linux-kernel - Re: [PATCH] fault-inject: support systematic fault injection

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Date:   Tue, 28 Mar 2017 15:02:36 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Akinobu Mita <akinobu.mita@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] fault-inject: support systematic fault injection

On Sat, Mar 25, 2017 at 10:54 AM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> 2017-03-25 5:08 GMT+09:00 Dmitry Vyukov <dvyukov@...gle.com>:
>> Add /sys/kernel/debug/fail_once file that allows failing 0-th, 1-st, 2-nd
>> and so on calls systematically. Excerpt from the added documentation:
>>
>> ===
>> 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.
>> ===
>
> The "/sys/kernel/debug/fail_once" contains per-task data.
>
> Should we introduce new per-task file like "/proc/<pid>/fail-nth"
> instead of adding a single global debugfs file?

Mailed v2 that uses /proc/self/task/tid/fail-nth.


>> 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).