Re: [Qemu-devel] [RFC] qid path collision issues in 9pfs

[Antonios Motakis]

On 13-Jan-18 00:14, Greg Kurz wrote:
> On Fri, 12 Jan 2018 19:32:10 +0800
> Antonios Motakis <address@hidden> wrote:
> 
>> Hello all,
>>
> 
> Hi Antonios,
> 
> I see you have attached a patch to this email... this really isn't the 
> preferred
> way to do things since it prevents to comment the patch (at least with my mail
> client). The appropriate way would have been to send the patch with a cover
> letter, using git-send-email for example.

I apologize for attaching the patch, I should have known better!

> 
>> We have found an issue in the 9p implementation of QEMU, with how qid paths 
>> are generated, which can cause qid path collisions and several issues caused 
>> by them. In our use case (running containers under VMs) these have proven to 
>> be critical.
>>
> 
> Ouch...
> 
>> In particular, stat_to_qid in hw/9pfs/9p.c generates a qid path using the 
>> inode number of the file as input. According to the 9p spec the path should 
>> be able to uniquely identify a file, distinct files should not share a path 
>> value.
>>
>> The current implementation that defines qid.path = inode nr works fine as 
>> long as there are not files from multiple partitions visible under the 9p 
>> share. In that case, distinct files from different devices are allowed to 
>> have the same inode number. So with multiple partitions, we have a very high 
>> probability of qid path collisions.
>>
>> How to demonstrate the issue:
>> 1) Prepare a problematic share:
>>  - mount one partition under share/p1/ with some files inside
>>  - mount another one *with identical contents* under share/p2/
>>  - confirm that both partitions have files with same inode nr, size, etc
>> 2) Demonstrate breakage:
>>  - start a VM with a virtio-9p pointing to the share
>>  - mount 9p share with FSCACHE on
>>  - keep open share/p1/file
>>  - open and write to share/p2/file
>>
>> What should happen is, the guest will consider share/p1/file and 
>> share/p2/file to be the same file, and since we are using the cache it will 
>> not reopen it. We intended to write to partition 2, but we just wrote to 
>> partition 1. This is just one example on how the guest may rely on qid paths 
>> being unique.
>>
>> In the use case of containers where we commonly have a few containers per 
>> VM, all based on similar images, these kind of qid path collisions are very 
>> common and they seem to cause all kinds of funny behavior (sometimes very 
>> subtle).
>>
>> To avoid this situation, the device id of a file needs to be also taken as 
>> input for generating a qid path. Unfortunately, the size of both inode nr + 
>> device id together would be 96 bits, while we have only 64 bits for the qid 
>> path, so we can't just append them and call it a day :(
>>
>> We have thought of a few approaches, but we would definitely like to hear 
>> what the upstream maintainers and community think:
>>
>> * Full fix: Change the 9p protocol
>>
>> We would need to support a longer qid path, based on a virtio feature flag. 
>> This would take reworking of host and guest parts of virtio-9p, so both QEMU 
>> and Linux for most users.
>>
> 
> I agree for a longer qid path, but we shouldn't tie it to a virtio flag since
> 9p is transport agnostic. And it happens to be used with a variety of 
> transports.
> QEMU has both virtio-9p and a Xen backend for example.
> 
>> * Fallback and/or interim solutions
>>
>> A virtio feature flag may be refused by the guest, so we think we still need 
>> to make collisions less likely even with 64 bit paths. E.g.
> 
> In all cases, we would need a fallback solution to support current
> guest setups. Also there are several 9p server implementations out
> there (ganesha, diod, kvmtool) that are currently used with linux
> clients... it will take some time to get everyone in sync :-\
> 
>> 1. XOR the device id with inode nr to produce the qid path (we attach a 
>> proof of concept patch)
> 
> Hmm... this would still allow collisions. Not good for fallback.
> 
>> 2. 64 bit hash of device id and inode nr
> 
> Same here.
> 
>> 3. other ideas, such as allocating new qid paths and keep a look up table of 
>> some sorts (possibly too expensive)
>>
> 
> This would be acceptable for fallback.

Maybe we can use the QEMU hash table 
(https://github.com/qemu/qemu/blob/master/util/qht.c) but I wonder if it scales 
to millions of entries. Do you think it is appropriate in this case?

I was thinking on how to implement something like this, without having to 
maintain millions of entries... One option we could consider is to split the 
bits into a directly-mapped part, and a lookup part. For example:

Inode =
[ 10 first bits ] + [ 54 lowest bits ]

A hash table maps [ inode 10 first bits ] + [ device id ] => [ 10 bit prefix ]
The prefix is uniquely allocated for each input.

Qid path = 
[ 10 bit prefix ] + [ inode 54 lowest bits ]

Since inodes are not completely random, and we usually have a handful of device 
IDs, we get a much smaller number of entries to track in the hash table.

So what this would give:
(1)     Would be faster and take less memory than mapping the full 
inode_nr,devi_id tuple to unique QID paths
(2)     Guaranteed not to run out of bits when inode numbers stay below the 
lowest 54 bits and we have less than 1024 devices.
(3)     When we get beyond this this limit, there is a chance we run out of 
bits to allocate new QID paths, but we can detect this and refuse to serve the 
offending files instead of allowing a collision.

We could tweak the prefix size to match the scenarios that we consider more 
likely, but I think close to 10-16 bits sounds reasonable enough. What do you 
think?

> 
>> With our proof of concept patch, the issues caused by qid path collisions go 
>> away, so it can be seen as an interim solution of sorts. However, the chance 
>> of collisions is not eliminated, we are just replacing the current strategy, 
>> which is almost guaranteed to cause collisions in certain use cases, with 
>> one that makes them more rare. We think that a virtio feature flag for 
>> longer qid paths is the only way to eliminate these issues completely.
>>
>> This is the extent that we were able to analyze the issue from our side, we 
>> would like to hear if there are some better ideas, or which approach would 
>> be more appropriate for upstream.
>>
>> Best regards,
>>
> 
> Cheers,
> 
> --
> Greg
> 

-- 
Antonios Motakis
Virtualization Engineer
Huawei Technologies Duesseldorf GmbH
European Research Center
Riesstrasse 25, 80992 München