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

[Ian Kent <raven@themaw.net>]

On Wed, 2008-06-11 at 16:47 +0400, Pavel Emelyanov wrote:
> Ian Kent wrote:
> > On Wed, 2008-06-11 at 13:54 +0400, Pavel Emelyanov wrote:
> >> From: Konstantin Khlebnikov <khlebnikov@openvz.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@openvz.org>
> >> Acked-by: Pavel Emelyanov <xemul@openvz.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)

Oh .. so maybe the answer to my question is yes?

What about other arches offer 32-bit within a 64-bit environment (has
this been the case on sparc64 at some point)?
What about the compiler padding on these?

Don't get me wrong, I'm not against fixing this, in fact I'd like to,
but I'm concerned it may end up a bit of a can of worms.

> >> +		/*
> >> +		 * 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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