===========================================
  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_futex
  
    injects futex deadlock and uaddr fault errors.
  
  - 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. (generic_make_request())
  
  - 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.
  
  - 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.
  
  
  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 'N', setting it to 'Y' won't inject failures into
  	highmem/user allocations.
  
  - /sys/kernel/debug/failslab/ignore-gfp-wait:
  - /sys/kernel/debug/fail_page_alloc/ignore-gfp-wait:
  
  	Format: { 'Y' | 'N' }
  
  	default is 'N', setting it to 'Y' will inject failures
  	only into non-sleep allocations (GFP_ATOMIC 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_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/<functiuon-name>/retval:
  
  	specifies the "error" return value to inject to the given
  	function for given function. This will be created when
  	user specifies new injection entry.
  
  Boot option
  ^^^^^^^^^^^
  
  In order to inject faults while debugfs is not available (early boot time),
  use the boot option::
  
  	failslab=
  	fail_page_alloc=
  	fail_make_request=
  	fail_futex=
  	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.
  
  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, 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 1 > /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 1 > /sys/kernel/debug/$FAILTYPE/ignore-gfp-wait
      echo 1 > /sys/kernel/debug/$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
      echo -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
  
  
  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
  ", 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