l4-hackers July 2019

l4-hackers@os.inf.tu-dresden.de

2 participants
3 discussions

Filesystem Architectures with L4Re and L4-Based Systems
by Paul Boddie 02 Dec '19

02 Dec '19

Hello, As this list can attest, I previously spent a bit of time diving into the way dynamic linking is supported by L4Re to get shared mode programs working. This turned out to have a relatively simple fix related to symbols employed by GCC- generated code. But during this exercise, I gained some familiarity with the way libraries are loaded, and this appears to involve the virtual filesystem support, with the loader perusing the filesystem to access libraries, and with these being provided as modules by the "rom" filesystem. Since then, I've spent some time looking at how files are provided by filesystems and accessed by code that uses the conventional C or POSIX library interfaces. It raises a few questions about why things are done in a particular way in L4Re, and which approaches are in use for other L4-based systems, especially those that seek to provide general-purpose, multi-user solutions. What I think I now understand about the virtual filesystem support in L4Re is as follows. The virtual filesystem itself is a "client-side" construct, meaning that it resides within any given program. That programs have their own view of a filesystem is not too different from what the documentation about systems like the Hurd describes and advocates, and I suppose it is a tempting approach because it gives each program the flexibility to be customised in this regard. However, it seems that beyond the internal "mount tree" representation, configuration of the namespace hierarchy is largely done using the L4Re namespace concept, with namespaces acting as directories, and non-namespace objects within the hierarchy interpreted as particular file types. Apart from the interpretation of namespaces as directories, it seems that for the most part, dataspaces are employed to be interpreted as file objects. I will admit that I didn't really look very hard at how different systems provide file access before now, but what surprised me slightly was the apparent lack of delegation in L4Re. As far as I can tell, on systems based on kernels like Linux, the open library function will employ a system call to delegate the matter of finding the file and obtaining a way of accessing it to the familiar monolithic-kernel-plus-filesystems arrangement. On more "exotic" systems (than Linux) like Minix 3 or the Hurd, it appears that delegation to a virtual filesystem server [1] or servers [2] occurs to mediate access to specific files. Some systems like Inferno and Plan 9 employ protocols [3] to formalise the client-server relationship. I had almost expected to find similar things in L4Re, but I could only confirm their absence after reviewing the various VFS abstractions. One thing I wonder about is whether the current L4Re approach is able to satisfactorily deal with filesystem content that needs to serve different user entities, with content having different permission and ownership rights. It seems that the filesystem logic has to be available as a library within a program, which is not necessarily a problem. But to avoid filesystem content being completely exposed to the discretion of user programs (reminiscent of the problems with early Network File System implementations), the library would need to call out to other entities, which doesn't appear to be done with the current L4Re abstractions. One could implement support for, say, the ext3 filesystem in a library, operating on a dataspace provided at some kind of mountpoint in a directory (with the directory being provided by a namespace, of course) but it would be like giving a user program access to a block device in a traditional Unix-like system. Are there any articles about the design of L4Re that might explain the motivations here? And are there any accessible-but-detailed articles about other microkernel-based systems and the way in which they structure their filesystem architectures? I looked around on the TU-Dresden site for materials, but I didn't immediately find anything obviously relevant. Paul [1] https://wiki.minix3.org/doku.php?id=developersguide:vfsinternals [2] https://www.gnu.org/software/hurd/hurd-talk.html#pat [3] http://doc.cat-v.org/inferno/4th_edition/styx

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New L4Re component released: virtio-net
by Jakub Jermar 29 Jul '19

29 Jul '19

Dear L4 hackers, last week we released virtio-net on GitHub[0]. Together with the previously released L4Re components, virtio-net can be used to interconnect two VMs with a virtual network. We also wrote a little tutorial which illustrates that[1]. Happy hacking, Jakub [0] https://github.com/kernkonzept/virtio-net [1] https://github.com/kernkonzept/manifest/wiki/MultipleVMs -- Kernkonzept GmbH at Dresden, Germany, HRB 31129, CEO Dr.-Ing. Michael Hohmuth

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kernel error
by ssrmurthy saka 04 Jul '19

04 Jul '19

Hi team, I am using "4.13.0-36-generic" kernel and i exported cross compiler of version "gcc-linaro-7.4.1-2019.02-i686_arm-linux-gnueabihf". when i try to compile new odule i am facing below error. can you please help on this. make -C /lib/modules/4.13.0-36-generic/build/ M=/home/tessolve/c modules make[1]: Entering directory '/usr/src/linux-headers-4.13.0-36-generic' CC [M] /home/tessolve/c/vm.o In file included from ./include/asm-generic/int-ll64.h:10:0, from ./arch/arm/include/uapi/asm/types.h:4, from ./include/uapi/linux/types.h:4, from ./include/linux/compiler.h:231, from ./include/linux/init.h:4, from /home/tessolve/c/vm.c:1: ./include/uapi/asm-generic/int-ll64.h:11:10: fatal error: asm/bitsperlong.h: No such file or directory #include <asm/bitsperlong.h> ^~~~~~~~~~~~~~~~~~~ compilation terminated. scripts/Makefile.build:315: recipe for target '/home/tessolve/c/vm.o' failed make[2]: *** [/home/tessolve/c/vm.o] Error 1 Makefile:1550: recipe for target '_module_/home/tessolve/c' failed make[1]: *** [_module_/home/tessolve/c] Error 2 make[1]: Leaving directory '/usr/src/linux-headers-4.13.0-36-generic' Makefile:4: recipe for target 'all' failed make: *** [all] Error 2 Thanks, murthy

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l4-hackers July 2019