On Tue, 28 Mar 2017 15:01:28 +0200 Dmitry Vyukov <dvyukov@xxxxxxxxxx> 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:
>
> ===
> 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).
Seems reasonable.
> --- 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
Nit: fail_nth should really be unsigned. And make_it_fail could be
made a single bit which shares storage with brk_randomized (for
example).
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