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.Dd May 10, 2009
.Dt FAIL 9
.Os
.Sh NAME
.Nm KFAIL_POINT_CODE ,
.Nm KFAIL_POINT_RETURN ,
.Nm KFAIL_POINT_RETURN_VOID ,
.Nm KFAIL_POINT_ERROR ,
.Nm KFAIL_POINT_GOTO ,
.Nm fail_point ,
.Nm DEBUG_FP
.Nd fail points
.Sh SYNOPSIS
.In sys/fail.h
.Fn KFAIL_POINT_CODE "parent" "name" "code"
.Fn KFAIL_POINT_RETURN "parent" "name"
.Fn KFAIL_POINT_RETURN_VOID "parent" "name"
.Fn KFAIL_POINT_ERROR "parent" "name" "error_var"
.Fn KFAIL_POINT_GOTO "parent" "name" "error_var" "label"
.Sh DESCRIPTION
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
.Xr sysctl 9
MIB, and a parser for that MIB that describes how the error
injection code should fire.
.Pp
The base fail point macro is
.Fn KFAIL_POINT_CODE
where
.Fa parent
is a sysctl tree (frequently
.Sy DEBUG_FP
for kernel fail points, but various subsystems may wish to provide
their own fail point trees), and
.Fa name
is the name of the MIB in that tree, and
.Fa code
is the error injection code.
The
.Fa code
argument does not require braces, but it is considered good style to
use braces for any multi-line code arguments.
Inside the
.Fa code
argument, the evaluation of
.Sy RETURN_VALUE
is derived from the
.Fn return
value set in the sysctl MIB.
See
.Sx SYSCTL VARIABLES
below.
.Pp
The remaining
.Fn KFAIL_POINT_*
macros are wrappers around common error injection paths:
.Bl -inset
.It Fn KFAIL_POINT_RETURN parent name
is the equivalent of
.Sy KFAIL_POINT_CODE(..., return RETURN_VALUE)
.It Fn KFAIL_POINT_RETURN_VOID parent name
is the equivalent of
.Sy KFAIL_POINT_CODE(..., return)
.It Fn KFAIL_POINT_ERROR parent name error_var
is the equivalent of
.Sy KFAIL_POINT_CODE(..., error_var = RETURN_VALUE)
.It Fn KFAIL_POINT_GOTO parent name error_var label
is the equivalent of
.Sy KFAIL_POINT_CODE(..., { error_var = RETURN_VALUE; goto label;})
.El
.Sh SYSCTL VARIABLES
The
.Fn KFAIL_POINT_*
macros add sysctl MIBs where specified.
Many base kernel MIBs can be found in the
.Sy debug.fail_point
tree (referenced in code by
.Sy DEBUG_FP ) .
.Pp
The sysctl variable may be set using the following grammar:
.Bd -literal
  <fail_point> ::
      <term> ( "->" <term> )*

  <term> ::
      ( (<float> "%") | (<integer> "*" ) )*
      <type>
      [ "(" <integer> ")" ]
      [ "[pid " <integer> "]" ]

  <float> ::
      <integer> [ "." <integer> ] |
      "." <integer>

  <type> ::
      "off" | "return" | "sleep" | "panic" | "break" | "print"
.Ed
.Pp
The <type> argument specifies which action to take:
.Bl -tag -width ".Dv return"
.It Sy off
Take no action (does not trigger fail point code)
.It Sy return
Trigger fail point code with specified argument
.It Sy sleep
Sleep the specified number of milliseconds
.It Sy panic
Panic
.It Sy break
Break into the debugger, or trap if there is no debugger support
.It Sy print
Print that the fail point executed
.El
.Pp
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".
.Pp
The operator -> can be used to express cascading terms.
If you specify <term1>-><term2>, it means that if <term1> does not
.Ql 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.
.Sh EXAMPLES
.Bl -tag -width Sy
.It Sy sysctl debug.fail_point.foobar="2.1%return(5)"
21/1000ths of the time, execute
.Fa code
with RETURN_VALUE set to 5.
.It Sy sysctl debug.fail_point.foobar="2%return(5)->5%return(22)"
2/100ths of the time, execute
.Fa code
with RETURN_VALUE set to 5.
If that does not happen, 5% of the time execute
.Fa code
with RETURN_VALUE set to 22.
.It Sy 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.
.It Sy sysctl debug.fail_point.foobar="0.1%5*return(5)"
Return 5 for 1 in 1000 executions, but only 5 times total.
.It Sy sysctl debug.fail_point.foobar="1%*sleep(50)"
1/100th of the time, sleep 50ms.
.It Sy sysctl debug.fail_point.foobar="1*return(5)[pid 1234]"
Return 5 once, when pid 1234 executes the fail point.
.El
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Zach Loafman Aq zml@FreeBSD.org .
.Sh CAVEATS
It is easy to shoot yourself in the foot by setting fail points too
aggressively or setting too many in combination.
For example, forcing
.Fn malloc
to fail consistently is potentially harmful to uptime.
.Pp
The
.Fn sleep
sysctl setting may not be appropriate in all situations.
Currently,
.Fn fail_point_eval
does not verify whether the context is appropriate for calling
.Fn msleep .