[Bug 2007856] [NEW] snmpd fails to recognize Docker's overlay filesystem

[István Papp]

Public bug reported:

Ubuntu release: 22.04 (Jammy)
Package: https://packages.ubuntu.com/jammy/snmpd

The net-snmp project switched to a new implementation of the fsys
hardware module at their 5.8 version. This was not mentioned in the
release notes, because the decision was made earlier through the build
system. The new version did not recognize the /etc/mtab entry of
"overlay" as a supported filesystem, so monitoring / inside a Docker
container fails after Bionic (5.7.3), likely in every Ubuntu since Focal
(5.8).

The fix happened in this commit, already on their master: https://github.com/net-snmp/net-snmp/commit/798f206561b7de4bc54453c01ffd21de8ae59c87
Since it is an added item in a static list, it would be easy to backport to Jammy's 5.9.1, or most other releases really. A refactor happened at some point that makes backporting before a certain version a bit more interesting, but still easy based on the suggestions in this similar issue: https://github.com/net-snmp/net-snmp/issues/268

Steps to reproduce:
1. Extract the attached zip with a Dockerfile, a patch, and an snmpd.conf
2. Run "docker build -t repro . && docker run --rm -p 161:161/udp repro" to compile and start a Jammy container with a 5.9.1 snmpd inside.
3. Query the disk table from another terminal: "snmpwalk -v2c -c public 0.0.0.0:161 .1.3.6.1.4.1.2021.9"

There should be no OID found. Repeat steps 2 and 3 after uncommenting
the two lines in the Dockerfile that apply the patch. Now proper data
should arrive about the root filesystem.

I would like to get an SRU with this patch for Jammy.

There should be no risk except for snmpd users. The patch is recent and
not released, so it is not widely tested yet. It should only impact
snmpd users with Docker containers, since it only expands the list of
recognized filesystems with "overlay". This feature was broken since
Focal, so I assume there aren't many such users.

** Affects: net-snmp (Ubuntu)
     Importance: Undecided
         Status: New

** Attachment added: "Files for reproduction and verification"
   https://bugs.launchpad.net/bugs/2007856/+attachment/5648641/+files/repro.zip

** Package changed: unattended-upgrades (Ubuntu) => net-snmp (Ubuntu)

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https://bugs.launchpad.net/bugs/2007856

Title:
  snmpd fails to recognize Docker's overlay filesystem

Status in net-snmp package in Ubuntu:
  New

Bug description:
  Ubuntu release: 22.04 (Jammy)
  Package: https://packages.ubuntu.com/jammy/snmpd

  The net-snmp project switched to a new implementation of the fsys
  hardware module at their 5.8 version. This was not mentioned in the
  release notes, because the decision was made earlier through the build
  system. The new version did not recognize the /etc/mtab entry of
  "overlay" as a supported filesystem, so monitoring / inside a Docker
  container fails after Bionic (5.7.3), likely in every Ubuntu since
  Focal (5.8).

  The fix happened in this commit, already on their master: https://github.com/net-snmp/net-snmp/commit/798f206561b7de4bc54453c01ffd21de8ae59c87
  Since it is an added item in a static list, it would be easy to backport to Jammy's 5.9.1, or most other releases really. A refactor happened at some point that makes backporting before a certain version a bit more interesting, but still easy based on the suggestions in this similar issue: https://github.com/net-snmp/net-snmp/issues/268

  Steps to reproduce:
  1. Extract the attached zip with a Dockerfile, a patch, and an snmpd.conf
  2. Run "docker build -t repro . && docker run --rm -p 161:161/udp repro" to compile and start a Jammy container with a 5.9.1 snmpd inside.
  3. Query the disk table from another terminal: "snmpwalk -v2c -c public 0.0.0.0:161 .1.3.6.1.4.1.2021.9"

  There should be no OID found. Repeat steps 2 and 3 after uncommenting
  the two lines in the Dockerfile that apply the patch. Now proper data
  should arrive about the root filesystem.

  I would like to get an SRU with this patch for Jammy.

  There should be no risk except for snmpd users. The patch is recent
  and not released, so it is not widely tested yet. It should only
  impact snmpd users with Docker containers, since it only expands the
  list of recognized filesystems with "overlay". This feature was broken
  since Focal, so I assume there aren't many such users.

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