FAIL(9)

💖 NAME 💖

okay so this is FAIL(9) from the Kernel Developer’s Manual, and it’s literally all about fail points, bestie. The full name squad on this page is:

• DEBUG_FP
• KFAIL_POINT_CODE
• KFAIL_POINT_CODE_FLAGS
• KFAIL_POINT_CODE_COND
• KFAIL_POINT_ERROR
• KFAIL_POINT_EVAL
• KFAIL_POINT_DECLARE
• KFAIL_POINT_DEFINE
• KFAIL_POINT_GOTO
• KFAIL_POINT_RETURN
• KFAIL_POINT_RETURN_VOID
• KFAIL_POINT_SLEEP_CALLBACKS
• fail_point

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✨ SYNOPSIS ✨

#include <sys/fail.h>

KFAIL_POINT_CODE(parent, name, code);

KFAIL_POINT_CODE_FLAGS(parent, name, flags, code);

KFAIL_POINT_CODE_COND(parent, name, cond, flags, code);

KFAIL_POINT_ERROR(parent, name, error_var);

KFAIL_POINT_EVAL(name, code);

KFAIL_POINT_DECLARE(name);

KFAIL_POINT_DEFINE(parent, name, flags);

KFAIL_POINT_GOTO(parent, name, error_var, label);

KFAIL_POINT_RETURN(parent, name);

KFAIL_POINT_RETURN_VOID(parent, name);

KFAIL_POINT_SLEEP_CALLBACKS(parent, name, pre_func, pre_arg, post_func,
    post_arg, code);

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🦋 DESCRIPTION 🦋

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

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), 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.

Additionally, KFAIL_POINT_CODE_FLAGS() provides a flags argument which controls the fail point’s behaviour. This can be used to e.g., mark the fail point’s context as non-sleepable, which causes the sleep action to be coerced to a busy wait. The supported flags are:

• FAIL_POINT_USE_TIMEOUT_PATH — Rather than sleeping on a sleep() call, just fire the post-sleep function after a timeout fires.
• FAIL_POINT_NONSLEEPABLE — Mark the fail point as being in a non-sleepable context, which coerces sleep() calls to delay() calls.

Likewise, KFAIL_POINT_CODE_COND() supplies a cond argument, which allows you to set the condition under which the fail point’s code may fire. This is equivalent to:

if (cond)
        KFAIL_POINT_CODE_FLAGS(...);

See SYSCTL VARIABLES below, bestie.

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)
• KFAIL_POINT_RETURN_VOID(parent, name) 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;})

You can also introduce fail points by separating the declaration, definition, and evaluation portions.

• KFAIL_POINT_DECLARE(name) is used to declare the fail_point struct.
• KFAIL_POINT_DEFINE(parent, name, flags) defines and initializes the fail_point and sets up its sysctl(9).
• KFAIL_POINT_EVAL(name, code) is used at the point that the fail point is executed.

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🌸 SYSCTL VARIABLES 🌸

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 in a number of ways:

[<pct>%][<cnt>*]<type>[(args...)][-><more terms>]

The <type> argument specifies which action to take; it can be one of:

• off — Take no action (does not trigger fail point code)
• return — Trigger fail point code with specified argument
• sleep — Sleep the specified number of milliseconds
• panic — Panic
• break — Break into the debugger, or trap if there is no debugger support
• print — Print that the fail point executed
• pause — Threads sleep at the fail point until the fail point is set to off
• yield — Thread yields the cpu when the fail point is evaluated
• delay — Similar to sleep, but busy waits the cpu. (Useful in non-sleepable contexts.)

The <pct>% and <cnt>* modifiers prior to <type> control when <type> is executed. The <pct>% form (e.g. "1.2%") can be used to specify a probability that <type> will execute. This is a decimal in the range (0, 100] which can specify up to 1/10,000% precision. The <cnt>* 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.

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🎀 EXAMPLES 🎀

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.

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👑 AUTHORS 👑

This manual page was written by:

• Matthew Bryan <matthew.bryan@isilon.com>
• and Zach Loafman <zml@FreeBSD.org>.

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💀 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 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(). You can force it to evaluate a sleep action as a delay action by specifying the FAIL_POINT_NONSLEEPABLE flag at the point the fail point is declared.

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💖 okay that’s the whole page, bestie — FAIL(9) from Debian, dated June 6, 2019, signing off 💖