bcron documentation

• Introduction
• Design Notes
• Configuration
• Implementation Notes

1 Introduction

Name comes from: Bruce's / Better / Busy cron.

• Problems
• Requirements
• Design Choices
• vixie-cron Patches

1.1 Problems With Other cron Systems

• Problems with vixie-cron
• Problems with fcron
• Problems with anacron
• Problems with dcron

1.1.1 vixie-cron

• Too many bugs The last official release of vixie-cron was in December of 1993. Since then, many security holes and other bugs have been found, requiring every vendor to distribute a multiply-patched version of this system. Most vendors have a different set of patches, making this a bit of a moving target.
• Can't handle DST changes vixie-cron's mode of operations is to wake up every minute and determine, with no history, what jobs to run. When DST changes, this causes some jobs to be either skipped or run twice.

1.1.2 fcron

• Parsing issues If a job starts with a word (terminated by any non-word character, like /) that happens to also be a username, fcron interprets this word as the userid to run the job under, even for non-root users. This causes problems for jobs like bin/something when user bin exists.
• No support for /etc/cron.d
• No support for /etc/crontab
• Gratuitious incompatibilities with vixie-cron Jobs are run with the login shell for the user instead of /bin/sh. The default mode of execution prevents the same job from running multiple times (a good option, but bad default). MAILTO can only contain a username, not a full email address.

1.1.3 anacron

Anacron is only really useful for running jobs once a day or less frequently. From what I've seen, it's good at what it does, just not useful at much else.

1.1.4 dcron

• No support for /etc/cron.d
• No support for /etc/crontab
• No easy way of emulating support for system crontabs dcron's crontab format does not include a “username” column, and as such makes it nearly impossible to even emulate system crontabs.
• Unusual or broken handling of DST changes From reading the source, dcron only handles cases where the linear time jumps. When DST changes, linear time does not change, and so dcron effectively does not handle this situation.

1.2 Requirements

• Security

  I am choosing to make a number of relatively unorthodox choices in order to avoid many of the security issues that have plagued vixie-cron and other related systems.

• External compatability with vixie-cron

  In particular, the system must support:

  • crontabs in the same file format as vixie-cron
  • crontabs submitted via a command-line compatable crontab program
  • mail is sent to $MAILTO if the job produces any output (and possibly if the job exits non-zero)
• Support for system crontabs

  This means that the system MUST support:

  • System crontab entries in /etc/crontab
  • System crontab entries in /etc/cron.d/*
  System crontab entries are additionally differentiated from normal ones by having a “username” column immediately preceding the command.
• Intelligent handling of DST time changes

  One of the biggest frustrations I have had with dealing with vixie-cron is its complete inability to deal with time jumps in an intelligent manner. In particular, when DST changes happen, jobs will either get skipped (when time jumps forward) or executed twice (when time jumps backwards). This is unacceptable.

• Allow setting ulimits before executing commands

  In one of the target installations, we need to set up ulimits (limiting CPU time and memory) before executing commands. It would be easy enough to ulimit the entire daemon, but then the daemon itself would be vulnerable to getting killed when it has run for long enough. Our current setup is to run jobs through a global wrapper script, which can set any necessary limits (or anything else) and then execute the job.

1.3 Design Choices

Use local socket to submit files.

There are two basic methods of submitting crontab files:

1. Use a setuid program to write directly into spool files.
2. Set up a local socket to submit jobs to a daemon.

¹ Using a setuid submission agent was discarded to prevent the possibility of all the bugs that have plagued other setuid submission agents. The socket protocol is deliberately very simple, to make the submission agent foolproof.

Multiple process daemon.

By seperating job submission from job execution, exploiting the system to run arbitrary jobs as privileged users is made even harder. It also makes the design of those individual programs much simpler.

1.4 vixie-cron Patches

This section lists all the non-trivial patches found for vixie-cron, what problem they appear to address, and (if appropriate) how bcron will avoid the same problem. The patches listed come from multiple sources, including the latest RPM (Fedora Core IIRC).

