[Bug 1782152] Re: GDM blocks SIGUSR1 used in PAM scripts

Eric Desrochers eric.desrochers at canonical.com
Fri Aug 17 13:46:03 UTC 2018

** Changed in: gdm3 (Ubuntu Xenial)
       Status: New => In Progress

** Changed in: gdm3 (Ubuntu Xenial)
   Importance: Undecided => Medium

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  GDM blocks SIGUSR1 used in PAM scripts

Status in gdm3 package in Ubuntu:
  Fix Released
Status in gdm3 source package in Xenial:
  In Progress
Status in gdm3 source package in Bionic:
  In Progress
Status in gdm3 source package in Cosmic:
  Fix Released
Status in gdm3 package in Debian:
  Fix Released

Bug description:

  GDM blocks SIGUSR1 for it's processes, since this is used in communication with X. This signal is later unblocked, however it happens after PAM
  interaction, so if PAM depends on this signal in any way it will get blocked.
  The issue has been fixed upstream.

  [Test Case]
  1. Prepare a setup described in Other Info using the attached scripts.
  2. Log in.
  3. Check logs /tmp/auth.log.

  Expected result: SIGUSR1 has been received.
  Actual result: SIGUSR1 never reaches the process.

  [Regression Potential]
  If there were components depending on SIGUSR1 their behavior may change - features that were inactive before may be triggered.

  [Other Info]

   Original bug description:

  In case of the following scenario:
  1. PAM configured to run auth and session with pam_exec scripts synchronizing via SIGUSR1
  2. Using GDM as the login manager causes SIGUSR1 never reaches the target scripts.

  a) Use SIGUSR2 in the scripts.
  b) Comment out block_sigusr1() call in daemon/main.c.

  To reproduce add the following entries:
  auth	optional	pam_exec.so log=/tmp/auth.log expose_authtok quiet /usr/local/bin/auth.py

  session	optional  pam_exec.so   log=/tmp/session.log  /usr/local/bin/session.py

  Attaching example scripts.
  When using SIGUSR1 - sigusr1_handler is never called, with SIGUSR2 it is called without issues.

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