[PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth

[PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth

[Akinobu Mita]

The fault-inject-make-fail-nth-read-write-interface-symmetric.patch in
-mm tree allows users to set task->fail_nth for non current task by procfs.
On the other hand, the current task's fail_nth is decreased to zero in
fault-injection path without any specific locks.

So we need to prevent the task->fail_nth from being unexpected value by
data races (for example, setting task->fail_nth to zero while decreasing
the current->fail_nth).  In this fix, we use READ_ONCE() and WRITE_ONCE()
to prevent the compiler from creating unsolicited accesses.

Cc: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
 fs/proc/base.c     | 5 +++--
 lib/fault-inject.c | 7 +++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ecc8a25..719c2e9 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1370,7 +1370,7 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
 	task = get_proc_task(file_inode(file));
 	if (!task)
 		return -ESRCH;
-	task->fail_nth = n;
+	WRITE_ONCE(task->fail_nth, n);
 	put_task_struct(task);
 
 	return count;
@@ -1386,7 +1386,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
 	task = get_proc_task(file_inode(file));
 	if (!task)
 		return -ESRCH;
-	len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
+	len = snprintf(numbuf, sizeof(numbuf), "%u\n",
+			READ_ONCE(task->fail_nth));
 	len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
 	put_task_struct(task);
 
diff --git a/lib/fault-inject.c b/lib/fault-inject.c
index 09ac73c1..7d315fd 100644
--- a/lib/fault-inject.c
+++ b/lib/fault-inject.c
@@ -107,9 +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)
+	if (in_task()) {
+		unsigned int fail_nth = READ_ONCE(current->fail_nth);
+
+		if (fail_nth && !WRITE_ONCE(current->fail_nth, fail_nth - 1))
 			goto fail;
+
 		return false;
 	}
 
-- 
2.7.4

Re: [PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth

[Lu Fengqi]

On Fri, Jul 14, 2017 at 01:14:52AM +0900, Akinobu Mita wrote:
>The fault-inject-make-fail-nth-read-write-interface-symmetric.patch in
>-mm tree allows users to set task->fail_nth for non current task by procfs.
>On the other hand, the current task's fail_nth is decreased to zero in
>fault-injection path without any specific locks.
>
>So we need to prevent the task->fail_nth from being unexpected value by
>data races (for example, setting task->fail_nth to zero while decreasing
>the current->fail_nth).  In this fix, we use READ_ONCE() and WRITE_ONCE()
>to prevent the compiler from creating unsolicited accesses.
>
>Cc: Dmitry Vyukov <dvyukov@google.com>
>Reported-by: Dmitry Vyukov <dvyukov@google.com>
>Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
>---
> fs/proc/base.c     | 5 +++--
> lib/fault-inject.c | 7 +++++--
> 2 files changed, 8 insertions(+), 4 deletions(-)
>
>diff --git a/fs/proc/base.c b/fs/proc/base.c
>index ecc8a25..719c2e9 100644
>--- a/fs/proc/base.c
>+++ b/fs/proc/base.c
>@@ -1370,7 +1370,7 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
> 	task = get_proc_task(file_inode(file));
> 	if (!task)
> 		return -ESRCH;
>-	task->fail_nth = n;
>+	WRITE_ONCE(task->fail_nth, n);
> 	put_task_struct(task);
> 
> 	return count;
>@@ -1386,7 +1386,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
> 	task = get_proc_task(file_inode(file));
> 	if (!task)
> 		return -ESRCH;
>-	len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
>+	len = snprintf(numbuf, sizeof(numbuf), "%u\n",
>+			READ_ONCE(task->fail_nth));
> 	len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
> 	put_task_struct(task);
> 
>diff --git a/lib/fault-inject.c b/lib/fault-inject.c
>index 09ac73c1..7d315fd 100644
>--- a/lib/fault-inject.c
>+++ b/lib/fault-inject.c
>@@ -107,9 +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)
>+	if (in_task()) {
>+		unsigned int fail_nth = READ_ONCE(current->fail_nth);
>+
>+		if (fail_nth && !WRITE_ONCE(current->fail_nth, fail_nth - 1))
> 			goto fail;
>+
> 		return false;
> 	}
> 
>-- 
>2.7.4
>
>
>
hi

I'm a btrfs developer. I found that fail_make_request didn't produce the
expected IO ERROR when running xfstests on linux 4.13-rc1.

That testcase enable fail_make_request by the following commands:
# echo 100 > /sys/kernel/debug/fail_make_request/probability
# echo 2 > /sys/kernel/debug/fail_make_request/times
# echo 0 > /sys/kernel/debug/fail_make_request/verbose
# echo 1 > /sys/block/sda/sda1/make-it-fail
# dd if=/dev/zero of=/dev/sda1 bs=128K count=1 oflag=direct

As I understand it, after applying this patch, I have to write
/proc/<dd pid>/file-nth firstly so that dd process can catch the IO ERROR.
However, the dd process is so fast that I can't write file-nth.

