SoLo — a .so loader for static Linux binaries

Ship one musl-linked executable. At runtime, load the user's existing glibc-linked GPU driver. No container, no AppImage, and no second libc in the process.

Static binaries are a wonderfully boring way to deploy software on Linux: one file, no dependencies, nothing to break. We build ours with IX, a source-first build system for producing fully static Linux binaries. The boredom ends the moment the application needs the GPU: Vulkan and OpenGL drivers are supplied by the host as shared objects, usually built against glibc, and a fully static musl binary cannot normally dlopen() them.

SoLo crosses that boundary. It provides a dlfcn-style source API backed by its own ELF loader (x86-64 and aarch64) and a glibc ABI bridge implemented on top of musl. The result is still one ordinary static executable, but it can use the graphics driver already installed on the machine.

The repository includes an end-to-end Vulkan proof: a fully static executable loads the host's unmodified Vulkan driver, runs a compute shader, and writes the result to a PNG. Tested on AMD radv, radeonsi, Intel, and NVIDIA GPUs under Linux, and on Apple M1 under Asahi Linux.

The host keeps the hardware-specific code. You ship everything else.

And not on a demo's word alone: on every commit, CI loads the shared libraries of the 1,000 most-installed Debian packages — over 2,100 host objects — through SoLo, on both x86-64 and aarch64.

Grab the prebuilt binary — no clone, no toolchain, any Linux with a Vulkan driver installed (mesa-vulkan-drivers is enough):

curl -LO https://github.com/pg83/solo/releases/latest/download/vulkan-x86_64
chmod +x vulkan-x86_64
./vulkan-x86_64 hello.png

vulkan-aarch64 is the same demo for arm64 machines. The command discovers the distro-installed Vulkan ICD in the usual way and produces a 512×512 RGBA image. This is how we build the Shitty release binaries—a blazingly fast terminal emulator, BTW! To force a particular driver:

./vulkan-x86_64 --driver /usr/share/vulkan/icd.d/radeon_icd.x86_64.json radeon.png
./vulkan-x86_64 --driver /usr/share/vulkan/icd.d/lvp_icd.json lavapipe.png

ICD manifest names vary slightly between distributions. Passing no --driver lets the embedded Khronos loader perform its normal discovery.

You can verify that the executable itself is not dynamically linked:

readelf -lW ./vulkan-x86_64 | grep INTERP    # no output
readelf -dW ./vulkan-x86_64                  # "There is no dynamic section"

Or build the same demo from source, with Python 3 and a C/C++ compiler in PATH:

git clone https://github.com/pg83/solo.git
cd solo
./build vulkan
./vulkan hello.png

This is not a toy call to vkCreateInstance. The demo:

1. enters the statically linked Khronos Vulkan loader;
2. loads the host's Vulkan ICD and its non-glibc dependencies through SoLo;
3. creates a device, storage buffer, descriptor set, and compute pipeline;
4. dispatches a checked-in SPIR-V shader;
5. maps the result and writes it through statically linked libpng.

The complete example is in bin/vulkan, and the Vulkan program itself is in main.cpp.

┌──────────────────── fully static executable ────────────────────┐
│                                                                 │
│  application → embedded Vulkan loader → SoLo dlopen/dlsym       │
│                                           ├─ x86-64 ELF mapper  │
│                                           └─ glibc ABI → musl   │
│                                           │                     │
└───────────────────────────────────────────┬─────────────────────┘
                                            │ maps at runtime
                                            ▼
                              system Mesa/Vulkan ICD.so + DSOs

elf_loader.cpp maps ELF segments, walks DT_NEEDED, resolves versioned symbols, applies x86-64 relocations, supports ELF TLS and TLSDESC, materializes IFUNCs, applies RELRO, and runs initializers. Dependencies that are themselves ELF DSOs are loaded recursively.

glibc is deliberately not loaded. Imports such as malloc@GLIBC_2.2.5 are resolved by glibc_shim.cpp to ABI-correct adapters over the process's existing musl runtime. Unsupported glibc functions have unique generated stubs that fail loudly with the exact symbol and version if they are ever called, instead of silently corrupting the process.

