error-injection: Support fault injection framework

Support in-kernel fault-injection framework via debugfs.
This allows you to inject a conditional error to specified
function using debugfs interfaces.

Here is the result of test script described in
Documentation/fault-injection/fault-injection.txt

  ===========
  # ./test_fail_function.sh
  1+0 records in
  1+0 records out
  1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0227404 s, 46.1 MB/s
  btrfs-progs v4.4
  See http://btrfs.wiki.kernel.org for more information.

  Label:              (null)
  UUID:               bfa96010-12e9-4360-aed0-42eec7af5798
  Node size:          16384
  Sector size:        4096
  Filesystem size:    1001.00MiB
  Block group profiles:
    Data:             single            8.00MiB
    Metadata:         DUP              58.00MiB
    System:           DUP              12.00MiB
  SSD detected:       no
  Incompat features:  extref, skinny-metadata
  Number of devices:  1
  Devices:
     ID        SIZE  PATH
      1  1001.00MiB  /dev/loop2

  mount: mount /dev/loop2 on /opt/tmpmnt failed: Cannot allocate memory
  SUCCESS!
  ===========

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/fault-injection/fault-injection.txt

@@ -30,6 +30,12 @@ o fail_mmc_request
   injects MMC data errors on devices permitted by setting
   debugfs entries under /sys/kernel/debug/mmc0/fail_mmc_request
 
+o 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.
+
 Configure fault-injection capabilities behavior
 -----------------------------------------------
 
@@ -123,6 +129,29 @@ configuration of fault-injection capabilities.
 	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.
+
 o Boot option
 
 In order to inject faults while debugfs is not available (early boot time),
@@ -268,6 +297,45 @@ trap "echo 0 > /sys/kernel/debug/$FAILTYPE/probability" SIGINT SIGTERM EXIT
 echo "Injecting errors into the module $module... (interrupt to stop)"
 sleep 1000000
 
+------------------------------------------------------------------------------
+
+o 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