linux-ext4 - Re: Regular ext4 error warning with HD in USB dock

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Date:	Sun, 9 Jan 2011 09:58:38 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Rogier Wolff <R.E.Wolff@...wizard.nl>
Cc:	Amir Goldstein <amir73il@...il.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	Con Kolivas <kernel@...ivas.org>, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org
Subject: Re: Regular ext4 error warning with HD in USB dock

On Sun, Jan 09, 2011 at 09:12:49AM +0100, Rogier Wolff wrote:
> > No.  The superblock nor its offset will never change.  It's like the
> > syscall ABI, only worse.  If we changed it would break *everybody*.
> > Fortunately there is a huge amount of space left over in the 1024 byte
> > superblock.
> 
> It's called defensive programming. It prevents bugs before they
> happen. By your reasoning you could've written 2048 or 0x800 there.

Defensive programming would be something like

	  BUG_ON(sizeof(struct ext4_super_block) != 1024);

(unfortunately #error sizeof(struct ext4_super_block) != 1024 won't
work since #error is handled by the preprocessor, and I don't think we
can trigger a compile-time warning for a structure size issue).

We could add that, if people like.  I do have regression tests (i.e.,
boot a system with ext4) which would die if anything like that
changed, though.

And yes, I have similar regression tests in e2fsprogs that would
trigger if the superblock size were to ever change.

	       		       	       - Ted

P.S.  The only way I can think of to do it at compile time would be to
build a test .o file with -g, and then use a program like pahole that
pulls the information out of the DWARF information.  Might actually be
a good thing to do that, since it could also be useful for automating
searches for unoptimize structures.  Unfortunately, many developers
don't have the DWARF utilities installed, so that would add a
dependency on the kernel build.

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