status of bugzilla #99171 - mdraid broken for O_DIRECT

[Roland <devzero@web.de>]

sorry, resend in text format as mail contained html and bounced from ML.


Am 14.10.25 um 08:31 schrieb Hannes Reinecke:
> Hmm. I still would argue that the testcase quoted is invalid.
>
> What you do is to issue writes of a given buffer, while at the
> same time modifying the contents of that buffer.
>
> As we're doing zerocopy with O_DIRECT the buffer passed to pwrite
> is _the same_ buffer used when issuing the write to disk. The
> block layer now assumes that the buffer will _not_ be modified
> when writing to disk (ie between issuing 'pwrite' and the resulting
> request being send to disk).
> But that's not the case here; it will be modified, and consequently
> all sorts of issues will pop up.
> We have had all sorts of fun some years back with this issue until
> we fixed up all filesystems to do this correctly; if interested
> dig up the threads regarding 'stable pages' on linux-fsdevel.
>
> I would think you will end up with a corrupted filesystem if you
> run this without mdraid by just using btrfs with data checksumming.
>
yes, it's correct. you also end up with corrupted btrfs with this tool, 
see https://bugzilla.kernel.org/show_bug.cgi?id=99171#c16

> So really I'm not sure how to go from here; I would declare this
> as invalid, but what do I know ...
>
> Cheers,
>
> Hannes 

anyhow, i don't see why this testcase is invalid, especially when zfs 
seems not to be affected.

please look at this issue from a security perspective.

if you can break or corrupt your raid mirror from userspace even from an 
insulated layer/environment, i would better consider this "testcase" to 
be "malicious code" , which is able to subvert the 
virtualization/block/fs layer stack.

how could we prevent, that non-trused users in a vm or container 
environment can execute this "invalid" code ?


how can we prevent, that they do harm on the underlying mirror in a 
hosting environment for example ?

not using it in a hosting environment is a little bit weird strategy for 
a linux basic technoligy which exists for years.

and let it up to the hoster to remember he needs to disable direct-io 
for the hypervisor - is dissatisfying and error-prone, too.

roland