So, could you tell me how to produce IO ERROR under these circumstances?

A response would be very much appreciated. Thank you for your time.

-- 
Thanks,
Lu

Re: [PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth

[Dmitry Vyukov]

On Tue, Aug 1, 2017 at 3:09 PM, Lu Fengqi <lufq.fnst@cn.fujitsu.com> wrote:
> On Fri, Jul 14, 2017 at 01:14:52AM +0900, Akinobu Mita wrote:
>>The fault-inject-make-fail-nth-read-write-interface-symmetric.patch in
>>-mm tree allows users to set task->fail_nth for non current task by procfs.
>>On the other hand, the current task's fail_nth is decreased to zero in
>>fault-injection path without any specific locks.
>>
>>So we need to prevent the task->fail_nth from being unexpected value by
>>data races (for example, setting task->fail_nth to zero while decreasing
>>the current->fail_nth).  In this fix, we use READ_ONCE() and WRITE_ONCE()
>>to prevent the compiler from creating unsolicited accesses.
>>
>>Cc: Dmitry Vyukov <dvyukov@google.com>
>>Reported-by: Dmitry Vyukov <dvyukov@google.com>
>>Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
>>---
>> fs/proc/base.c     | 5 +++--
>> lib/fault-inject.c | 7 +++++--
>> 2 files changed, 8 insertions(+), 4 deletions(-)
>>
>>diff --git a/fs/proc/base.c b/fs/proc/base.c
>>index ecc8a25..719c2e9 100644
>>--- a/fs/proc/base.c
>>+++ b/fs/proc/base.c
>>@@ -1370,7 +1370,7 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
>>       task = get_proc_task(file_inode(file));
>>       if (!task)
>>               return -ESRCH;
>>-      task->fail_nth = n;
>>+      WRITE_ONCE(task->fail_nth, n);
>>       put_task_struct(task);
>>
>>       return count;
>>@@ -1386,7 +1386,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
>>       task = get_proc_task(file_inode(file));
>>       if (!task)
>>               return -ESRCH;
>>-      len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
>>+      len = snprintf(numbuf, sizeof(numbuf), "%u\n",
>>+                      READ_ONCE(task->fail_nth));
>>       len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
>>       put_task_struct(task);
>>
>>diff --git a/lib/fault-inject.c b/lib/fault-inject.c
>>index 09ac73c1..7d315fd 100644
>>--- a/lib/fault-inject.c
>>+++ b/lib/fault-inject.c
>>@@ -107,9 +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)
>>+      if (in_task()) {
>>+              unsigned int fail_nth = READ_ONCE(current->fail_nth);
>>+
>>+              if (fail_nth && !WRITE_ONCE(current->fail_nth, fail_nth - 1))
>>                       goto fail;
>>+
>>               return false;
>>       }
>>
>>--
>>2.7.4
>>
>>
>>
> hi
>
> I'm a btrfs developer. I found that fail_make_request didn't produce the
> expected IO ERROR when running xfstests on linux 4.13-rc1.
>
> That testcase enable fail_make_request by the following commands:
> # echo 100 > /sys/kernel/debug/fail_make_request/probability
> # echo 2 > /sys/kernel/debug/fail_make_request/times
> # echo 0 > /sys/kernel/debug/fail_make_request/verbose
> # echo 1 > /sys/block/sda/sda1/make-it-fail
> # dd if=/dev/zero of=/dev/sda1 bs=128K count=1 oflag=direct
>
> As I understand it, after applying this patch, I have to write
> /proc/<dd pid>/file-nth firstly so that dd process can catch the IO ERROR.
> However, the dd process is so fast that I can't write file-nth.
>
> So, could you tell me how to produce IO ERROR under these circumstances?

Hi,

fail-nth is orthogonal to the existing mechanisms, so if you have a
setup that fails all sites with certain probability, that should
continue to work.

