linux-kernel - Re: [PATCH] autofs4: fix 32-bit userspace vs 64-bit kernel communications

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Date:	Wed, 11 Jun 2008 16:47:10 +0400
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Ian Kent <raven@...maw.net>
CC:	autofs mailing list <autofs@...ux.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Subject: Re: [PATCH] autofs4: fix 32-bit userspace vs 64-bit kernel	communications

Ian Kent wrote:
> On Wed, 2008-06-11 at 13:54 +0400, Pavel Emelyanov wrote:
>> From: Konstantin Khlebnikov <khlebnikov@...nvz.org>
>>
>> The struct autofs_v5_packet has only one difference between
>> 32-bit and 64-bit versions - on 64-bit gcc aligns its size and
>> it is 4 bytes larger that it is on 32-bit kernel. This confuses
>> 32-bit user-space daemon, when talking to 64-bit kernel.
> 
> No, I don't think that's quite right.
> 
> As far as I know this issue arises when a user space program, compiled
> as a 32-bit application is executed within 64-bit user space.

What program do mean here? The problem arises right on the kernel-daemon
boundary - the latter refuses to accept the message with larger length.
No other software required.

>> This is very critical for containerized setups, when containers
>> with <different>-bit tolls are used.
> 
> Have you tested different situations with this change?
> Will this affect the existing check and adjustment the version 5
> automount daemon does now?

64-bit daemons *still* work OK and 32-bit *start* to after this fix :)

>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>
>> Acked-by: Pavel Emelyanov <xemul@...nvz.org>
>>
>> ---
>>
>> diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
>> index 1e4a539..9855b6e 100644
>> --- a/fs/autofs4/waitq.c
>> +++ b/fs/autofs4/waitq.c
>> @@ -132,6 +132,14 @@ static void autofs4_notify_daemon(struct autofs_sb_info *sbi,
>>  		struct autofs_v5_packet *packet = &pkt.v5_pkt.v5_packet;
>>  
>>  		pktsz = sizeof(*packet);
>> +#if defined(CONFIG_X86_64) && defined(CONFIG_IA32_EMULATION)
>> +		/*
>> +		 * On x86_64 autofs_v5_packet struct is padded with 4 bytes
>> +		 * which breaks 32-bit autofs daemon.
>> +		 */
>> +		if (test_thread_flag(TIF_IA32))
>> +			pktsz -= 4;
>> +#endif
>>  
>>  		packet->wait_queue_token = wq->wait_queue_token;
>>  		packet->len = wq->len;
>>
> 
> --
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