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I've checked /proc/sys/kernel/yama/ptrace_scope in the container and on the host - both report the value as zero but when attached to pid one gdb reports 

Reading symbols from /opt/my-web-proxy/bin/my-web-proxy...done.
Attaching to program: /opt/my-web-proxy/bin/my-web-proxy, process 1
ptrace: Operation not permitted.

I've also tried attached to the container with the privileged flag

docker exec --privileged -it mywebproxy_my-proxy_1 /bin/bash

Host OS is Fedora 25 with docker from their repos and container is a official centos6.8

Adrian Cornish
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1 Answers1

94

I discovered the answer - the container needs to be started with strace capabilities

Adding this to my docker-compose.yml file allows GDB to work

cap_add:
    - SYS_PTRACE

Or it can also be passed on the docker command line with --cap-add=SYS_PTRACE

Adrian Cornish
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    This is also relevant when trying to take a heap dump for a java program with jmap or jvisualvm and getting `sun.jvm.hotspot.debugger.DebuggerException: Can't attach to the process: ptrace(PTRACE_ATTACH, ..) failed for 1: Operation not permitted` as an error – beerbajay Nov 01 '18 at 23:48