If you are writing a new facility and want to use fail-nth, then the
test process itself needs to cooperate and write fail-nth accordingly.
See the original patch for an example of how to do it:
https://groups.google.com/d/msg/syzkaller/DbB4rjYd82s/3MHDwtcqCAAJ

Re: [PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth

[Akinobu Mita]

2017-08-01 22:45 GMT+09:00 Dmitry Vyukov <dvyukov@google.com>:
> On Tue, Aug 1, 2017 at 3:09 PM, Lu Fengqi <lufq.fnst@cn.fujitsu.com> wrote:
>> On Fri, Jul 14, 2017 at 01:14:52AM +0900, Akinobu Mita wrote:
>>>The fault-inject-make-fail-nth-read-write-interface-symmetric.patch in
>>>-mm tree allows users to set task->fail_nth for non current task by procfs.
>>>On the other hand, the current task's fail_nth is decreased to zero in
>>>fault-injection path without any specific locks.
>>>
>>>So we need to prevent the task->fail_nth from being unexpected value by
>>>data races (for example, setting task->fail_nth to zero while decreasing
>>>the current->fail_nth).  In this fix, we use READ_ONCE() and WRITE_ONCE()
>>>to prevent the compiler from creating unsolicited accesses.
>>>
>>>Cc: Dmitry Vyukov <dvyukov@google.com>
>>>Reported-by: Dmitry Vyukov <dvyukov@google.com>
>>>Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
>>>---
>>> fs/proc/base.c     | 5 +++--
>>> lib/fault-inject.c | 7 +++++--
>>> 2 files changed, 8 insertions(+), 4 deletions(-)
>>>
>>>diff --git a/fs/proc/base.c b/fs/proc/base.c
>>>index ecc8a25..719c2e9 100644
>>>--- a/fs/proc/base.c
>>>+++ b/fs/proc/base.c
>>>@@ -1370,7 +1370,7 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
>>>       task = get_proc_task(file_inode(file));
>>>       if (!task)
>>>               return -ESRCH;
>>>-      task->fail_nth = n;
>>>+      WRITE_ONCE(task->fail_nth, n);
>>>       put_task_struct(task);
>>>
>>>       return count;
>>>@@ -1386,7 +1386,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
>>>       task = get_proc_task(file_inode(file));
>>>       if (!task)
>>>               return -ESRCH;
>>>-      len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
>>>+      len = snprintf(numbuf, sizeof(numbuf), "%u\n",
>>>+                      READ_ONCE(task->fail_nth));
>>>       len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
>>>       put_task_struct(task);
>>>
>>>diff --git a/lib/fault-inject.c b/lib/fault-inject.c
>>>index 09ac73c1..7d315fd 100644
>>>--- a/lib/fault-inject.c
>>>+++ b/lib/fault-inject.c
>>>@@ -107,9 +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)
>>>+      if (in_task()) {
>>>+              unsigned int fail_nth = READ_ONCE(current->fail_nth);
>>>+
>>>+              if (fail_nth && !WRITE_ONCE(current->fail_nth, fail_nth - 1))
>>>                       goto fail;
>>>+
>>>               return false;
>>>       }
>>>
>>>--
>>>2.7.4
>>>
>>>
>>>
>> hi
>>
>> I'm a btrfs developer. I found that fail_make_request didn't produce the
>> expected IO ERROR when running xfstests on linux 4.13-rc1.
>>
>> That testcase enable fail_make_request by the following commands:
>> # echo 100 > /sys/kernel/debug/fail_make_request/probability
>> # echo 2 > /sys/kernel/debug/fail_make_request/times
>> # echo 0 > /sys/kernel/debug/fail_make_request/verbose
>> # echo 1 > /sys/block/sda/sda1/make-it-fail
>> # dd if=/dev/zero of=/dev/sda1 bs=128K count=1 oflag=direct
>>
>> As I understand it, after applying this patch, I have to write
>> /proc/<dd pid>/file-nth firstly so that dd process can catch the IO ERROR.
>> However, the dd process is so fast that I can't write file-nth.
>>
>> So, could you tell me how to produce IO ERROR under these circumstances?
>
> Hi,
>
> fail-nth is orthogonal to the existing mechanisms, so if you have a
> setup that fails all sites with certain probability, that should
> continue to work.

Lu's setting for fail_make_request is fine before introducing systematic
fault injection and they want to inject fail_make_request only.

So I think we need a global parameter to turn on/off the systematic fault
injection.  (e.g. /sys/kernel/debug/systematic-fault-inject/enable)

> If you are writing a new facility and want to use fail-nth, then the
> test process itself needs to cooperate and write fail-nth accordingly.
> See the original patch for an example of how to do it:
> https://groups.google.com/d/msg/syzkaller/DbB4rjYd82s/3MHDwtcqCAAJ

Re: [PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth

[Akinobu Mita]

