Erlexec not found when running app in Docker

Just started working with Elixir a month or so ago and am trying to package an application in Docker for the first time. It builds perfectly fine, but it crashes when ran with the following error:

/opt/sergei/releases/0.1.0/../../erts-13.1.3/bin/erl: exec: line 12: /opt/sergei/erts-13.1.3/bin/erlexec: not found

I’ve shelled into the docker container; the file does exist.

Here’s my Dockerfile

FROM elixir:alpine AS build_stage

# Config
WORKDIR /opt/build

# Dependendies
COPY mix.* ./
COPY config ./config

RUN mix local.hex --force && \
  mix local.rebar --force && \
  mix deps.get --only prod && \
  mix deps.compile

# Build project
COPY . .
RUN mix release sergei

FROM alpine:latest

WORKDIR /opt/sergei
COPY --from=build_stage /opt/build/_build/prod/rel/sergei /opt/sergei

ENTRYPOINT ["/opt/sergei/bin/sergei"]
CMD ["start"]

I did some work to reproduce this locally and while I can’t prove it exactly, I’m willing to bet that there’s a versioning difference between elixir:alpine and alpine:latest. If I change the second FROM statement to elixir:alpine, it works fine. I’ll do some more digging (because I’m curious), but that seems like a plausible reason to me. Not sure why you’re getting a “no such file or directory” error, though.

I think I know the problem here. alpine uses musl OOTB, while most others use glibc. I cannot reproduce your problem exactly (the only way I was able to do so was by setting the build_stage FROM to elixir, which is actually Debian based), however, in that situation, I was able to do some poking. If I strace erlexec – since as you point out, it does exist – I see the following

execve("/opt/elixir_hello_world/erts-13.1.4/bin/erlexec", ["/opt/elixir_hello_world/erts-13."...], 0x7ffd6d09ac30 /* 6 vars */) = -1 ENOENT (No such file or directory)

Looking at the man page for execve, ENOENT can actually be returned under two situations. One, if the executable doesn’t exist, or two, if the loader doesn’t exist for that ELF binary (see here c - execve file not found when stracing the very same file! - Stack Overflow).

What I suspect is going on (but is hard to pin down given I can’t fully recreate your problem) is that glibc is being brought into your container somehow (are you perhaps copying _build from your local machine when you build the release? it may be worth adding that to .dockerignore), which is causing you problems when you move onto a glibc-less system (i.e. alpine:latest). When I build the release on elixir:alpine, I see that erlexec is correctly linked to libmusl

/opt/elixir_hello_world/erts-13.1.4/bin # ldd erlexec
	/lib/ (0x7faa179c8000) => /lib/ (0x7faa179c8000)

This was the issue, thank you so much!

1 Like