The Mount Namespace Reality for Older ELF Binaries
Strictly confined snaps execute in an isolated mount namespace where host directories like /lib, /usr/lib, and /usr/lib/x86_64-linux-gnu are entirely absent from the dynamic linker's search path. This architectural boundary is absolute. Legacy ELF binaries compiled against a host distro's libstdc++.so.6 will immediately fail at ld.so execution when confinement is set to strict. The host system's libraries simply do not exist from the perspective of the confined application.
Many packaging efforts stall at this exact point. The immediate reaction is often to request classic confinement to regain access to the host filesystem. A path forward is to stage the C++ runtime directly inside the snap, maintain strict confinement, and treat the first AppArmor denial as the start of your work queue.
Three specific checks determine if a legacy C++ snap will successfully launch under these conditions. First, every SONAME the binary requires must be primed into the $SNAP directory. Second, the declarations in apps.*.plugs must match the application's real file and socket utilization. Third, snappy-debug must be attached and monitoring the system before you begin guessing at which interfaces the application needs.
Strict Confinement Launch Criteria
The strict confinement mount namespace exclusion of host /usr/lib/x86_64-linux-gnu forces all dependency resolution inward. If ldd cannot resolve a library within the snap's own squashfs payload, the application will not start.
Structuring the YAML for Pre-Compiled Trees
Early packaging attempts frequently rely on classic confinement and the dump plugin to bypass dependency resolution. This approach can mask missing libraries on the build machine, causing silent segmentation faults on end-user systems. A stricter procedure uses plugin: nil combined with override-build and override-prime under strict confinement.
Start from a minimal recipe. Define the name, version, summary, and description. Set the base to core22 or core24 to match the ABI you intend to stage. Declare the application with an explicit command pointing at the primed ELF. Include only the plugs you can already justify based on the application's core function, such as home, network, or x11, rather than appending a kitchen-sink list of permissions.
Build-time dependencies must be strictly separated from runtime stage-packages. Packages such as g++, make, cmake, libc6-dev, and libstdc++-dev belong exclusively in the build environment. The runtime stage-packages list should only contain the compiled libraries: libstdc++6, libc6, and libgcc-s1. Mixing these lists inflates the final snap size and introduces unnecessary security surface area.
Prime filters and the after keyword are used to strip headers,.a static archives, and compiler wrappers from the final squashfs payload. When the source tree still builds, select the appropriate part plugin like cmake or autotools. When you only possess a prebuilt binary and a lib directory, rely on plugin: nil and handle the file placement manually during the override steps. Consult the Snapcraft YAML reference for the exact syntax required to implement these prime filters.
Re-Routing the Dynamic Linker with Rpath and Layouts
Old C++ applications often hardcode /usr/lib or embed an rpath pointing into /opt directly within the ELF header. Under strict confinement, these paths lead nowhere. You must fix the search order so the linker looks under $SNAP/lib and $SNAP/usr/lib.
Executing patchelf --set-rpath during the override-prime step forces the dynamic linker to prioritize the snap's internal directories over any hardcoded /opt or /usr/lib paths. Run ldd and readelf -d on the main ELF and every bundled plugin shared object. Map each SONAME to the Ubuntu or Debian package that provides it. List those specific packages under stage-packages so they are pulled from the same archive as your chosen base.
Staging libstdc++.so.6 and libgcc_s.so.1 directly from the core22 or core24 base archive ensures the GCC ABI inside the snap matches the compiled binary. This alignment prevents the subtle symbol lookup errors that occur when a binary compiled on an older distribution attempts to link against a newer C++ standard library.
When the binary attempts to open a fixed path such as /usr/share/app-name or /usr/lib/vendor-name, use layouts to remap that path onto $SNAP. Layouts create bind mounts within the snap's namespace, making the application behave as if it were installed in its traditional host location.
Layouts mapping fixed paths like /usr/share/app-name to $SNAP fail to resolve when the legacy application relies on hardcoded absolute symlinks that traverse outside the snap's confined root filesystem. If the application attempts to follow a symlink pointing to /etc/opt/vendor, the layout bind mount will not save it. You must patch the application to use relative paths or restructure the directory layout during the prime step.
Translating AppArmor Denials into Interface Plugs
The AppArmor denial resolution loop for legacy C++ ELF binaries requires methodical analysis. Install snappy-debug and run snappy-debug.security scanlog in a dedicated terminal. Launch the snap and reproduce the crash or hang. Work through the output one denial at a time.
Linker and loader AppArmor denials frequently target /etc/ld.so.cache, /proc/self/maps, and /dev/shm during initialization. These files are heavily used by the glibc dynamic linker to map shared libraries into memory and manage shared memory segments. Denials on these paths often indicate that the application is attempting to access host resources that are intentionally blocked.
Resolving /tmp access denials typically requires an application wrapper script that explicitly exports TMPDIR=$SNAP_USER_DATA before invoking the main ELF. Legacy C++ applications frequently hardcode /tmp for temporary file storage, lock files, or IPC sockets. Redirecting this environment variable ensures the application writes to a writable, confined directory specific to the snap.
Translate typical C++ denials into their corresponding interface plugs. Use the home plug for configuration files stored in the user's home directory. Apply network or network-bind for applications opening sockets. Assign desktop alongside wayland or x11 for GUI toolkits. Use opengl for GPU acceleration. Add removable-media only if the application truly reads USB paths, and audio-playback if it opens PulseAudio or PipeWire connections. Apply a layout or system-files exception only after all standard plugs are exhausted.
Validating the Confinement Boundary on a Clean Virtual Machine
Testing must occur on a clean virtual machine lacking the legacy application's original distro packages to prevent host libraries from masking missing stage-packages. A developer workstation often contains hundreds of installed libraries that can inadvertently satisfy a dependency if confinement is temporarily lowered or misconfigured. A pristine environment ensures that the snap is entirely self-sufficient.
Executing snap run --shell app-name provides an interactive environment to run ldd and confirm that every shared library resolves exclusively to paths under $SNAP. This shell drops you directly into the application's confined mount namespace. Running ldd against your binary here reveals exactly what the dynamic linker sees at runtime. If any library reports as "not found," you must return to your YAML and add the missing stage-package.
On a virtual machine that does not have the legacy application's distro packages installed, run sudo snap install snappy-debug, pack your application with confinement: strict, install the resulting.snap file with the --dangerous flag if it is unsigned, start the scanlog utility, and launch the application. Do not publish the snap, request classic confinement, or add extra plugs until that first denial is gone and ldd inside the snap shows every library resolving correctly. Once the scanlog stays quiet through the application's real workflow, tag the YAML, rebuild the package, and upload it to your designated store channel.