2017-08-02 0:54 GMT+09:00 Akinobu Mita <akinobu.mita@gmail.com>:
> 2017-08-01 22:45 GMT+09:00 Dmitry Vyukov <dvyukov@google.com>:
>> On Tue, Aug 1, 2017 at 3:09 PM, Lu Fengqi <lufq.fnst@cn.fujitsu.com> wrote:
>>> On Fri, Jul 14, 2017 at 01:14:52AM +0900, Akinobu Mita wrote:
>>>>The fault-inject-make-fail-nth-read-write-interface-symmetric.patch in
>>>>-mm tree allows users to set task->fail_nth for non current task by procfs.
>>>>On the other hand, the current task's fail_nth is decreased to zero in
>>>>fault-injection path without any specific locks.
>>>>
>>>>So we need to prevent the task->fail_nth from being unexpected value by
>>>>data races (for example, setting task->fail_nth to zero while decreasing
>>>>the current->fail_nth).  In this fix, we use READ_ONCE() and WRITE_ONCE()
>>>>to prevent the compiler from creating unsolicited accesses.
>>>>
>>>>Cc: Dmitry Vyukov <dvyukov@google.com>
>>>>Reported-by: Dmitry Vyukov <dvyukov@google.com>
>>>>Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
>>>>---
>>>> fs/proc/base.c     | 5 +++--
>>>> lib/fault-inject.c | 7 +++++--
>>>> 2 files changed, 8 insertions(+), 4 deletions(-)
>>>>
>>>>diff --git a/fs/proc/base.c b/fs/proc/base.c
>>>>index ecc8a25..719c2e9 100644
>>>>--- a/fs/proc/base.c
>>>>+++ b/fs/proc/base.c
>>>>@@ -1370,7 +1370,7 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
>>>>       task = get_proc_task(file_inode(file));
>>>>       if (!task)
>>>>               return -ESRCH;
>>>>-      task->fail_nth = n;
>>>>+      WRITE_ONCE(task->fail_nth, n);
>>>>       put_task_struct(task);
>>>>
>>>>       return count;
>>>>@@ -1386,7 +1386,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
>>>>       task = get_proc_task(file_inode(file));
>>>>       if (!task)
>>>>               return -ESRCH;
>>>>-      len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
>>>>+      len = snprintf(numbuf, sizeof(numbuf), "%u\n",
>>>>+                      READ_ONCE(task->fail_nth));
>>>>       len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
>>>>       put_task_struct(task);
>>>>
>>>>diff --git a/lib/fault-inject.c b/lib/fault-inject.c
>>>>index 09ac73c1..7d315fd 100644
>>>>--- a/lib/fault-inject.c
>>>>+++ b/lib/fault-inject.c
>>>>@@ -107,9 +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)
>>>>+      if (in_task()) {
>>>>+              unsigned int fail_nth = READ_ONCE(current->fail_nth);
>>>>+
>>>>+              if (fail_nth && !WRITE_ONCE(current->fail_nth, fail_nth - 1))
>>>>                       goto fail;
>>>>+
>>>>               return false;
>>>>       }
>>>>
>>>>--
>>>>2.7.4
>>>>
>>>>
>>>>
>>> hi
>>>
>>> I'm a btrfs developer. I found that fail_make_request didn't produce the
>>> expected IO ERROR when running xfstests on linux 4.13-rc1.
>>>
>>> That testcase enable fail_make_request by the following commands:
>>> # echo 100 > /sys/kernel/debug/fail_make_request/probability
>>> # echo 2 > /sys/kernel/debug/fail_make_request/times
>>> # echo 0 > /sys/kernel/debug/fail_make_request/verbose
>>> # echo 1 > /sys/block/sda/sda1/make-it-fail
>>> # dd if=/dev/zero of=/dev/sda1 bs=128K count=1 oflag=direct
>>>
>>> As I understand it, after applying this patch, I have to write
>>> /proc/<dd pid>/file-nth firstly so that dd process can catch the IO ERROR.
>>> However, the dd process is so fast that I can't write file-nth.
>>>
>>> So, could you tell me how to produce IO ERROR under these circumstances?
>>
>> Hi,
>>
>> fail-nth is orthogonal to the existing mechanisms, so if you have a
>> setup that fails all sites with certain probability, that should
>> continue to work.
>
> Lu's setting for fail_make_request is fine before introducing systematic
> fault injection and they want to inject fail_make_request only.
>
> So I think we need a global parameter to turn on/off the systematic fault
> injection.  (e.g. /sys/kernel/debug/systematic-fault-inject/enable)

Oops.  That is simply a bug in my patch.  Correct should_fail() is below.

bool should_fail(struct fault_attr *attr, ssize_t size)
{
        if (in_task()) {
                unsigned int fail_nth = READ_ONCE(current->fail_nth);

                if (fail_nth) {
                        if (!WRITE_ONCE(current->fail_nth, fail_nth - 1))
                                goto fail;

                        return false;
                }
        }
...


>> If you are writing a new facility and want to use fail-nth, then the
>> test process itself needs to cooperate and write fail-nth accordingly.
>> See the original patch for an example of how to do it:
>> https://groups.google.com/d/msg/syzkaller/DbB4rjYd82s/3MHDwtcqCAAJ