[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Qemu-devel] Allow to use virtfs as overlayfs upper dir

From: Piotr Jurkiewicz
Subject: [Qemu-devel] Allow to use virtfs as overlayfs upper dir
Date: Wed, 7 Feb 2018 04:48:57 +0100
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0


in order to obtain Docker-like experience I am trying to set up overlayfs merged directory inside VM. Both upper and lower directories are mounted with virtfs/9pfs from outside of the VM. However, current implementation of virtfs makes its usage as upper filesystem impossible.

1. Upper filesystem must support the creation of trusted.* extended attributes.

9pfs has support for getting/setting xattrs, but calls operating on attributes other than user.* and system.posix_acl_* are dropped.

2. Upper filesystem must provide valid d_type in readdir responses.

This works, but only in case of 'passtrough' and 'none' security models. In the case of 'mapped-xattr' and 'mapped-file' models, d_type is being zeroes to DT_UNKNOWN during readdir() call.

All these limitations can be resolved pretty easily, but requires some design decisions. I can prepare appropriate patches.

Ad. 1.

Why are operations on attributes other than than user.* and system.posix_acl_* forbidden? Is this due to security reasons?

If so, can we map all of them to user.virtfs namespace, similarly as system.posix_acl_* are being mapped to user.virtfs.system.posix_acl_* in 'mapping' mode already? This way any trusted/security/system attributes will be effective only when mounted via virtfs inside VM.

Ad. 2.

local_readdir() can fill entry->d_type with the right DT_* value by obtaining file type from mapping and translating it with IFTODT() macro. This would, however, require reading 'user.virtfs.mode' for each direntry during readdir() call, what can affect performance. If so, this behavior would probably need to be controlled with some runtime option.

'mapped-xattr' and 'mapped-file' models are essential for running qemu with overlayfs as non-root, because overlayfs creates device nodes, what is possible for unprivileged user only with these models.

I am looking forward for your opinions.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]