0days.patch

This patch modifies the crontab.5 man page to remove allowing ‘0’ for day of month or month numbers.

badsig.patch

On some systems, signal handlers are single-shot. This patch modifies the SIGHUP handler to reactivate itself before it returns. bcron will use the bglibs signal functions, which use sigaction to create suitable handlers, where appropriate. bcron doesn't use signals for any purpose.

buffer.patch

This patch increases the maximum username length from 20 to 32, and modifies calls to strcpy to use strncpy to ensure all string copies are length bounded. bcron uses dynamically allocated strings to eliminate the possibility of buffer overflows.

close_stdin.diff

This patch modifies the cron daemon to close stdin, stdout, and stderr on startup, and to reopen them as /dev/null. The bcron daemons run under supervise, and have no need of such handling.

crond.patch

Adds support for /etc/cron.d

cront_stdin.patch

Appears to modify crontab's command-line handling such that no argument is interpreted as to read the crontab from standard input.

crontab.5.diff

Documents several builtin macros to replace the first 5 fields. This macros consist of: ‘@reboot’, ‘@yearly’, ‘@annually’, ‘@monthly’, ‘@weekly’, ‘@daily’, ‘@midnight’, and ‘@hourly’. bcron will not, at least initially, support these macros.

crontab.patch

Modifies crontab to use strncpy and snprintf when writing into length-bounded strings.

crontabloc.patch

Patches the crontab man page to reference /etc/crontab.

dst.patch

Patches the crontab man page to point out that DST may cause jobs to be skipped or repeated.

name.patch

Appears to modify how the cron daemon handles sending messages to syslog. bcron will log messages to stderr, avoiding syslog entirely.

nodot.patch

Adds ‘-i’ to the list of arguments sent to sendmail (result is ‘-FCronDaemon -i -odi -oem’). Only useful for sendmail, but still needed.

root_-u-85879.patch

Sanity checks the use of ‘-u’ against UID and/or root.

security2.patch

Does some sanity checking on mailto, and does a setuid before sending mail. bcron plays safe with mailto by putting it into a message header, and always drops root privileges before executing commands.

sigchld.patch

Return the SIGCHLD handler to its default state before executing commands.

sprintf.patch

More sprintf -> snprintf conversions.

time.patch

Sync all the crontabs before sleeping to handle changes in the system time.

timeaftertime.patch

The previous patch created double execution issues with small backwards adjustments in the clock time.

2 Design Notes

• Fundamental Operations
• Programs
• Files
• Inter-Process Communication

2.1 Fundamental Operations

The following is a list of all the “core” operations that must be provided based on the requirements.

Execute jobs

Strictly speaking, this is the only role that requires superuser privileges. Every other job should run as non-root.

Schedule jobs

Scan all the known jobs, determine which one needs to be executed next, and sleep until that time arrives.

Accept new user crontabs

Listen for connections on a socket and accept job data.

Parse crontabs into internal job format

Read the submitted files and parse their contents into a structured format.

Check for new system crontabs

Check /etc/crontab and /etc/cron.d/* every minute for modifications. If any files are changed, added, or deleted, add the listed jobs. On systems with tightened security, these files may only be readable by ‘root’.

Manage saved state

All jobs need to be saved to disk along with when they were last executed, in order to determine when they should be next executed.

2.2 Programs

bcron-sched

Top-level scheduler agent. Starts up as root, runs bcron-exec, and then drops root permanently.

bcron-exec

Accepts jobs to run from bcron-sched on stdin, writes exit status back to stdout.

bcron-spool

Manages the cron spool: receives jobs submitted from users, writes them to files in /var/spool/cron/crontabs, and notifies bcron-sched. This needs to be run from unixserver in order to securely determine the invoking UID. This program will optionally run the file through an external filter, specified on the command line, before installing the job.

bcron-update

Watches for changes to the system crontabs and notifies bcron-sched.

2.3 Files

• File Hierarchy

2.3.1 File Hierarchy

/etc/cron.d/

/etc/cron.d/*

/etc/crontab

The above three items are read

/var/spool/cron/

/var/spool/cron/crontabs/

Directory containing raw (text) crontab files.

/var/spool/cron/crontabs/:etc:cron.d:*

Colon is chosen as a seperator because usernames cannot contain colons due to the format of /etc/passwd.

