===========================================
Fault injection capabilities infrastructure
===========================================

See also drivers/md/md-faulty.c and "every_nth" module option for scsi_debug.


Available fault injection capabilities
--------------------------------------

- failslab

  injects slab allocation failures. (kmalloc(), kmem_cache_alloc(), ...)

- fail_page_alloc

  injects page allocation failures. (alloc_pages(), get_free_pages(), ...)

- fail_usercopy

  injects failures in user memory access functions. (copy_from_user(), get_user(), ...)

- fail_futex

  injects futex deadlock and uaddr fault errors.

- fail_sunrpc

  injects kernel RPC client and server failures.

- fail_make_request

  injects disk IO errors on devices permitted by setting
  /sys/block/<device>/make-it-fail or
  /sys/block/<device>/<partition>/make-it-fail. (submit_bio_noacct())

- fail_mmc_request

  injects MMC data errors on devices permitted by setting
  debugfs entries under /sys/kernel/debug/mmc0/fail_mmc_request

- fail_function

  injects error return on specific functions, which are marked by
  ALLOW_ERROR_INJECTION() macro, by setting debugfs entries
  under /sys/kernel/debug/fail_function. No boot option supported.

- fail_skb_realloc

  inject skb (socket buffer) reallocation events into the network path. The
  primary goal is to identify and prevent issues related to pointer
  mismanagement in the network subsystem.  By forcing skb reallocation at
  strategic points, this feature creates scenarios where existing pointers to
  skb headers become invalid.

  When the fault is injected and the reallocation is triggered, cached pointers
  to skb headers and data no longer reference valid memory locations. This
  deliberate invalidation helps expose code paths where proper pointer updating
  is neglected after a reallocation event.

  By creating these controlled fault scenarios, the system can catch instances
  where stale pointers are used, potentially leading to memory corruption or
  system instability.

  To select the interface to act on, write the network name to
  /sys/kernel/debug/fail_skb_realloc/devname.
  If this field is left empty (which is the default value), skb reallocation
  will be forced on all network interfaces.

  The effectiveness of this fault detection is enhanced when KASAN is
  enabled, as it helps identify invalid memory references and use-after-free
  (UAF) issues.

- NVMe fault injection

  inject NVMe status code and retry flag on devices permitted by setting
  debugfs entries under /sys/kernel/debug/nvme*/fault_inject. The default
  status code is NVME_SC_INVALID_OPCODE with no retry. The status code and
  retry flag can be set via the debugfs.

- Null test block driver fault injection

  inject IO timeouts by setting config items under
  /sys/kernel/config/nullb/<disk>/timeout_inject,
  inject requeue requests by setting config items under
  /sys/kernel/config/nullb/<disk>/requeue_inject, and
  inject init_hctx() errors by setting config items under
  /sys/kernel/config/nullb/<disk>/init_hctx_fault_inject.

Configure fault-injection capabilities behavior
-----------------------------------------------

debugfs entries
^^^^^^^^^^^^^^^

fault-inject-debugfs kernel module provides some debugfs entries for runtime
configuration of fault-injection capabilities.

- /sys/kernel/debug/fail*/probability:

	likelihood of failure injection, in percent.

	Format: <percent>

	Note that one-failure-per-hundred is a very high error rate
	for some testcases.  Consider setting probability=100 and configure
	/sys/kernel/debug/fail*/interval for such testcases.

- /sys/kernel/debug/fail*/interval:

	specifies the interval between failures, for calls to
	should_fail() that pass all the other tests.

	Note that if you enable this, by setting interval>1, you will
	probably want to set probability=100.

- /sys/kernel/debug/fail*/times:

	specifies how many times failures may happen at most. A value of -1
	means "no limit".

- /sys/kernel/debug/fail*/space:

	specifies an initial resource "budget", decremented by "size"
	on each call to should_fail(,size).  Failure injection is
	suppressed until "space" reaches zero.