Because musl sizes its synchronization objects to the glibc ABI of each architecture, the bridge does not shadow them: a pthread_mutex_t a driver creates is used in place. A lock is therefore one lock for both the loaded DSO and the static executable that may share it, and glibc's static recursive and error-check initializers are adopted on first use.

Before loading a DSO from disk, SoLo checks its static provider registry. This lets an application satisfy a dependency—Wayland, for example—with functions already linked into the executable. LD_LIBRARY_PATH and DL_ELF_LIBRARY_PATH are honored for libraries outside the standard system directories.

The interesting pieces are small enough to read:

• lib/dlfcn.cpp — dlopen, dlsym, errors, and static providers
• lib/elf_loader.cpp — ELF mapping, symbols, relocations, and TLS
• lib/glibc_shim.cpp — implemented glibc ABI adapters
• lib/glibc_stubs.cpp — explicit fallbacks for the rest of the ABI

The default target builds the standalone archive:

./build

The published ./dlfcn symlink points to the resulting libdlfcn.a. Include lib/dlfcn.h, link the archive into a musl-static application, and ordinary dlopen()/dlsym() calls are redirected to SoLo. The source tree is intentionally self-contained and suitable for copying into another static build graph.

./build test          # load an Arch glibc DSO closure in the smoke test
./build vulkan_test   # build the static demo and verify a native Lavapipe PNG

CI performs the native build and test on Alpine/musl with GCC, Fedora with GCC, and Ubuntu with Clang. The Vulkan test installs each distribution's own Lavapipe package; it does not run the driver from an Arch sysroot.

Every build input for the standalone Vulkan executable is vendored under bin/vulkan. build.py compiles those sources directly: upstream CMake, Meson, configure, and Make build systems are not invoked.

In the general case, only SoLo lets a static application tell the dynamic loader: "for this system DSO's libwayland dependency, use the symbols already linked into my executable." This lets the application embed the newest libwayland instead of targeting the oldest version available on every supported system.

And the boundary between the two worlds is not a thin dlsym shim — it carries the parts that make foreign code actually behave:

• C++ exceptions cross it in both directions. A throw in the static world unwinds through glibc-compiled frames into a glibc catch, and the other way around, destructors running on both sides: the guests' _Unwind_* imports are bound to the one unwinder in the executable, so there is a single exception machinery in the process instead of two fighting ones.
• All four TLS models, without wrappers or code patching. General- and local-dynamic through __tls_get_addr, TLSDESC through its custom-ABI resolver, and initial-exec — whose GOT slots are plain thread-pointer-relative offsets no loader can intercept — served from a surplus arena that rides in the executable's own static TLS, so one process-wide offset is valid in every thread and unmodified musl does the per-thread layout.
• ld.so's binding semantics, not an approximation. Global-scope interposition, RTLD_DEEPBIND, DT_SYMBOLIC, symbol versioning with the unversioned-provider compatibility rule, lazy PLT binding with the argument registers preserved through the resolver, GNU and SysV hash lookups, ifunc resolvers handed their hwcaps, /etc/ld.so.cache.
• Cross-world introspection. backtrace() walks static and glibc frames alike and names both through one dladdr; dl_iterate_phdr, dladdr1, and the link_map facade let unwinders and profilers see every image; the file-backed mappings keep real paths in /proc/self/maps for debuggers.
• The stateful corners of glibc, for real. getcontext / makecontext / swapcontext in assembly against glibc's mcontext layouts on both architectures, the pre-2.34 pthread ABIs, GNU obstacks, the fortified _chk family, and the inline-stdio ABI — musl's FILE is deliberately laid out so glibc's inlined putc_unlocked compiles against it — down to _IO_2_1_stdout_ resolving to musl's own stream.

