linux-ext4 - Re: v4.7--v4.10+: ext4: repeatable inline-data oops (and fs corruption) caused by msync() of shared writable mmap (with recipe)

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Message-ID: <20170313005232.GA593@zzz>
Date:   Sun, 12 Mar 2017 17:52:32 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Nick Alcock <nick.alcock@...cle.com>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: v4.7--v4.10+: ext4: repeatable inline-data oops (and fs
 corruption) caused by msync() of shared writable mmap (with recipe)

On Wed, Mar 01, 2017 at 11:45:52AM +0000, Nick Alcock wrote:
> [Resend, after the first attempt, from my home address, failed with
>  endless greylisting followed by "4.5.0 Interactive router timed out"
>  from all but the lowest-priority MX for vger, and "Name server:
>  bl-ckh-le.kernel.org.: host not found" for the apparently-nonexistent
>  lowest-priority MX. Maybe it'll work better from here.]
> 
> I first spotted this -- or it spotted me -- back in the v4.7.x days. It
> is still present in v4.10.
> 
> Here's a replication recipe, given a reasonable rootfs with a compiler
> on it, and assuming a blank virtio disk on /dev/vdb:
> 

Hi Nick, thanks for reporting this.  I've sent a patch which should fix this,
and Cc'ed you.  This actually seems to been a bug for a very long time, maybe
even ever since the inline_data feature was introduced.  (I was able to
reproduce it in a 3.18 kernel, at least.)  I'm not sure why it didn't get
noticed earlier --- maybe hardly anyone ever writes to small files with mmap...

- Eric