.18.0",
)

load("@com_github_rules_patchelf//:patchelf_dependencies.bzl", "patchelf_dependencies")

patchelf_dependencies()

register_toolchains(
    "@com_github_rules_patchelf//:patchelf_toolchain",  # This toolchain is build-host specific
)

Motivation

The tool is broadly useful outside of the Nix community, and this is a bazel rule that allows users to interact with it in a bazel-native C/C++ build.

Use Cases

Personally, I use this to rewrite the ELF sections to use $ORIGIN as a source for some shared objects, allowing the linker to pull dependent shared libraries from the directory of the toplevel shared library. Patchelf neatly solves this with its --set-rpath flag

Renaming shared libraries causes the linker to fail, but patchelf solves this through its --set-soname flag

I use both of these features heavily in order to implement JNI native libraries for java rules, and pack the shared objects into a JAR file on the program’s classpath. Extracting these at runtime and loading them with System.loadLibrary simplifies JVM deployments.

Examples

renaming a shared library

patchelf(
    name = "example_set_soname",
    objs = [":my_shared_object"], # generated from a bazel rule
    command = "--set-soname libexample.so"
)

genrule(
    name = "do_so_rename",
    srcs = [":example_set_soname"],
    outs = ["libexample.so"],
    cmd = "cat $< > $@" # bazel's way of addressing the sole input and output of a genrule
)

allowing shared objects to depend on libraries in the current directory

patchelf(
    name = "example_set_rpath",
    objs = [":my_shared_object"],
    command = "--set-rpath '$ORIGIN' libdepends.so"
)

Gotchas

I haven’t figured out a nice way to make the DEBUG error around sha256’s not being included in the http_archive in patchelf_dependencies. Suggestions as to how this should work would be good.

This repo also doen’t have a LICENCE yet, soon to be a oss liscence