/var/spool/cron/crontabs/:etc:crontab

/var/spool/cron/bcrontabs/

Directory containing pre-parsed (aka compiled) crontab files (Not yet implemented).

/var/spool/cron/tmp/

Temporary directory for files as they are written.

/var/spool/cron/trigger

Named pipe used to tell bcron-sched to rescan the crontabs.

2.4 Inter-Process Communication

All communication between programs is done in terms of either “packets” or “lines”. A packet is formatted as a netstring. That is, a packet of length N is encoded as the ASCII decimal value of N, ‘:’, N bytes of data, terminated by ‘,’. A line is simply a series of non-NUL bytes terminated by a NUL byte.

• Job Submission Protocol
• bcron-exec Protocol

2.4.1 Job Submission Protocol

Client sends a packet containing a single byte command followed by the username. If the command requires additional data, it is seperated from the username by a NUL byte. Server responds with a packet containing a response byte followed by a text message.

Client command codes are:

S

Submit a user crontab file. The content string contains the entire crontab file.

L

List the current crontab file. No content string.

R

Remove any previously submitted crontab file. No content string.

Y

List all system crontabs. No content string. This command is only available to users ‘root’ and ‘cron’.

Server response codes are:

K

Command was successful; file was parsed and accepted.

D

File could not be parsed.

Z

Temporary internal error.

2.4.2 bcron-exec Protocol

Input packets contain a series of four or more NUL-terminated lines:

ID

username

command

environment

The environment is optional. If the environment contains SHELL, it replaces the default shell (‘/bin/sh’). If the environment contains MAILTO, it overrides the default mailing address derived from the username.

Output packet:

ID

NUL

response code

text message

3 Configuration

• Environment Variables

3.1 Environment Variables

BCRON_SPOOL

The base directory for bcron's files. Defaults to /var/spool/cron.

BCRON_USER

The non-root user name to switch to for all processes that don't require root privileges. Defaults to ‘cron’.

BCRON_MAXSIZE

The maximum size (in bytes) of a single user crontab. Defaults to unlimited.

BCRON_MAXJOBS

The maximum number of jobs in a single user crontab. Defaults to unlimited.

BCRON_SOCKET

The full path to the UNIX-domain socket used to submit crontabs. Defaults to /var/run/bcron-spool.

4 Implementation Notes

• Job Scheduler

4.1 Job Scheduler

Getting the job scheduler to work correctly for all possible cases consumed more time than all the other parts of the program put together, and this is all because of the problems that daylight savings causes.

The ultimate goal is this: given a linear timestamp, determine the next linear timestamp after which a job must be run. “Linear time” means the number of seconds since an epoch, in this case the UNIX epoch, midnight GMT January 1st, 1970. To contrast, “local time” means the time in terms of years, months, days, hours, minutes, and seconds in the current locale.

The time specification for jobs is composed of a set of local time bitmaps quantifying under what time conditions the job should run, assuming the conditions are evaluated every minute. The maximum scheduling resolution is one minute.

The effective algorithm is to step through every possible minute until the local time matches all the conditions in the time specification. This would result in an algorithm that could take up to 527,040-1 steps to complete, which is far too big a number to evaluate on every job. So, the algorithm optimizes away all impossible cases to make much larger time jumps. For example, if the job cannot be scheduled on the current day, skip over the entire day instead of just the current minute.

There are two ways I approached this task. First, do all calculations in terms of local time, and return the linear time at the last step. Second, do as many calculations as possible in terms of linear time, which can be returned directly. Both methods start with the same input data: The current linear time, and a set of bitmasks representing under what time conditions the job should run.

The first method was the most straightforward to get mostly working, until I started to consider the implications of DST. During the transition from normal time to DST, an hour is skipped. This is no big deal, as mktime will (or can be made to) compensate by picking the next valid hour when presented with a “missing” hour. On the other hand, there are many gotchas when dealing with the duplicated hour. For example, hourly jobs need to get scheduled on both, but daily jobs on only one.

The second method was harder to get working initially, as the math is more complicated. Despite doing many calculations in terms of linear time, this method still needs to keep track of the local time, in order to check against the bitmaps as well as to determine things like when the next day or month should start. This approach proved to be much easier to work with, once the initial math was done, and easier to make work correctly with regards to DST transitions.