Date:   Sat, 8 Apr 2017 01:47:01 +0900
From:   Akinobu Mita <akinobu.mita@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        "open list:ABI/API" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2] fault-inject: support systematic fault injection

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.

>> ===
>> 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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