- /sys/kernel/debug/fail*/verbose

	Format: { 0 | 1 | 2 }

	specifies the verbosity of the messages when failure is
	injected.  '0' means no messages; '1' will print only a single
	log line per failure; '2' will print a call trace too -- useful
	to debug the problems revealed by fault injection.

- /sys/kernel/debug/fail*/task-filter:

	Format: { 'Y' | 'N' }

	A value of 'N' disables filtering by process (default).
	Any positive value limits failures to only processes indicated by
	/proc/<pid>/make-it-fail==1.

- /sys/kernel/debug/fail*/require-start,
  /sys/kernel/debug/fail*/require-end,
  /sys/kernel/debug/fail*/reject-start,
  /sys/kernel/debug/fail*/reject-end:

	specifies the range of virtual addresses tested during
	stacktrace walking.  Failure is injected only if some caller
	in the walked stacktrace lies within the required range, and
	none lies within the rejected range.
	Default required range is [0,ULONG_MAX) (whole of virtual address space).
	Default rejected range is [0,0).

- /sys/kernel/debug/fail*/stacktrace-depth:

	specifies the maximum stacktrace depth walked during search
	for a caller within [require-start,require-end) OR
	[reject-start,reject-end).

- /sys/kernel/debug/fail_page_alloc/ignore-gfp-highmem:

	Format: { 'Y' | 'N' }

	default is 'Y', setting it to 'N' will also inject failures into
	highmem/user allocations (__GFP_HIGHMEM allocations).

- /sys/kernel/debug/failslab/cache-filter
	Format: { 'Y' | 'N' }

        default is 'N', setting it to 'Y' will only inject failures when
        objects are requests from certain caches.

        Select the cache by writing '1' to /sys/kernel/slab/<cache>/failslab:

- /sys/kernel/debug/failslab/ignore-gfp-wait:
- /sys/kernel/debug/fail_page_alloc/ignore-gfp-wait:

	Format: { 'Y' | 'N' }

	default is 'Y', setting it to 'N' will also inject failures
	into allocations that can sleep (__GFP_DIRECT_RECLAIM allocations).

- /sys/kernel/debug/fail_page_alloc/min-order:

	specifies the minimum page allocation order to be injected
	failures.

- /sys/kernel/debug/fail_futex/ignore-private:

	Format: { 'Y' | 'N' }

	default is 'N', setting it to 'Y' will disable failure injections
	when dealing with private (address space) futexes.

- /sys/kernel/debug/fail_sunrpc/ignore-client-disconnect:

	Format: { 'Y' | 'N' }

	default is 'N', setting it to 'Y' will disable disconnect
	injection on the RPC client.

- /sys/kernel/debug/fail_sunrpc/ignore-server-disconnect:

	Format: { 'Y' | 'N' }

	default is 'N', setting it to 'Y' will disable disconnect
	injection on the RPC server.

- /sys/kernel/debug/fail_sunrpc/ignore-cache-wait:

	Format: { 'Y' | 'N' }

	default is 'N', setting it to 'Y' will disable cache wait
	injection on the RPC server.

- /sys/kernel/debug/fail_function/inject:

	Format: { 'function-name' | '!function-name' | '' }

	specifies the target function of error injection by name.
	If the function name leads '!' prefix, given function is
	removed from injection list. If nothing specified ('')
	injection list is cleared.

- /sys/kernel/debug/fail_function/injectable:

	(read only) shows error injectable functions and what type of
	error values can be specified. The error type will be one of
	below;
	- NULL:	retval must be 0.
	- ERRNO: retval must be -1 to -MAX_ERRNO (-4096).
	- ERR_NULL: retval must be 0 or -1 to -MAX_ERRNO (-4096).

- /sys/kernel/debug/fail_function/<function-name>/retval:

	specifies the "error" return value to inject to the given function.
	This will be created when the user specifies a new injection entry.
	Note that this file only accepts unsigned values. So, if you want to
	use a negative errno, you better use 'printf' instead of 'echo', e.g.:
	$ printf %#x -12 > retval

- /sys/kernel/debug/fail_skb_realloc/devname:

        Specifies the network interface on which to force SKB reallocation.  If
        left empty, SKB reallocation will be applied to all network interfaces.

