FAIL(9) FreeBSD Kernel Developer's Manual FAIL(9)




#include < sys/fail.h>

KFAIL_POINT_CODE( parent, name, code);

KFAIL_POINT_RETURN( parent, name);


KFAIL_POINT_ERROR( parent, name, error_var);

KFAIL_POINT_GOTO( parent, name, error_var, label);


Fail points are used to add code points where errors may be injected in a user controlled fashion. Fail points provide a convenient wrapper around user-provided error injection code, providing a sysctl(9) MIB, and a parser for that MIB that describes how the error injection code should fire.

The base fail point macro is KFAIL_POINT_CODE() where parent is a sysctl tree (frequently DEBUG_FP for kernel fail points, but various subsystems may wish to provide their own fail point trees), and name is the name of the MIB in that tree, and code is the error injection code. The code argument does not require braces, but it is considered good style to use braces for any multi-line code arguments. Inside the code argument, the evaluation of RETURN_VALUE is derived from the return() value set in the sysctl MIB. See SYSCTL VARIABLES below.

The remaining KFAIL_POINT_*() macros are wrappers around common error injection paths:

KFAIL_POINT_RETURN( parent, name)
is the equivalent of KFAIL_POINT_CODE(..., return RETURN_VALUE)
is the equivalent of KFAIL_POINT_CODE(..., return)
KFAIL_POINT_ERROR( parent, name, error_var)
is the equivalent of KFAIL_POINT_CODE(..., error_var = RETURN_VALUE)
KFAIL_POINT_GOTO( parent, name, error_var, label)
is the equivalent of KFAIL_POINT_CODE(..., { error_var = RETURN_VALUE; goto label;})


The KFAIL_POINT_*() macros add sysctl MIBs where specified. Many base kernel MIBs can be found in the debug.fail_point tree (referenced in code by DEBUG_FP).

The sysctl variable may be set using the following grammar:

  <fail_point> :: 
      <term> ( "->" <term> )* 
  <term> :: 
      ( (<float> "%") | (<integer> "*" ) )* 
      [ "(" <integer> ")" ] 
      [ "[pid " <integer> "]" ] 
  <float> :: 
      <integer> [ "." <integer> ] | 
      "." <integer> 
  <type> :: 
      "off" | "return" | "sleep" | "panic" | "break" | "print"

The <type> argument specifies which action to take:

Take no action (does not trigger fail point code)
Trigger fail point code with specified argument
Sleep the specified number of milliseconds
Break into the debugger, or trap if there is no debugger support
Print that the fail point executed

The <float>% and <integer>* modifiers prior to <type> control when <type> is executed. The <float>% form (e.g. "1.2%") can be used to specify a probability that <type> will execute. The <integer>* form (e.g. "5*") can be used to specify the number of times <type> should be executed before this <term> is disabled. Only the last probability and the last count are used if multiple are specified, i.e. "1.2%2%" is the same as "2%". When both a probability and a count are specified, the probability is evaluated before the count, i.e. "2%5*" means "2% of the time, but only 5 times total".

The operator -> can be used to express cascading terms. If you specify <term1>-><term2>, it means that if <term1> does not ‘ execute’, <term2> is evaluated. For the purpose of this operator, the return() and print() operators are the only types that cascade. A return() term only cascades if the code executes, and a print() term only cascades when passed a non-zero argument. A pid can optionally be specified. The fail point term is only executed when invoked by a process with a matching p_pid.


sysctl debug.fail_point.foobar="2.1%return(5)"
21/1000ths of the time, execute code with RETURN_VALUE set to 5.
sysctl debug.fail_point.foobar="2%return(5)->5%return(22)"
2/100ths of the time, execute code with RETURN_VALUE set to 5. If that does not happen, 5% of the time execute code with RETURN_VALUE set to 22.
sysctl debug.fail_point.foobar="5*return(5)->0.1%return(22)"
For 5 times, return 5. After that, 1/1000th of the time, return 22.
sysctl debug.fail_point.foobar="0.1%5*return(5)"
Return 5 for 1 in 1000 executions, but only 5 times total.
sysctl debug.fail_point.foobar="1%*sleep(50)"
1/100th of the time, sleep 50ms.
sysctl debug.fail_point.foobar="1*return(5)[pid 1234]"
Return 5 once, when pid 1234 executes the fail point.


This manual page was written by Zach Loafman <zml@FreeBSD.org>.


It is easy to shoot yourself in the foot by setting fail points too aggressively or setting too many in combination. For example, forcing malloc() to fail consistently is potentially harmful to uptime.

The sleep() sysctl setting may not be appropriate in all situations. Currently, fail_point_eval() does not verify whether the context is appropriate for calling msleep().

May 10, 2009 FreeBSD