linux-kernel - Re: [PATCH] Force same size of struct autofs_v5_packet on x86 and x86

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Date:	Mon, 19 Sep 2011 11:30:42 +0200
From:	Thomas Meyer <thomas@...3r.de>
To:	Al Viro <viro@...IV.linux.org.uk>, Ian Kent <raven@...maw.net>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dhowells@...hat.com
Subject: Re: [PATCH] Force same size of struct autofs_v5_packet on x86 and x86_64


Am 19.09.2011 um 08:10 schrieb Al Viro:

> On Mon, Sep 19, 2011 at 11:52:48AM +0800, Ian Kent wrote:
> 
>>> btw. where are the padding bytes are added? how to tell gcc to display
>>> this?
>> 
>> It's also been a long time since I looked into this so I don't remember
>> the processor alignment details.
>> 
>> Not sure if there is a compiler option that would report this, sorry,
>> anyone else?
> 
> Padding is added in the very end.  The thing is, it's not a misaligned
> field per se; they all have a natural alignment in there.  It's that
> __u64 ino has alignment 8 on amd64 and alignment 4 on i386.  So the alignment
> requirements for the entire structure are different - on amd64 the address
> of the entire thing must be a multiple of 8 and on i386 it can be any multiple
> of 4.  IOW, on i386 you can just put them one after another in e.g. an array
> (the size without any padding is 300 bytes) and on amd64 you need 4 bytes
> of padding added between them.  I.e. sizeof needs to go up by 4, with the
> padding added in the end.

okay. that's what I thought.

@Ian: Is the read() from the pipe fd the only way to consume this structure from userspace?

if so: why not align the size of this structure for x86 and x86_64 to the same size?

the systemd compiled for x86_64 will probably work with this change, but does maybe report something like a short read or so, as it currently expects 304 bytes to read.
i didn't test this yet.

@al: what other autofs4 users are out there besides systemd?

with kind regards
thomas--
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