        Example usage::

          # Force skb reallocation on eth0
          echo "eth0" > /sys/kernel/debug/fail_skb_realloc/devname

          # Clear the selection and force skb reallocation on all interfaces
          echo "" > /sys/kernel/debug/fail_skb_realloc/devname

Boot option
^^^^^^^^^^^

In order to inject faults while debugfs is not available (early boot time),
use the boot option::

	failslab=
	fail_page_alloc=
	fail_usercopy=
	fail_make_request=
	fail_futex=
	fail_skb_realloc=
	mmc_core.fail_request=<interval>,<probability>,<space>,<times>

proc entries
^^^^^^^^^^^^

- /proc/<pid>/fail-nth,
  /proc/self/task/<tid>/fail-nth:

	Write to this file of integer N makes N-th call in the task fail.
	Read from this file returns a integer value. A value of '0' indicates
	that the fault setup with a previous write to this file was injected.
	A positive integer N indicates that the fault wasn't yet injected.
	Note that this file enables all types of faults (slab, futex, etc).
	This setting takes precedence over all other generic debugfs settings
	like probability, interval, times, etc. But per-capability settings
	(e.g. fail_futex/ignore-private) take precedence over it.

	This feature is intended for systematic testing of faults in a single
	system call. See an example below.


Error Injectable Functions
--------------------------

This part is for the kernel developers considering to add a function to
ALLOW_ERROR_INJECTION() macro.

Requirements for the Error Injectable Functions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Since the function-level error injection forcibly changes the code path
and returns an error even if the input and conditions are proper, this can
cause unexpected kernel crash if you allow error injection on the function
which is NOT error injectable. Thus, you (and reviewers) must ensure;

- The function returns an error code if it fails, and the callers must check
  it correctly (need to recover from it).

- The function does not execute any code which can change any state before
  the first error return. The state includes global or local, or input
  variable. For example, clear output address storage (e.g. `*ret = NULL`),
  increments/decrements counter, set a flag, preempt/irq disable or get
  a lock (if those are recovered before returning error, that will be OK.)

The first requirement is important, and it will result in that the release
(free objects) functions are usually harder to inject errors than allocate
functions. If errors of such release functions are not correctly handled
it will cause a memory leak easily (the caller will confuse that the object
has been released or corrupted.)

The second one is for the caller which expects the function should always
does something. Thus if the function error injection skips whole of the
function, the expectation is betrayed and causes an unexpected error.

Type of the Error Injectable Functions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Each error injectable functions will have the error type specified by the
ALLOW_ERROR_INJECTION() macro. You have to choose it carefully if you add
a new error injectable function. If the wrong error type is chosen, the
kernel may crash because it may not be able to handle the error.
There are 4 types of errors defined in include/asm-generic/error-injection.h

EI_ETYPE_NULL
  This function will return `NULL` if it fails. e.g. return an allocated
  object address.

EI_ETYPE_ERRNO
  This function will return an `-errno` error code if it fails. e.g. return
  -EINVAL if the input is wrong. This will include the functions which will
  return an address which encodes `-errno` by ERR_PTR() macro.

EI_ETYPE_ERRNO_NULL
  This function will return an `-errno` or `NULL` if it fails. If the caller
  of this function checks the return value with IS_ERR_OR_NULL() macro, this
  type will be appropriate.

EI_ETYPE_TRUE
  This function will return `true` (non-zero positive value) if it fails.

If you specifies a wrong type, for example, EI_TYPE_ERRNO for the function
which returns an allocated object, it may cause a problem because the returned
value is not an object address and the caller can not access to the address.


How to add new fault injection capability
-----------------------------------------

- #include <linux/fault-inject.h>

- define the fault attributes

  DECLARE_FAULT_ATTR(name);

  Please see the definition of struct fault_attr in fault-inject.h
  for details.

