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Date:   Thu, 12 Aug 2021 08:05:32 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] mke2fs: warn about missing y2038 support when formatting
 fresh ext4 fs

On Wed, Aug 11, 2021 at 09:25:18PM -0400, Theodore Ts'o wrote:
> On Wed, Aug 11, 2021 at 04:32:53PM -0700, Darrick J. Wong wrote:
> > +/*
> > + * Returns true if the user is forcing an old format (e.g. ext2, ext3).
> > + *
> > + * If there is no fs_types list, the user invoked us with no explicit type and
> > + * gets the default (ext4) format.  If we find the latest format (ext4) in the
> > + * type list, some combination of program name and -T argument put us in ext4
> > + * mode.  Anything else (ext2, ext3, hurd) and we return false.
> > + */
> 
> So that's not actually quite right.  Even if the user has no explicit
> type, mke2fs will assign a default type --- and it's not necessarily
> ext4.  You can see what the contents of the fs_types list using the -v
> option:

Ok, fair, I'll fix the message.  "If the user invoked us with no
explicit type, mke2fs adds some variant of ext* to the type list by
default."

> % /bin/rm /tmp/foo.img ; mke2fs -vq /tmp/foo.img 8m
> fs_types for mke2fs.conf resolution: 'ext2', 'small'
> % /bin/rm /tmp/foo.img ; mke2fs -vq -T news /tmp/foo.img 8m
> fs_types for mke2fs.conf resolution: 'ext2', 'news'
> % /bin/rm /tmp/foo.img ; mkfs.ext4 -vq /tmp/foo.img 8m
> fs_types for mke2fs.conf resolution: 'ext4', 'small'
> % /bin/rm /tmp/foo.img ; mkfs.ext4 -T huge -vq /tmp/foo.img 8m
> fs_types for mke2fs.conf resolution: 'ext4', 'huge'
> % /bin/rm /tmp/foo.img ; mkfs.ext4 -o hurd -vq /tmp/foo.img 8m
> fs_types for mke2fs.conf resolution: 'ext2', 'small', 'hurd'
> 
> Also note that the ext2/ext3/ext4 fs_type will always be in
> fs_types[0], so it's not necessary to search the entire list, as the
> patch is currently doing:

But that's not quite right either:

# mke2fs -t small -T ext4 -vqn a.img
fs_types for mke2fs.conf resolution: 'small', 'ext4'

So to find the 'ext4' here, you /do/ have to iterate the whole list.

--D

> 
> > +	for (i = 0; fs_types[i]; i++)
> > +		if (!strcmp(fs_types[i], "ext4"))
> > +			found_ext4 = 1;
> 
> 
> Cheers,
> 
> 						- Ted
> 
> P.S.  Although I'm not aware of anyone actually doing this, if there
> mke2fs is installed as mke3fs or mke4fs, that's the equivalent of
> mkfs.ext3 and mkfs.ext4.  (See the logic in the parse_fs_type
> function.)  Although perhaps there is some obscure distro somewhere
> out there that I don't know about....

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