Date:   Sat, 8 Apr 2017 19:40:01 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Akinobu Mita <akinobu.mita@...il.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "open list:ABI/API" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2] fault-inject: support systematic fault injection

On Fri, Apr 7, 2017 at 6:47 PM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> 2017-04-07 3:33 GMT+09:00 Michal Hocko <mhocko@...nel.org>:
>> [Let's add linux-api - please always cc this list when adding/modifying
>> user visible interfaces]
>>
>> On Tue 28-03-17 15:01:28, Dmitry Vyukov wrote:
>>> Add /proc/self/task/<current-tid>/fail-nth file that allows failing
>>> 0-th, 1-st, 2-nd and so on calls systematically.
>>> Excerpt from the added documentation:
>>
>> I didn't really get to read through details here but it just feels wrong
>> to add this debugging only feature into proc. It also smells like one
>> off thing as well.
>
> We have 'sched' (CONFIG_SCHED_DEBUG), 'latency' (CONFIG_LATENCYTOP)
> and 'make-it-fail' as debugging per-process proc files.  So it doesn't
> look very wrong to me.  But I would like to avoid per-process proc
> directory becoming messy. Do you think introducing /proc/<pid>/debug/
> directory for debugging stuff makes sense?
>
> Side note: 'fail-nth' was originally a single debugfs file
> /sys/kernel/debug/fail_once.  But it actually read/write current task's
> fail_nth field, so I suggested changing per process procfs file.i
> This change enables to inject N-th fail to kernel threads, too.


/sys/kernel/debug/fail_once (or fail_nth) looks more appropriate to me
for a optional testing feature. The fact that it currently
reads/writes a task_struct field is merely an implementation detail.
I would also prefer ioctl's. Then we don't need to preserve "symmetry"
for no useful reason and deal with nonsensical uses like setting it
for a non-current task and running cat on it.



