The Design and Implementation of the Anykernel and Rump Kernels

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Due to how etfs is implemented as part of the file system lookup routine, the mapped
filenames is not browseable (i.e. readdir). However, it does not affect the intended
use cases such as access to firmware images, since the pathnames are hardcoded into
the kernel.
In addition to taking a lookup key and the backing file path, the etfs interface
takes an argument controlling how the mapped path is presented inside the rump
kernel. The following three options are valid for non-directory host files: regular file,
character device or block device. The main purpose of the type mapping feature is to
be able to present a regular file on the host as a block device in the rump kernel. This
mapping addresses an implementation detail in the NetBSD kernel: the only valid
backends for disk file systems are block devices. On a regular system the mapping
is done using the vnconfig utility to map regular files to /dev/vndxx block device
nodes which can be mounted 11 . Avoiding vnconfig on the host is beneficial since
using it requires root privileges regardless of the permissions of the backing file.
With the etfs interface, a rump kernel requires only the minimal host privileges
which allow it to read or write the backing file and therefore more finegrained access
control is possible.
In addition to mapping files, it is possible to map directories. There are two options:
a single-level mapping or the mapping of the whole directory subtree. For example,
if /rump_a from the host is directory mapped to /a in the rump kernel, it is possible
to access /rump_a/b from /a/b in both single-level and subtree mappings. However,
/rump_a/b/c is visible at /a/b/c only if the directory subtree was mapped.
Directory mappings do not allow the use of the type and offset/size translations,
but allow mappings without having to explicitly add them for every single file. The
original use case for the directory mapping functionality was to get the kernel mod-
11 For disk images with a partition table vnconfig provides multiple block devices in /dev. The
withsize variant of the etfs interface can be used to map a range of the host file corresponding to
the desired partition. The p2k and ukfs libraries’ interfaces for mounting disk file systems provide
support for this variant (manual pages at A–12 and A–38, respectively).