How can I substitute lsof inside a Docker (native, not LXC-based)

(NOTE: it is unclear in the question how the asker is entering the docker container. I'm assuming docker exec -it CONTAINER bash was used.)

I had this problem when using a docker image based on centos:7 with docker version 1.9.0 and to overcome this, I just ran:

docker exec --privileged -it CONTAINER bash

Note the inclusion of --privileged.

My naive understanding of the reason this is required: it seems that docker makes an effort to have the container be more "secure", as documented here.

Hah, the plot thickens. If someone has a better answer please write it up and I'll accept it, if acceptable. But here the apparent reason. How negligent of me to ignore the log files on the host:

Jun 12 01:29:46 hostmachine kernel: [140235.718807] audit_printk_skb: 183 callbacks suppressed
Jun 12 01:29:46 hostmachine kernel: [140235.718810] type=1400 audit(1402536586.521:477): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="trace" denied_mask="trace" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.718860] type=1400 audit(1402536586.521:478): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.718886] type=1400 audit(1402536586.521:479): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.718899] type=1400 audit(1402536586.521:480): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.718921] type=1400 audit(1402536586.521:481): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.718954] type=1400 audit(1402536586.521:482): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.719001] type=1400 audit(1402536586.521:483): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.719043] type=1400 audit(1402536586.521:484): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.719086] type=1400 audit(1402536586.521:485): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"
Jun 12 01:29:46 hostmachine kernel: [140235.719126] type=1400 audit(1402536586.521:486): apparmor="DENIED" operation="ptrace" profile="docker-default" pid=3782 comm="lsof" requested_mask="read" denied_mask="read" peer="docker-default"

So apparmor appears to be the culprit, although I'll have to figure out how to convince it to allow this without compromising host/container security or to see whether it's at all possible without compromising security.

Another possibility, this time with a more finegrained security setting: give the SYS_PTRACE privilege to the docker container:

docker run --cap-add=SYS_PTRACE ...