>>> ===
>>> Write to this file of integer N makes N-th call in the current task fail
>>> (N is 0-based). Read from this file returns a single char 'Y' or 'N'
>>> that says if the fault setup with a previous write to this file was
>>> injected or not, and disables the fault if it wasn't yet injected.
>>> Note that this file enables all types of faults (slab, futex, etc).
>>> This setting takes precedence over all other generic 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.
>>> ===
>>>
>>> Why adding new setting:
>>> 1. Existing settings are global rather than per-task.
>>>    So parallel testing is not possible.
>>> 2. attr->interval is close but it depends on attr->count
>>>    which is non reset to 0, so interval does not work as expected.
>>> 3. Trying to model this with existing settings requires manipulations
>>>    of all of probability, interval, times, space, task-filter and
>>>    unexposed count and per-task make-it-fail files.
>>> 4. Existing settings are per-failure-type, and the set of failure
>>>    types is potentially expanding.
>>> 5. make-it-fail can't be changed by unprivileged user and aggressive
>>>    stress testing better be done from an unprivileged user.
>>>    Similarly, this would require opening the debugfs files to the
>>>    unprivileged user, as he would need to reopen at least times file
>>>    (not possible to pre-open before dropping privs).
>>>
>>> The proposed interface solves all of the above (see the example).
>>>
>>> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
>>> Cc: Akinobu Mita <akinobu.mita@...il.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: linux-kernel@...r.kernel.org
>>> Cc: linux-mm@...ck.org
>>>
>>> ---
>>> We want to integrate this into syzkaller fuzzer.
>>> A prototype has found 10 bugs in kernel in first day of usage:
>>> https://groups.google.com/forum/#!searchin/syzkaller/%22FAULT_INJECTION%22%7Csort:relevance
>>>
>>> Changes since v1:
>>>  - change file name from /sys/kernel/debug/fail_once
>>>    to /proc/self/task/<current-tid>/fail-nth as per
>>>    Akinobu suggestion
>>>
>>> ---
>>>  Documentation/fault-injection/fault-injection.txt | 78 +++++++++++++++++++++++
>>>  fs/proc/base.c                                    | 52 +++++++++++++++
>>>  include/linux/sched.h                             |  1 +
>>>  kernel/fork.c                                     |  4 ++
>>>  lib/fault-inject.c                                |  7 ++
>>>  5 files changed, 142 insertions(+)
>>>
>>> diff --git a/Documentation/fault-injection/fault-injection.txt b/Documentation/fault-injection/fault-injection.txt
>>> index 415484f3d59a..192d8cbcc5f9 100644
>>> --- a/Documentation/fault-injection/fault-injection.txt
>>> +++ b/Documentation/fault-injection/fault-injection.txt
>>> @@ -134,6 +134,22 @@ use the boot option:
>>>       fail_futex=
>>>       mmc_core.fail_request=<interval>,<probability>,<space>,<times>
>>>
>>> +o proc entries
>>> +
>>> +- /proc/self/task/<current-tid>/fail-nth:
>>> +
>>> +     Write to this file of integer N makes N-th call in the current task fail
>>> +     (N is 0-based). Read from this file returns a single char 'Y' or 'N'
>>> +     that says if the fault setup with a previous write to this file was
>>> +     injected or not, and disables the fault if it 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
>>>  -----------------------------------------
>>>
>>> @@ -278,3 +294,65 @@ 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 = 0;; i++) {
>>> +             sprintf(buf, "%d", i);
>>> +             write(fail_nth, buf, strlen(buf));
>>> +             res = socketpair(AF_LOCAL, SOCK_STREAM, 0, fds);
>>> +             err = errno;
>>> +             read(fail_nth, buf, 1);
>>> +             if (res == 0) {
>>> +                     close(fds[0]);
>>> +                     close(fds[1]);
>>> +             }
>>> +             printf("%d-th fault %c: res=%d/%d\n", i, buf[0], res, err);
>>> +             if (buf[0] != 'Y')
>>> +                     break;
>>> +     }
>>> +     return 0;
>>> +}
>>> +
>>> +An example output:
>>> +
>>> +0-th fault Y: res=-1/23
>>> +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
>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>> index 6e8655845830..66001172249b 100644
>>> --- a/fs/proc/base.c
>>> +++ b/fs/proc/base.c
>>> @@ -1353,6 +1353,53 @@ static const struct file_operations proc_fault_inject_operations = {
>>>       .write          = proc_fault_inject_write,
>>>       .llseek         = generic_file_llseek,
>>>  };
>>> +
>>> +static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
>>> +                                size_t count, loff_t *ppos)
>>> +{
>>> +     struct task_struct *task;
>>> +     int err, n;
>>> +
>>> +     task = get_proc_task(file_inode(file));
>>> +     if (!task)
>>> +             return -ESRCH;
>>> +     put_task_struct(task);
>>> +     if (task != current)
>>> +             return -EPERM;
>>> +     err = kstrtoint_from_user(buf, count, 10, &n);
>>> +     if (err)
>>> +             return err;
>>> +     if (n < 0 || n == INT_MAX)
>>> +             return -EINVAL;
>>> +     current->fail_nth = n + 1;
>>> +     return len;
>>> +}
>>> +
>>> +static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
>>> +                               size_t count, loff_t *ppos)
>>> +{
>>> +     struct task_struct *task;
>>> +     int err;
>>> +
>>> +     task = get_proc_task(file_inode(file));
>>> +     if (!task)
>>> +             return -ESRCH;
>>> +     put_task_struct(task);
>>> +     if (task != current)
>>> +             return -EPERM;
>>> +     if (count < 1)
>>> +             return -EINVAL;
>>> +     err = put_user((char)(current->fail_nth ? 'N' : 'Y'), buf);
>>> +     if (err)
>>> +             return err;
>>> +     current->fail_nth = 0;
>>> +     return 1;
>>> +}
>>> +
>>> +static const struct file_operations proc_fail_nth_operations = {
>>> +     .read           = proc_fail_nth_read,
>>> +     .write          = proc_fail_nth_write,
>>> +};
>>>  #endif
>>>
>>>
>>> @@ -3296,6 +3343,11 @@ static const struct pid_entry tid_base_stuff[] = {
>>>  #endif
>>>  #ifdef CONFIG_FAULT_INJECTION
>>>       REG("make-it-fail", S_IRUGO|S_IWUSR, proc_fault_inject_operations),
>>> +     /*
>>> +      * Operations on the file check that the task is current,
>>> +      * so we create it with 0666 to support testing under unprivileged user.
>>> +      */
>>> +     REG("fail-nth", 0666, proc_fail_nth_operations),
>>>  #endif
>>>  #ifdef CONFIG_TASK_IO_ACCOUNTING
>>>       ONE("io",       S_IRUSR, proc_tid_io_accounting),
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index 543e0ea82684..7b50221fea51 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -1897,6 +1897,7 @@ struct task_struct {
>>>  #endif
>>>  #ifdef CONFIG_FAULT_INJECTION
>>>       int make_it_fail;
>>> +     int fail_nth;
>>>  #endif
>>>       /*
>>>        * when (nr_dirtied >= nr_dirtied_pause), it's time to call
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index 61284d8122fa..869c97a0a930 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -545,6 +545,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>>>
>>>       kcov_task_init(tsk);
>>>
>>> +#ifdef CONFIG_FAULT_INJECTION
>>> +     tsk->fail_nth = 0;
>>> +#endif
>>> +
>>>       return tsk;
>>>
>>>  free_stack:
>>> diff --git a/lib/fault-inject.c b/lib/fault-inject.c
>>> index 6a823a53e357..d6516ba64d33 100644
>>> --- a/lib/fault-inject.c
>>> +++ b/lib/fault-inject.c
>>> @@ -107,6 +107,12 @@ static inline bool fail_stacktrace(struct fault_attr *attr)
>>>
>>>  bool should_fail(struct fault_attr *attr, ssize_t size)
>>>  {
>>> +     if (in_task() && current->fail_nth) {
>>> +             if (--current->fail_nth == 0)
>>> +                     goto fail;
>>> +             return false;
>>> +     }
>>> +
>>>       /* No need to check any other properties if the probability is 0 */
>>>       if (attr->probability == 0)
>>>               return false;
>>> @@ -134,6 +140,7 @@ bool should_fail(struct fault_attr *attr, ssize_t size)
>>>       if (!fail_stacktrace(attr))
>>>               return false;
>>>
>>> +fail:
>>>       fail_dump(attr);
>>>
>>>       if (atomic_read(&attr->times) != -1)
>>> --
>>> 2.12.2.564.g063fe858b8-goog
>>>
>>> --
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>>
>> --
>> Michal Hocko
>> SUSE Labs

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