Running User Mode Linux - Part I

Recently, I have been trying to run User Mode Linux (UML) on our monitor. Running UML is very challenging, because it performs many low level operations that need special handling. For example, UML reads CPU Time Stamp Counters. Reading these counters is done at user-level (using rdtsc) and without invoking any system call. Hence, the monitor is not informed of this operation and cannot send identical results to all the variants. This can cause discrepancies among the variants. We replace the rdtsc with gettimeofday so that we can catch the system call and send the same time to all variants.

Processes executed on top of UML are supervised using ptrace. In fact, UML runs processes in debugging mode. Therefore, it is essential that UML should be able to run ptrace successfully and attach to the processes. Our monitor used to attach to all the variants and all their children preventing UML from performing correctly. Now that the variants are UML, the monitor should not attach to the children. To support this, we have added a command line argument that can be used when running UML. The argument causes the monitor to trace the variants only and not their children.
(We should investigate if this kind of monitoring is enough.)

Another challenge when running UML is supporting mmap. Our monitor does most of the file operations itself and just sends the results to the variants, however, if variants try to mmap a file that is not opened by themselves, mmap fails. This is the case when loading dynamic libraries. They are opened as read-only and then are mmaped. To solve this, we allow the variants to open files only if the requested permission is read-only. This solves the problem of loading dynamic libraries, but UML goes further. It opens temporary files in /dev/shm/ with full permission (read/write/execute) and then mmaps them. Since the files are opened with full permission, they are opened only by the monitor not by the variants and, therefore, mmap fails. This is a problem that should be addressed.