Re: [Qemu-block] [PATCH v3] block/file-posix: do not fail on unlock bytes

[Vladimir Sementsov-Ogievskiy]

29.03.2019 22:32, Kevin Wolf wrote:
> Am 29.03.2019 um 19:00 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> 29.03.2019 20:58, Vladimir Sementsov-Ogievskiy wrote:
>>> 29.03.2019 20:44, Max Reitz wrote:
>>>> On 29.03.19 18:40, Kevin Wolf wrote:
>>>>> Am 29.03.2019 um 18:30 hat Max Reitz geschrieben:
>>>>>> On 29.03.19 18:24, Kevin Wolf wrote:
>>>>>>> Am 29.03.2019 um 18:15 hat Max Reitz geschrieben:
>>>>>>>> On 29.03.19 12:04, Vladimir Sementsov-Ogievskiy wrote:
>>>>>>>>> bdrv_replace_child() calls bdrv_check_perm() with error_abort on
>>>>>>>>> loosening permissions. However file-locking operations may fail even
>>>>>>>>> in this case, for example on NFS. And this leads to Qemu crash.
>>>>>>>>>
>>>>>>>>> Let's avoid such errors. Note, that we ignore such things anyway on
>>>>>>>>> permission update commit and abort.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>>>>>>>> ---
>>>>>>>>>    block/file-posix.c | 12 ++++++++++++
>>>>>>>>>    1 file changed, 12 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/block/file-posix.c b/block/file-posix.c
>>>>>>>>> index db4cccbe51..1cf4ee49eb 100644
>>>>>>>>> --- a/block/file-posix.c
>>>>>>>>> +++ b/block/file-posix.c
>>>>>>>>> @@ -815,6 +815,18 @@ static int raw_handle_perm_lock(BlockDriverState 
>>>>>>>>> *bs,
>>>>>>>>>        switch (op) {
>>>>>>>>>        case RAW_PL_PREPARE:
>>>>>>>>> +        if ((s->perm | new_perm) == s->perm &&
>>>>>>>>> +            (s->shared_perm & new_shared) == s->shared_perm)
>>>>>>>>> +        {
>>>>>>>>> +            /*
>>>>>>>>> +             * We are going to unlock bytes, it should not fail. If 
>>>>>>>>> it fail due
>>>>>>>>> +             * to some fs-dependent permission-unrelated reasons 
>>>>>>>>> (which occurs
>>>>>>>>> +             * sometimes on NFS and leads to abort in 
>>>>>>>>> bdrv_replace_child) we
>>>>>>>>> +             * can't prevent such errors by any check here. And we 
>>>>>>>>> ignore them
>>>>>>>>> +             * anyway in ABORT and COMMIT.
>>>>>>>>> +             */
>>>>>>>>> +            return 0;
>>>>>>>>> +        }
>>>>>>>>>            ret = raw_apply_lock_bytes(s, s->fd, s->perm | new_perm,
>>>>>>>>>                                       ~s->shared_perm | ~new_shared,
>>>>>>>>>                                       false, errp);
>>>>>>>>
>>>>>>>> Help me understand the exact issue, please.  I understand that there 
>>>>>>>> are
>>>>>>>> operations like bdrv_replace_child() that pass &error_abort to
>>>>>>>> bdrv_check_perm() because they just loosen the permissions, so it 
>>>>>>>> should
>>>>>>>> not fail.
>>>>>>>>
>>>>>>>> However, if the whole effect really would be to loosen permissions,
>>>>>>>> raw_apply_lock_bytes() wouldn't have failed here in PREPARE anyway:
>>>>>>>> @unlock is passed as false, so no bytes will be unlocked.  And if
>>>>>>>> permissions are just loosened (as your condition checks), it should not
>>>>>>>> lock any bytes.
>>>>>>>>
>>>>>>>> So why does it attempt lock any bytes in the first place?  There must 
>>>>>>>> be
>>>>>>>> some discrepancy between s->perm and s->locked_perm, or ~s->shared_perm
>>>>>>>> and s->locked_shared_perm.  How does that occur?
>>>>>>>
>>>>>>> I suppose raw_check_lock_bytes() is what is failing, not
>>>>>>> raw_apply_lock_bytes().
>>>>>>
>>>>>> Hm, maybe in Vladimir's case, but not in e.g.
>>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1652572 .
>>>>>
>>>>> This is reported against 3.0, which didn't avoid re-locking permissions
>>>>> that we already hold, so there raw_apply_lock_bytes() can still fail.
>>>>
>>>> That makes sense.  Which leaves the question why Vladimir still seems to
>>>> see the error there...?
>>>>
>>>
>>> I'm sorry :(. I'm trying to fix bug based on 2.10, and now I see that is 
>>> already fixed
>>>    upstream. I don't have a reproducer, only old coredumps.
>>>
>>> So, now it looks like we don't need this patch, as on permission loosening 
>>> file-posix
>>> don't call any FS apis, yes?
>>>
>>
>>
>> Ah, you mentioned, that raw_check_lock_bytes is still buggy.
> 
> I haven't tried it out, but from looking at the code it seems so. Maybe
> you can reproduce on master just to be sure?
> 

I don't have a reproducer :(

-- 
Best regards,
Vladimir