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