DinD commands exiting with non-zero return code add "exit status" to stderr text

[cmcfadden]

Description

We have an environment in which we're using docker-in-docker to run some command line tools. Then we parse the stdout and stderr to evaluate failures.

Starting with Docker 28.0, if the inner command fails, the actual "exit status" text is appended to the stderr output.

That is to say, for example, if I'm invoking ffmpeg with "docker run ffmpeg -i " inside a container, the returned stderr text will be:

<snip>
  Duration: 00:19:55.18, start: 0.000000, bitrate: 48 kb/s
  Stream #0:0: Audio: mp3, 24000 Hz, mono, fltp, 48 kb/s
At least one output file must be specified
**exit status 1**
</snip>

This is a change with 28.0, but I couldn't find anything indicating it's intentional? It seems like the message itself is from go.

Reproduce

I'm working on a set of containers to make it easy to reproduce, assuming this isn't an intentional change.

Expected behavior

Docker shouldn't modify the stderr content from a command.

docker version

Client: Docker Engine - Community
 Version:           28.0.0
 API version:       1.47 (downgraded from 1.48)
 Go version:        go1.23.6
 Git commit:        f9ced58
 Built:             Wed Feb 19 22:10:56 2025
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Desktop 4.38.0 (181591)
 Engine:
  Version:          27.5.1
  API version:      1.47 (minimum version 1.24)
  Go version:       go1.22.11
  Git commit:       4c9b3b0
  Built:            Wed Jan 22 13:41:25 2025
  OS/Arch:          linux/arm64
  Experimental:     false
 containerd:
  Version:          1.7.25
  GitCommit:        bcc810d6b9066471b0b6fa75f557a15a1cbf31bb
 runc:
  Version:          1.1.12
  GitCommit:        v1.1.12-0-g51d5e946
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client: Docker Engine - Community
 Version:    28.0.0
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.21.0
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.33.0
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 10
  Running: 4
  Paused: 0
  Stopped: 6
 Images: 53
 Server Version: 27.5.1
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: bcc810d6b9066471b0b6fa75f557a15a1cbf31bb
 runc version: v1.1.12-0-g51d5e946
 init version: de40ad0
 Security Options:
  seccomp
   Profile: unconfined
  cgroupns
 Kernel Version: 6.12.5-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: aarch64
 CPUs: 8
 Total Memory: 11.67GiB
 Name: docker-desktop
 ID: e3c61781-7d2a-4753-8d56-b01d662a7820
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 No Proxy: hubproxy.docker.internal
 Labels:
  com.docker.desktop.address=unix:///Users/colin/Library/Containers/com.docker.docker/Data/docker-cli.sock
 Experimental: false
 Insecure Registries:
  hubproxy.docker.internal:5555
  127.0.0.0/8
 Live Restore Enabled: false

Additional Info

No response

[cmcfadden]

realized this was intentional:
#5229

[vvoland]

Note: It will be reverted in 28.0.1: #5854

[thaJeztah]

Ah, beat me to it; yes, it was partially intentional, but not intended to print the custom "exit status" if the error is produced by the container itself (in which case the error output of the container should be printed as-is (even if "empty").