linux-ext4 - Re: regression: 4.13 cannot follow symlinks on some ext3 fs

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Date:   Fri, 24 Nov 2017 17:28:19 -0500
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Andreas Dilger <adilger@...ger.ca>,
        Andi Kleen <andi@...stfloor.org>
Cc:     Theodore Ts'o <tytso@....edu>, Tahsin Erdogan <tahsin@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: regression: 4.13 cannot follow symlinks on some ext3 fs

On Fri, 2017-11-24 at 15:03 -0700, Andreas Dilger wrote:
> On Nov 24, 2017, at 9:51 AM, Andi Kleen <andi@...stfloor.org> wrote:
> > 
> > 
> > > 
> > > We checked old kernels, and old e2fsprogs, and didn't see any
> > > cases
> > > where fast (<= 60 chars) symlinks were created using external
> > > blocks.
> > > It seems that _something_ did create them, and it would be good
> > > to
> > > figure that out so we can determine if it is a widespread problem
> > 
> > I assume it was the original kernel.
> > 
> > > 
> > > 
> > > I think e2fsck can fix this quite easily, and there really isn't
> > > an easy way to revert to the old method if the large xattr
> > > feature
> > > is enabled.  If you are willing to run a new kernel, you should
> > > also
> > > be willing to run a new e2fsck.
> > 
> > It's obviously not enabled on ext3.
> > 
> > > 
> > > We could probably add a fallback to the old mechanism (and print
> > > a one-time warning to upgrade to a newer e2fsck) if an external
> > > fast symlink is found and the large xattr  feature is not
> > > enabled, which would give more time to fix this (hopefully rare
> > > in the wild) case.
> > 
> > If the old kernel created it, then likely all the
> > /lib{,64}/ld-linux.so.2 symlinks have that, which breaks all ELF
> > executables. I suspect in these old file systems it's not
> > particularly rare.
> 
> Sure, but not many people are going to be running a 4.14 kernel with
> a 2007 system. 

I really disagree on this ... most of us who are doing kernel testing
will be running with older systems.  It's true, some of us do install
from scratch and then test, but most of us upgrade (which doesn't
necessarily modify the symlinks).  On your creation test, this is my
cloud system:

bedivere:~# dumpe2fs -h $(df -P / | awk '/dev/ { print $1 }') 2>&1 | grep created
Filesystem created:       Tue Mar 24 20:21:35 2009

Your find command turns up nothing untoward.

My older system is the home entertainment system, but that has an xfs
root dating back to 2005.

I bet I have a laptop even older (currently travelling, so can't
check).

James

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