Every one of these is exercised by a conformance battery compiled against real glibc headers at -O2, and by loading every shared object of the thousand most-installed Debian library packages in CI, on x86-64 and aarch64.

• gcompat is a distribution-level glibc API shim for running prebuilt glibc binaries on musl. Its loader stub re-executes the program through musl's dynamic linker with libgcompat.so preloaded; using it from a musl program requires linking that shared library or adding it to the loaded DSO's DT_NEEDED. It does not give a fully static musl process a dynamic loader. SoLo's self-contained model is stronger: the executable embeds both the ELF loader and ABI bridge, loads unchanged host DSOs without a system compatibility package, preserves the versions of their glibc imports, and lets unused unsupported functions remain behind symbol-specific, fail-loud stubs instead of blocking the entire DSO.
• Detour bootstraps the system's ld-linux and allows multiple C runtimes to coexist. SoLo takes the opposite route: it maps the required DSOs itself and translates their glibc imports onto musl, so a second libc and its TLS state never enter the process.
• Cosmopolitan Libc's cosmo_dlopen() follows the same split-runtime scheme as Detour, with all of its advantages and drawbacks: it bootstraps the host's ELF interpreter and libc, then delegates loading the target DSO to the host's dlopen().
• ClickHouse's experimental userspace dynamic loader currently maps ELF objects itself, but stops short of loading glibc. Its proposed path to real-world system libraries such as CUDA is Detour-like: bootstrap the system's ld.so, keep a second libc runtime, and swap the musl/glibc thread pointer at every boundary. SoLo instead implements the glibc ABI over the host's musl runtime and can satisfy DSO dependencies from providers already linked into the static executable.
• graphics.gd's musl + dlopen experiment follows the same split-runtime model as Detour: an embedded helper brings in the host's glibc loader, and assembly trampolines switch between musl and glibc TLS around foreign calls. This leaves two independent TLS worlds: every boundary crossing needs a trampoline, and a callback implemented in musl cannot be passed safely to glibc code because glibc invokes it while its own TLS is active. SoLo keeps a single musl TLS world instead.
• Flatpak, AppImage, and containers solve the problem by hiding a small Linux distribution inside or around your program. This works in roughly the same way that moving house solves a missing power adapter. The result is a huge blob full of duplicated libraries, mounts, namespaces, extraction tricks, and runtime indirection—all of which make profiling, debugging, and basic introspection worse. Shipping a distro because you need one system .so is not portability. SoLo ships one normal, inspectable executable and borrows the only component that genuinely belongs to the host: its hardware driver.

• Linux only, on x86-64 and aarch64. The loader, the TLSDESC and lazy-PLT resolvers, and the initial-exec arena cover both; the glibc symbol inventories are generated per architecture, so printf@GLIBC_2.2.5 on one is printf@GLIBC_2.17 on the other without a single translation rule in the code;
• focused on real Mesa/Vulkan ICD dependency closures, and driven by the top 1000 Debian library packages by popcon votes: the 885 of them that ship glibc-linked shared objects — about 2100 objects — all load through SoLo in CI on both architectures. Loading is the floor, not the claim: calls into the symbols the bridge still stubs abort loudly, and dev/abi-demand.txt is the remaining work, ranked by how many installations demand each symbol;
• a load-once runtime (dlclose succeeds but does not unload an image);
• supporting all four TLS models. Initial-exec variables are placed in a 16 KiB surplus arena that rides in the executable's own static TLS, so one process-wide offset is valid in every thread without patching musl. The one restriction: threads created before a dlopen see zero-initialized TLS for the modules it loaded, so load initial-exec libraries before spawning the threads that use them. An initial-exec module that does not fit the arena fails to load with an error naming the image and the byte counts;
• explicit about missing ABI coverage: an unimplemented glibc call aborts and names itself.

The goal is to turn the hard wall between “fully static” and “uses the system GPU” into a finite, testable compatibility layer. The Vulkan PNG is the first proof that the wall has a door.