docker cp archive/tar error with filenames greater than 100 characters

[mkozell]

• This is a bug report
• This is a feature request
• I searched existing issues before opening this one

Expected behavior

Using docker cp should copy files and directories from container to host with filenames greater than 100 characters for Docker version 18.06.1-ce and higher.

Actual behavior

Using docker cp container_name:/etc/resolv.conf a_very_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx_long_filename_that_is_101_characters in 18.03.1-ce works but results in the error below on 18.06.1-ce and higher.

archive/tar: cannot encode header: Format specifies USTAR; and USTAR cannot encode Name="a_very_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx_long_filename_that_is_101_characters"

Steps to reproduce the behavior

Output of docker version:

Server:
 Engine:
  Version:          18.06.1-ce
  API version:      1.38 (minimum version 1.12)
  Go version:       go1.10.3
  
Server:
 Engine:
  Version:      18.03.1-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.5

Additional environment details (AWS, VirtualBox, physical, etc.)

I was able to reproduce the issue in Docker 18.06.1-ce and 18.09.0-ce but not in Docker 18.03.1-ce. The commands below can reproduce the issue. I reproduced the issue using CentOS 7.5 and Amazon Linux 2 hosts. I then used Vagrant to confirm the issue locally.

docker pull centos:7
docker run -d -i --name docker_test centos:7
docker cp docker_test:/etc/resolv.conf a_very_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx_long_filename_that_is_100_characters
docker cp docker_test:/etc/resolv.conf a_very_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx_long_filename_that_is_101_characters

It appears a change to Golang v1.10 may have changed the default archive/tar format to USTAR which has a 100 character Linkname limit.

https://golang.org/doc/go1.10#archive/tar
https://golang.org/pkg/archive/tar/#Format

[thaJeztah]

ping @kolyshkin

[travisreed-wf]

I am running into this issue in 18.06.1-ce as well. I will downgrade to 18.03.1-ce but wanted to chime in that it is impacting us too.

[kolyshkin]

The problem is, docker daemon writes tar in PAX format, but since the reading side has to guess the format, and it mis-guesses it as USTAR. This issue is probably caused in archive/tar changes in Go 1.10 (docker 18.03 uses go-1.9.5, while 18.06 switched to go 1.10, which has many changes to archive/tar).

I have a fix in progress...

[kolyshkin]

Proposed fix: moby/moby#38634 (note it needs to be merged to moby/moby first and then vendored to docker/cli in order to take effect).