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Subject: Re: linux-next: umh: fix processed error when UMH_WAIT_PROC is used
 seems to break linux bridge on s390x (bisected)
From:   Christian Borntraeger <borntraeger@de.ibm.com>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     Christoph Hellwig <hch@infradead.org>, ast@kernel.org,
        axboe@kernel.dk, bfields@fieldses.org,
        bridge@lists.linux-foundation.org, chainsaw@gentoo.org,
        christian.brauner@ubuntu.com, chuck.lever@oracle.com,
        davem@davemloft.net, dhowells@redhat.com,
        gregkh@linuxfoundation.org, jarkko.sakkinen@linux.intel.com,
        jmorris@namei.org, josh@joshtriplett.org, keescook@chromium.org,
        keyrings@vger.kernel.org, kuba@kernel.org,
        lars.ellenberg@linbit.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-security-module@vger.kernel.org, nikolay@cumulusnetworks.com,
        philipp.reisner@linbit.com, ravenexp@gmail.com,
        roopa@cumulusnetworks.com, serge@hallyn.com, slyfox@gentoo.org,
        viro@zeniv.linux.org.uk, yangtiezhu@loongson.cn,
        netdev@vger.kernel.org, markward@linux.ibm.com,
        linux-s390 <linux-s390@vger.kernel.org>
References: <20200610154923.27510-5-mcgrof@kernel.org>
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 <20200624160953.GH4332@42.do-not-panic.com>
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Message-ID: <4d8fbcea-a892-3453-091f-d57c03f9aa90@de.ibm.com>
Date:   Wed, 24 Jun 2020 20:32:12 +0200
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On 24.06.20 20:09, Christian Borntraeger wrote:
> 
> 
> On 24.06.20 19:58, Christian Borntraeger wrote:
>>
>>
>> On 24.06.20 18:09, Luis Chamberlain wrote:
>>> On Wed, Jun 24, 2020 at 05:54:46PM +0200, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 24.06.20 16:43, Christoph Hellwig wrote:
>>>>> On Wed, Jun 24, 2020 at 01:11:54PM +0200, Christian Borntraeger wrote:
>>>>>> Does anyone have an idea why "umh: fix processed error when UMH_WAIT_PROC is used" breaks the
>>>>>> linux-bridge on s390?
>>>>>
>>>>> Are we even sure this is s390 specific and doesn't happen on other
>>>>> architectures with the same bridge setup?
>>>>
>>>> Fair point. AFAIK nobody has tested this yet on x86.
>>>
>>> Regardless, can you enable dynamic debug prints, to see if the kernel
>>> reveals anything on the bridge code which may be relevant:
>>>
>>> echo "file net/bridge/* +p" > /sys/kernel/debug/dynamic_debug/control
>>>
>>>   Luis
>>
>> When I start a guest the following happens with the patch:
>>
>> [   47.420237] virbr0: port 2(vnet0) entered blocking state
>> [   47.420242] virbr0: port 2(vnet0) entered disabled state
>> [   47.420315] device vnet0 entered promiscuous mode
>> [   47.420365] virbr0: port 2(vnet0) event 16
>> [   47.420366] virbr0: br_fill_info event 16 port vnet0 master virbr0
>> [   47.420373] virbr0: toggle option: 12 state: 0 -> 0
>> [   47.420536] virbr0: port 2(vnet0) entered blocking state
>> [   47.420538] virbr0: port 2(vnet0) event 16
>> [   47.420539] virbr0: br_fill_info event 16 port vnet0 master virbr0
>>
>> and the nothing happens.
>>
>>
>> without the patch
>> [   33.805410] virbr0: hello timer expired
>> [   35.805413] virbr0: hello timer expired
>> [   36.184349] virbr0: port 2(vnet0) entered blocking state
>> [   36.184353] virbr0: port 2(vnet0) entered disabled state
>> [   36.184427] device vnet0 entered promiscuous mode
>> [   36.184479] virbr0: port 2(vnet0) event 16
>> [   36.184480] virbr0: br_fill_info event 16 port vnet0 master virbr0
>> [   36.184487] virbr0: toggle option: 12 state: 0 -> 0
>> [   36.184636] virbr0: port 2(vnet0) entered blocking state
>> [   36.184638] virbr0: port 2(vnet0) entered listening state
>> [   36.184639] virbr0: port 2(vnet0) event 16
>> [   36.184640] virbr0: br_fill_info event 16 port vnet0 master virbr0
>> [   36.184645] virbr0: port 2(vnet0) event 16
>> [   36.184646] virbr0: br_fill_info event 16 port vnet0 master virbr0
>> [   37.805478] virbr0: hello timer expired
>> [   38.205413] virbr0: port 2(vnet0) forward delay timer
>> [   38.205414] virbr0: port 2(vnet0) entered learning state
>> [   38.205427] virbr0: port 2(vnet0) event 16
>> [   38.205430] virbr0: br_fill_info event 16 port vnet0 master virbr0
>> [   38.765414] virbr0: port 2(vnet0) hold timer expired
>> [   39.805415] virbr0: hello timer expired
>> [   40.285410] virbr0: port 2(vnet0) forward delay timer
>> [   40.285411] virbr0: port 2(vnet0) entered forwarding state
>> [   40.285418] virbr0: topology change detected, propagating
>> [   40.285420] virbr0: decreasing ageing time to 400
>> [   40.285427] virbr0: port 2(vnet0) event 16
>> [   40.285432] virbr0: br_fill_info event 16 port vnet0 master virbr0
>> [   40.765408] virbr0: port 2(vnet0) hold timer expired
>> [   41.805415] virbr0: hello timer expired
>> [   42.765426] virbr0: port 2(vnet0) hold timer expired
>> [   43.805425] virbr0: hello timer expired
>> [   44.765426] virbr0: port 2(vnet0) hold timer expired
>> [   45.805418] virbr0: hello timer expired
>>
>> and continuing....
> 
> Just reverting the umh.c parts like this makes the problem go away.
> 
> diff --git a/kernel/umh.c b/kernel/umh.c
> index f81e8698e36e..79f139a7ca03 100644
> --- a/kernel/umh.c
> +++ b/kernel/umh.c
> @@ -154,8 +154,8 @@ static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info)
>                  * the real error code is already in sub_info->retval or
>                  * sub_info->retval is 0 anyway, so don't mess with it then.
>                  */
> -               if (KWIFEXITED(ret))
> -                       sub_info->retval = KWEXITSTATUS(ret);
> +               if (ret)
> +                       sub_info->retval = ret;
>         }
>  
>         /* Restore default kernel sig handler */

I instrumented this:

[    5.118528] ret=0x100 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x1 
[    9.409235] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   10.114914] ret=0x100 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x1 
[   10.116308] ret=0x100 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x1 
[   10.117690] ret=0x100 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x1 
[   10.118732] ret=0x100 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x1 
[   10.119899] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   10.121365] ret=0x100 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x1 
[   10.128044] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   10.945814] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   10.962799] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   10.983755] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   10.992705] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.118877] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.124359] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.129364] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.142139] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.148385] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.153686] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.158861] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.164068] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.192445] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.200605] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.208493] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.216612] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.226467] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.525363] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.532758] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.535279] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.697660] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.701634] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.818652] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   11.836228] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   12.082266] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 
[   40.965898] ret=0x0 KWIFEXITED(ret) = 0x1 KWEXITSTATUS(ret)= 0x0 

So the translations look correct. But your change is actually a sematic change
if(ret) will only trigger if there is an error
if (KWIFEXITED(ret)) will always trigger when the process ends. So we will always overwrite -ECHILD
and we did not do it before.