linux-kernel - Re: compat: autofs v5 packet size ambiguity

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Date:	Sat, 25 Feb 2012 12:28:21 +0100
From:	Thomas Meyer <thomas@...3r.de>
To:	Ian Kent <raven@...maw.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	autofs@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: compat: autofs v5 packet size ambiguity - update

Am Mittwoch, den 22.02.2012, 13:57 +0800 schrieb Ian Kent:
> On Wed, 2012-02-22 at 13:53 +0800, Ian Kent wrote:
> > On Wed, 2012-02-22 at 13:43 +0800, Ian Kent wrote:
> > > On Tue, 2012-02-21 at 20:56 -0800, Linus Torvalds wrote:
> > 
> > Ahh ... forgot to set the file_operations structure member .. oops
> > 
> > >  
> > > +static int autofs4_root_dir_open(struct inode *inode, struct file *file)
> > > +{
> > > +	struct autofs_sb_info *sbi= autofs4_sbi(file->f_path.dentry->d_sb);
> > > +	if (sbi->compat_daemon < 0)
> > > +		sbi->compat_daemon = is_compat_task();
> > > +	return dcache_dir_open(inode, file);
> > > +}
> > > +
> > 
> 
> Lets try that again.
> 
> autofs: work around unhappy compat problem on x86-64
> 
> From: Ian Kent <raven@...maw.net>
> 
> When the autofs protocol version 5 packet type was added in commit
> 5c0a32fc2cd0 ("autofs4: add new packet type for v5 communications"), it
> obvously tried quite hard to be word-size agnostic, and uses explicitly
> sized fields that are all correctly aligned.
> 
> However, with the final "char name[NAME_MAX+1]" array at the end, the
> actual size of the structure ends up being not very well defined:
> because the struct isn't marked 'packed', doing a "sizeof()" on it will
> align the size of the struct up to the biggest alignment of the members
> it has.
> 
> And despite all the members being the same, the alignment of them is
> different: a "__u64" has 4-byte alignment on x86-32, but native 8-byte
> alignment on x86-64.  And while 'NAME_MAX+1' ends up being a nice round
> number (256), the name[] array starts out a 4-byte aligned.
> 
> End result: the "packed" size of the structure is 300 bytes: 4-byte, but
> not 8-byte aligned.
> 
> As a result, despite all the fields being in the same place on all
> architectures, sizeof() will round up that size to 304 bytes on
> architectures that have 8-byte alignment for u64.
> 
> Note that this is *not* a problem for 32-bit compat mode on POWER, since
> there __u64 is 8-byte aligned even in 32-bit mode.  But on x86, 32-bit
> and 64-bit alignment is different for 64-bit entities, and as a result
> the structure that has exactly the same layout has different sizes.
> 
> So on x86-64, but no other architecture, we will just subtract 4 from
> the size of the structure when running in a compat task.  That way we
> will write the properly sized packet that user mode expects.
> 
> Not pretty.  Sadly, this very subtle, and unnecessary, size difference
> has been encoded in user space that wants to read packets of *exactly*
> the right size, and will refuse to touch anything else.
> 
> Reported-and-tested-by: Thomas Meyer <thomas@...3r.de>
> Cc: Ian Kent <raven@...maw.net>
> ---

works for me on top of 3.2.7.


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