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Is this possible racing condition? #73

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hytgbn opened this issue Dec 15, 2020 · 2 comments
Closed

Is this possible racing condition? #73

hytgbn opened this issue Dec 15, 2020 · 2 comments

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@hytgbn
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hytgbn commented Dec 15, 2020

I recently found one of my host doesn't run specific cronjob file. The cronjob file is written when the host gets launched. (As part of UserData of AWS EC2 instance).

host: Amazon linux 4.14 assuming this supports inotify

Wondering if this race condition is ever possible:

  1. crond loads database without the cronjob file (or when the file doesn't have content) https://github.com/cronie-crond/cronie/blob/master/src/cron.c#L319
  2. Then the cronjob content is written.
  3. crond adds inotify watch https://github.com/cronie-crond/cronie/blob/master/src/cron.c#L333

If this is ever possible, would it make sense to do load_database after adds watch?
@t8m
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t8m commented Dec 16, 2020

What is the cronie version on the system? There were some other possible races solved before.
But yes, it the job file is written simultaneously with crond startup I think the race you described above is possible and easy fix should be to just switch the order of the initial database load and watch setup.

hytgbn added a commit to hytgbn/cronie that referenced this issue Dec 24, 2020
@hytgbn
Fix race condition between file update and load_database
e6d1eea 
Issue: cronie-crond#73

This CR is to fix race condition between file update and load_database
when INOTIFY is enabled.

Previously, if crontab file is simultaneously written when crond starts up,
there is chances to miss the notification. This is because load_database
was performed before inotify watches get registered.

This CR will invoke `load_database` after watches are registered, so
crond doesn't miss such updates.
@hytgbn hytgbn mentioned this issue Dec 24, 2020
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t8m pushed a commit that referenced this issue Jan 4, 2021
@hytgbn @t8m
Fix race condition between file update and load_database
46a2c8f 
Issue: #73

This CR is to fix race condition between file update and load_database
when INOTIFY is enabled.

Previously, if crontab file is simultaneously written when crond starts up,
there is chances to miss the notification. This is because load_database
was performed before inotify watches get registered.

This CR will invoke `load_database` after watches are registered, so
crond doesn't miss such updates.
@t8m
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t8m commented Jan 22, 2021

Fixed with the PR #74

@t8m t8m closed this as completed Jan 22, 2021
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@hytgbn @t8m