How to execute a binary file inside a snap

snapcraft
1

I have a binary file (written in C) and I’d like to build/wrap a snap of it. This binary file also uses some additional libraries (lib1.a, lib2.a, etc.). Is there a way I can do this? I tried to search on the canonical website but I couldn’t find much. Any suggestion on how to start?

2

You might find some useful information on the docs for building C applications here.

If it’s a pre-compiled binary, you can use the dump plugin and include libraries with the stage-packages keyword:

parts:
  part-1:
    source: .
    plugin: dump
    stage-packages:
      - lib1
      - lib2

Otherwise, I would recommend looking at the tutorials here: https://snapcraft.io/docs/creating-a-snap

3

Yes, it’s a pre-compiled binary and I already use the dump plugin. I have a directory like this:

project
  release
  snap
  snapcraft.yaml

and my snapcraft.yaml is:

name: examplebinarysnap
base: core20
version: '0.1'
summary: example snap
description: |
  This is description.

grade: devel
confinement: devmode
architectures:
   - build-on: amd64

parts:
  examplebin:
    plugin: dump
    source: release
    stage-packages:
      - lib1.a
      - lib2.a
      - lib3.a
      - c-binary #mybinary. Here?

apps:
  examplebin:
    command: examplebin

Should the c-binary be on the stage-packages section?

If I try to run “snapcraft” I get the error “Failed to fetch stage packages: Error downloading packages for part ‘fdoclientbin’: The package ‘lib1.a’ was not found…”

4

Are these libraries part of your project and in your release directory? Or are they in the Ubuntu repositories? The stage-packages keyword is fetches libraries with apt.

If everything (binary and libraries) is inside release/, then you don’t need the stage-packages keyword at all. The dump plugin copies the contents of release/ into the snap package.

5

These libraries are needed by the main binary (c-binary), they are not on the Ubuntu repositories. So if I understand correctly this is all I need in the parts section:

parts:
  examplebin:
    plugin: dump
    source: release

Correct?

Also, I get the error “Failed to generate snap metadata: The specified command ‘examplebin’ defined in the app ‘examplebin’ does not exist. Ensure that ‘examplebin’ is installed with the correct path.”

I suppose this is not the correct way to write the apps section…

6

Correct!

The apps section would look something like this:

apps:
  part-os-release:
    command: bin/os-release.sh

I pulled that from https://snapcraft.io/docs/defining-a-command.

You also may need to add an environment key with LD_LIBRARY_PATH in your snapcraft.yaml so your binary can load your libraries. You should be able to find some references for that on the forum or in the documentation.

7

Thank you very much. One last question though. What is the “bin/os-release.sh”? I do not have an .sh to run, just my binary in C.

8

bin/os-release.sh is just an example executable file to call. You can replace that to the filepath of your binary.

9

Thank you very much. Now the snapcraft process ends successfully. Then I install the snap with “snap install snap_name.snap --devmode” and I can run the binary with “snap_name.part-name”.

10

About the LD_LIBRARY_PATH , how I have to set this variable in the snapcraft.yaml?

apps:
  examplebin:
    command: mybinary
    environment:
      LD_LIBRARY_PATH: ???
11

Glad to hear you were able to package it!

Yes, that’s how I would recommend setting it. You can append to it with:

apps:
  examplebin:
    command: mybinary
    environment:
      LD_LIBRARY_PATH: $LD_LIBRARY_PATH:$SNAP/<path-to-your-custom-library-directory>

P.S. If you set the app name to the snap name, you can execute it directly.

i.e.

name: examplebinarysnap
...

apps:
  examplebinarysnap:
    command: bin/os-release.sh

Then you can execute it as examplebinarysnap instead of examplebinarysnap.<app-name>.

1 Like
12

All my custom libraries are in the “release” directory along with my binary so I suppose the correct directory for LD_LIBRARY_PATH is:

LD_LIBRARY_PATH: $LD_LIBRARY_PATH:$SNAP/release

Is it correct?

However, if I do so I get the error "CVE-2020-27348: A potentially empty LD_LIBRARY_PATH has been set for environment in… "

13

You can remove the self-reference, just LD_LIBRARY_PATH: $SNAP/release will be fine. If you include the self-reference, you’ll trigger the security warning above.

The other $LD_LIBRARY_PATH folders are handled elsewhere in runtime environment, so you’ll still get e.g $SNAP/lib/ & etc as appropriate.

2 Likes
14

Thank you very much for all your answers. I have one last problem I’m facing.

When I run the examplesnap executing my binary I get the error “error while loading shared libraries: libcurl-gnutls.so.4: cannot open shared object file: No such file or directory”. This “libcurl-gnutls.so.4” is not a library I have in my “release” directory (along with my binary) but I don’t understand why it needs it.

If I run my binary like “./mybinary” without any snap wrapping it works fine.

Any ideas?

15

You’d need to include libcurl-gnults (add it as a stage package) in your snap and update LD_LIBRARY_PATH to point at it.

With some specific libraries, you might find that they make a subdirectory in the usual folder, e.g $SNAP/lib/x86_64-linux-gnu/example-library, and place the .so files in there; where it wouldn’t be checked by default. Often this is because there’ll be a post-install hook in the apt package that handles symlinks in “real” environments, but those hooks aren’t executed for snaps. I don’t know if that library is one, but given the variety of implementations of TLS, I suspect it might be.

Aside from using $LD_LIBRARY_PATH to point to it directly, you could also consider modifying your snapcraft.yaml to move the file up a level into the normal search path; assuming that this might be the issue. Same end result but might be cleaner than having architecture-specific workarounds to set $LD_LIBRARY_PATH properly.