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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.
- 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". Note, though, that this file only accepts unsigned
values. So, if you want to specify -1, you better use 'printf' instead
of 'echo', e.g.: $ printf %#x -1 > times
- /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
- /sys/kernel/debug/fail*/require-start,
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
- /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
- /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_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
- 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
Boot option
In order to inject faults while debugfs is not available (early boot time),
use the boot option::
proc entries
- /proc/<pid>/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
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::
echo Y > /sys/kernel/debug/$FAILTYPE/task-filter
echo 10 > /sys/kernel/debug/$FAILTYPE/probability
echo 100 > /sys/kernel/debug/$FAILTYPE/interval
printf %#x -1 > /sys/kernel/debug/$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
bash -c "echo 1 > /proc/self/make-it-fail && exec $*"
if [ $# -eq 0 ]
echo "Usage: $0 modulename [ modulename ... ]"
exit 1
for m in $*
echo inserting $m...
faulty_system modprobe $m
echo removing $m...
faulty_system modprobe -r $m
- Inject page allocation failures only for a specific module::
if [ -z $module ]
echo "Usage: $0 <modulename>"
exit 1
modprobe $module
if [ ! -d /sys/module/$module/sections ]
echo Module $module is not loaded
exit 1
cat /sys/module/$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
printf %#x -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 $module... (interrupt to stop)"
sleep 1000000
- Inject open_ctree error while btrfs mount::
rm -f testfile.img
dd if=/dev/zero of=testfile.img bs=1M seek=1000 count=1
DEVICE=$(losetup --show -f testfile.img)
mkfs.btrfs -f $DEVICE
mkdir -p tmpmnt
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/$FAILTYPE/interval
printf %#x -1 > /sys/kernel/debug/$FAILTYPE/times
echo 0 > /sys/kernel/debug/$FAILTYPE/space
echo 1 > /sys/kernel/debug/$FAILTYPE/verbose
mount -t btrfs $DEVICE tmpmnt
if [ $? -ne 0 ]
echo "SUCCESS!"
echo "FAILED!"
umount tmpmnt
echo > /sys/kernel/debug/$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/ Please run a command
"./tools/testing/fault-injection/ --help" for more information and
see the following examples.
Run a command "make -C tools/testing/selftests/ run_tests" with injecting slab
allocation failure::
# ./tools/testing/fault-injection/ \
-- 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/ --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/ --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) {
printf("%d-th fault %c: res=%d/%d\n", i, atoi(buf) ? 'N' : 'Y',
res, err);
if (atoi(buf))
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