- provide a way to configure fault attributes

- boot option

  If you need to enable the fault injection capability from boot time, you can
  provide boot option to configure it. There is a helper function for it:

	setup_fault_attr(attr, str);

- debugfs entries

  failslab, fail_page_alloc, fail_usercopy, and fail_make_request use this way.
  Helper functions:

	fault_create_debugfs_attr(name, parent, attr);

- module parameters

  If the scope of the fault injection capability is limited to a
  single kernel module, it is better to provide module parameters to
  configure the fault attributes.

- add a hook to insert failures

  Upon should_fail() returning true, client code should inject a failure:

	should_fail(attr, size);

Application Examples
--------------------

- Inject slab allocation failures into module init/exit code::

    #!/bin/bash

    FAILTYPE=failslab
    echo Y > /sys/kernel/debug/$FAILTYPE/task-filter
    echo 10 > /sys/kernel/debug/$FAILTYPE/probability
    echo 100 > /sys/kernel/debug/$FAILTYPE/interval
    echo -1 > /sys/kernel/debug/$FAILTYPE/times
    echo 0 > /sys/kernel/debug/$FAILTYPE/space
    echo 2 > /sys/kernel/debug/$FAILTYPE/verbose
    echo Y > /sys/kernel/debug/$FAILTYPE/ignore-gfp-wait

    faulty_system()
    {
	bash -c "echo 1 > /proc/self/make-it-fail && exec $*"
    }

    if [ $# -eq 0 ]
    then
	echo "Usage: $0 modulename [ modulename ... ]"
	exit 1
    fi

    for m in $*
    do
	echo inserting $m...
	faulty_system modprobe $m

	echo removing $m...
	faulty_system modprobe -r $m
    done

------------------------------------------------------------------------------

- Inject page allocation failures only for a specific module::

    #!/bin/bash

    FAILTYPE=fail_page_alloc
    module=$1

    if [ -z $module ]
    then
	echo "Usage: $0 <modulename>"
	exit 1
    fi

    modprobe $module

    if [ ! -d /sys/module/$module/sections ]
    then
	echo Module $module is not loaded
	exit 1
    fi

    cat /sys/module/$module/sections/.text > /sys/kernel/debug/$FAILTYPE/require-start
    cat /sys/module/$module/sections/.data > /sys/kernel/debug/$FAILTYPE/require-end

    echo N > /sys/kernel/debug/$FAILTYPE/task-filter
    echo 10 > /sys/kernel/debug/$FAILTYPE/probability
    echo 100 > /sys/kernel/debug/$FAILTYPE/interval
    echo -1 > /sys/kernel/debug/$FAILTYPE/times
    echo 0 > /sys/kernel/debug/$FAILTYPE/space
    echo 2 > /sys/kernel/debug/$FAILTYPE/verbose
    echo Y > /sys/kernel/debug/$FAILTYPE/ignore-gfp-wait
    echo Y > /sys/kernel/debug/$FAILTYPE/ignore-gfp-highmem
    echo 10 > /sys/kernel/debug/$FAILTYPE/stacktrace-depth

    trap "echo 0 > /sys/kernel/debug/$FAILTYPE/probability" SIGINT SIGTERM EXIT

    echo "Injecting errors into the module $module... (interrupt to stop)"
    sleep 1000000

------------------------------------------------------------------------------

- Inject open_ctree error while btrfs mount::

    #!/bin/bash

    rm -f testfile.img
    dd if=/dev/zero of=testfile.img bs=1M seek=1000 count=1
    DEVICE=$(losetup --show -f testfile.img)
    mkfs.btrfs -f $DEVICE
    mkdir -p tmpmnt

