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Date:   Thu, 21 Nov 2019 22:56:54 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 2/2] ext4: simulate various I/O and checksum errors when
 reading metadata

On Thu, Nov 21, 2019 at 05:18:34PM -0800, Darrick J. Wong wrote:
> > So in theory, we could do that with dm_flakey --- but that's a pain in
> > the tuckus, since you have to specify the LBA for the directory blocks
> > that you might want to have fail.
> 
> Funny, I've been working on a fstests helper function to make it easy to
> set up dm-flakey based on fiemap/getfsmap output and such. :)

Ah, excellent, I'm looking forward to seeing it.  :-)

> > What might be interesting to do is some kind of eBPF hook where we
> > pass in the block #, inode #, and metadata type, and the ePBF program
> > could do use a much more complex set of criteria in terms of whether
> > or not to trigger an EIO, or how to fuzz a particular block to either
> > force a CRC failure, or to try to find bugs ala Hydra[1] (funded via a
> > Google Faculty Research Award grant), but using a much more glass-box
> > style test approach.
> 
> That would be fun.  Attach an arbitrary eBPF program to a range of
> sectors.  I wonder how loud the howls of protest would be for "can we
> let ebpf programs scribble on a kernel io buffer pleeze?"...

Well, because the eBPF hook would include the metadata type (an
allocation bitmap vs inode table vs htree index vs htree leaf block,
etc.) what I was thinking about has to be done in ext4 as a
ext4-specific hook.  And it would only be enabled if CONFIG_EXT4_DEBUG
or a separate Kconfig option is enabled --- and I *hope* no
distribution would be so silly/stupid enough to enable it on a product
kernel build.  :-)

						- Ted

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