Classic linter

The classic linter is a Snapcraft linter that is used to verify binary file parameters to ensure they are set appropriately for snaps using classic confinement.

The classic linter is only invoked when snap confinement is set to classic, or if libc is staged.

See Disabling linters for details on how to stop this linter running.


How the linter helps

Unlike un-snapped applications, snaps using classic confinement require dynamic executables to load shared libraries from the appropriate base snap rather from than the host’s root filesystem.

To prevent version and platform incompatibly issues, snap-based binaries need to be either built with appropriate linker parameters, or patched to allow loading shared libraries from their base snap. The classic linter helps

Linter warnings

The classic linter will issue a warning if the ELF binary its testing:

  1. does not use the correct dynamic linker (either from the base or the staged libc).
  2. does not have an rpath (run-time search path) set to a value needed to load shared library dependencies, either from the base or from the snap if the dependency is not part of the base.
Example classic linter output
Running linter: classic
Lint OK:
- classic: Snap confinement is set to classic.
Lint warnings:
- classic: usr/bin/toilet: ELF interpreter should be set to '/snap/core22/current/lib64/ld-linux-x86-64.so.2'.
- classic: usr/bin/toilet: ELF rpath should be set to '$ORIGIN/../lib/x86_64-linux-gnu:/snap/core22/current/lib/x86_64-linux-gnu'.
- classic: usr/lib/x86_64-linux-gnu/caca/libgl_plugin.so.0.0.0: ELF interpreter should be set to '/snap/core22/current/lib64/ld-linux-x86-64.so.2'.
- classic: usr/lib/x86_64-linux-gnu/caca/libgl_plugin.so.0.0.0: ELF rpath should be set to '$ORIGIN/..:/snap/core22/current/lib/x86_64-linux-gnu'.
- classic: usr/lib/x86_64-linux-gnu/caca/libx11_plugin.so.0.0.0: ELF interpreter should be set to '/snap/core22/current/lib64/ld-linux-x86-64.so.2'.
- classic: usr/lib/x86_64-linux-gnu/caca/libx11_plugin.so.0.0.0: ELF rpath should be set to '$ORIGIN/..:/snap/core22/current/lib/x86_64-linux-gnu'.
- classic: usr/lib/x86_64-linux-gnu/libcaca++.so.0.99.19: ELF interpreter should be set to '/snap/core22/current/lib64/ld-linux-x86-64.so.2'.
- classic: usr/lib/x86_64-linux-gnu/libcaca++.so.0.99.19: ELF rpath should be set to '$ORIGIN:/snap/core22/current/lib/x86_64-linux-gnu'.
- classic: usr/lib/x86_64-linux-gnu/libcaca.so.0.99.19: ELF interpreter should be set to '/snap/core22/current/lib64/ld-linux-x86-64.so.2'.
- classic: usr/lib/x86_64-linux-gnu/libcaca.so.0.99.19: ELF rpath should be set to '$ORIGIN:/snap/core22/current/lib/x86_64-linux-gnu'.
- classic: usr/lib/x86_64-linux-gnu/libslang.so.2.3.2: ELF interpreter should be set to '/snap/core22/current/lib64/ld-linux-x86-64.so.2'.
- classic: usr/lib/x86_64-linux-gnu/libslang.so.2.3.2: ELF rpath should be set to '/snap/core22/current/lib/x86_64-linux-gnu'.

Addressing issues

To address classic linter issues, the appropriate rpath can be set during build time, or existing binaries can be patched to have their rpath changed.

At build time

Setting rpath at build time requires linker parameters to be used. The linker is typically invoked indirectly via a compiler driver; with gcc, for example, case parameters can be passed to the linker using the -Wl option:

gcc -o foo foo.o -Wl,-rpath=\$ORIGIN/lib,--disable-new-dtags -Llib -lbar

A similar strategy can be used to set rpath in a cgo binary:

package main
/*
#cgo LDFLAGS: -L${SRCDIR}/lib -Wl,-rpath=\$ORIGIN/lib -Wl,--disable-new-dtags -lbar
#include "bar.h"
*/
import "C"

func main() {
    C.bar()
}

Linker argument -Wl,-dynamic-linker=... can be used to set the ELF interpreter.

Binary patching

A snap payload may also contain pre-built ELF binaries installed from arbitrary sources (typically from the distribution archive, after installing stage packages).

In these cases, rpath must be set by modifying the existing binary using a tool such as patchelf:

patchelf --force-rpath --set-rpath \$ORIGIN/lib foo

Or, to set the ELF interpreter, the following command can be used:

patchelf --set-interpreter /snap/core22/current/lib64/ld-linux-x86-64.so.2 foo

Automatic ELF file patching

Snapcraft 7.2 does not currently perform automatic ELF patching for base: core22 classic snaps, however this feature is now available in edge. To use it, declare:

 build-attributes:
  - enable-patchelf

in all parts that should have ELF binaries automatically patched.

If automatic ELF file patching is required in a stable channel, use base: core20 until Snapcraft 7.3 is released to stable.

1 Like

For setting the elf interpreter, surely it should be using something like /snap/core22/current/lib64/ld-linux-x86-64.so.2?

Probably also worth mentioning that you can use -Wl,-dynamic-linker=... to set the ELF interpreter while linking an executable, as an alternative to using patchelf.

1 Like

Thanks, that’s correct. I’ll fix the post accordingly, and add the interpreter note.