    FAILTYPE=fail_function
    FAILFUNC=open_ctree
    echo $FAILFUNC > /sys/kernel/debug/$FAILTYPE/inject
    printf %#x -12 > /sys/kernel/debug/$FAILTYPE/$FAILFUNC/retval
    echo N > /sys/kernel/debug/$FAILTYPE/task-filter
    echo 100 > /sys/kernel/debug/$FAILTYPE/probability
    echo 0 > /sys/kernel/debug/$FAILTYPE/interval
    echo -1 > /sys/kernel/debug/$FAILTYPE/times
    echo 0 > /sys/kernel/debug/$FAILTYPE/space
    echo 1 > /sys/kernel/debug/$FAILTYPE/verbose

    mount -t btrfs $DEVICE tmpmnt
    if [ $? -ne 0 ]
    then
	echo "SUCCESS!"
    else
	echo "FAILED!"
	umount tmpmnt
    fi

    echo > /sys/kernel/debug/$FAILTYPE/inject

    rmdir tmpmnt
    losetup -d $DEVICE
    rm testfile.img

------------------------------------------------------------------------------

- Inject only skbuff allocation failures ::

    # mark skbuff_head_cache as faulty
    echo 1 > /sys/kernel/slab/skbuff_head_cache/failslab
    # Turn on cache filter (off by default)
    echo 1 > /sys/kernel/debug/failslab/cache-filter
    # Turn on fault injection
    echo 1 > /sys/kernel/debug/failslab/times
    echo 1 > /sys/kernel/debug/failslab/probability


Tool to run command with failslab or fail_page_alloc
----------------------------------------------------
In order to make it easier to accomplish the tasks mentioned above, we can use
tools/testing/fault-injection/failcmd.sh.  Please run a command
"./tools/testing/fault-injection/failcmd.sh --help" for more information and
see the following examples.

Examples:

Run a command "make -C tools/testing/selftests/ run_tests" with injecting slab
allocation failure::

	# ./tools/testing/fault-injection/failcmd.sh \
		-- make -C tools/testing/selftests/ run_tests

Same as above except to specify 100 times failures at most instead of one time
at most by default::

	# ./tools/testing/fault-injection/failcmd.sh --times=100 \
		-- make -C tools/testing/selftests/ run_tests

Same as above except to inject page allocation failure instead of slab
allocation failure::

	# env FAILCMD_TYPE=fail_page_alloc \
		./tools/testing/fault-injection/failcmd.sh --times=100 \
		-- make -C tools/testing/selftests/ run_tests

Systematic faults using fail-nth
---------------------------------

The following code systematically faults 0-th, 1-st, 2-nd and so on
capabilities in the socketpair() system call::

  #include <sys/types.h>
  #include <sys/stat.h>
  #include <sys/socket.h>
  #include <sys/syscall.h>
  #include <fcntl.h>
  #include <unistd.h>
  #include <string.h>
  #include <stdlib.h>
  #include <stdio.h>
  #include <errno.h>

  int main()
  {
	int i, err, res, fail_nth, fds[2];
	char buf[128];

	system("echo N > /sys/kernel/debug/failslab/ignore-gfp-wait");
	sprintf(buf, "/proc/self/task/%ld/fail-nth", syscall(SYS_gettid));
	fail_nth = open(buf, O_RDWR);
	for (i = 1;; i++) {
		sprintf(buf, "%d", i);
		write(fail_nth, buf, strlen(buf));
		res = socketpair(AF_LOCAL, SOCK_STREAM, 0, fds);
		err = errno;
		pread(fail_nth, buf, sizeof(buf), 0);
		if (res == 0) {
			close(fds[0]);
			close(fds[1]);
		}
		printf("%d-th fault %c: res=%d/%d\n", i, atoi(buf) ? 'N' : 'Y',
			res, err);
		if (atoi(buf))
			break;
	}
	return 0;
  }

An example output::

	1-th fault Y: res=-1/23
	2-th fault Y: res=-1/23
	3-th fault Y: res=-1/12
	4-th fault Y: res=-1/12
	5-th fault Y: res=-1/23
	6-th fault Y: res=-1/23
	7-th fault Y: res=-1/23
	8-th fault Y: res=-1/12
	9-th fault Y: res=-1/12
	10-th fault Y: res=-1/12
	11-th fault Y: res=-1/12
	12-th fault Y: res=-1/12
	13-th fault Y: res=-1/12
	14-th fault Y: res=-1/12
	15-th fault Y: res=-1/12
	16-th fault N: res=0/12
