linux-ext4 - Re: RFC, 32-bit compat handlers for EXT4_IOC_GROUP

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Date:	Fri, 05 Dec 2008 18:02:04 -0700
From:	Andreas Dilger <adilger@....com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: RFC, 32-bit compat handlers for EXT4_IOC_GROUP_ADD

On Dec 04, 2008  16:37 -0600, Eric Sandeen wrote:
> It's too bad this wasn't caught sooner, but I wonder if maybe it's still
> not too late; can we change the structure in the kernel and have newer
> e2fsprogs try both the old & new?  The new interface would add 32 bits
> of padding to the structure; this would leave it unchanged on 64-bit
> boxes so everything would continue to work.  Newer e2fsprogs would try
> both, so still work on older kernels.  32-bit compat would have a simple
> handler, *but* this *would* break resize of ext4 on native 32-bit
> machines with older e2fsprogs (the kernel would have a padded struct;
> older userspace would not, and the ioctl would fail).
> 
> How far out of "dev" are we?  I'm leaning towards saying "oh well, would
> have been nicer the other way" but going ahead and just putting the
> compat handler into the kernel.

I would be OK with changing to the "proper" struct layout.  Not being able
to resize with an older e2fsprogs + newer kernel isn't going to cause any
serious problems (unlike e.g. not being able to mount or e2fsck "/").

If we are seriously worried about compatibility, we could add the compat
handler for 32-bit kernels (should have a different IOC number anyways
because of the struct size) and add some arbitrary check like:

#ifdef LINUX_KERNEL_VERSION > KERNEL_VERSION(2,6,40)
#warning remove this old compat code
#endif

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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