Monthly Archives: September 2006

Acrobat Reader and the decimal comma problem

When printing a PDF file, Acrobat Reader creates a Postscript and sends it to the printer spooler (or, optionally to a file). Nothing new here, as it’s a common way for Unix programs to print files.

However, I found that under certain circumstances the Postscript file was corrupt: it could not be printed or opened with Ghostscript, because it showed instead an “undefined command” error.

Looking deeper into the problem I found that the corrupt file had its decimal numbers separated by a comma instead of a dot. The comma is the decimal separator in many locales, but decimal numbers in Postscript files must be separated by a dot.

Investigating a bit more I found that this problem shows when the Postscript file uses vector primitives, and that’s why the problem appears to occur randomly.

So the problem is that Acrobat Reader relies incorrectly on the locale settings to create a Postscript file. This should be a trivial bug to fix, but as Acrobat Reader is closed software you have to live with it if you want to use this software (fortunately free PDF readers are constantly improving).

There’s a quick workaround, however. As /usr/bin/acroread is really a shell script that lauches the real binary, you can add a export LC_NUMERIC=C at the beginning of the file and solve the problem. This bug is already fixed in some unofficial Debian packages.

Hope it helps.

User-mode Linux and skas0

User-mode Linux (UML) is a port of Linux to its own system call interface. In short, it’s a system that allows to run Linux inside Linux.

UML is integrated in the standard Linux tree, so it’s possible to compile an UML kernel from any recent kernel sources (using ‘make ARCH=um‘).

Traditionally, UML had a working mode which was both slow and insecure, as each process inside the UML had write access to the kernel data. This mode is known as Tracing Thread (tt mode).

A new mode was added in order to solve those issues. It was called skas (for Separate Kernel Address Space). Now the UML kernel was totally inaccessible to UML processes, resulting in a far more secure environment. In skas mode the system ran noticeably faster too.

To enable skas mode the host kernel had to be patched. As of September 2006, the latest version of the patch is called skas3. The patch is small but hasn’t been merged in the standard Linux tree. The official UML site has a page about skas mode that explains all these issues more thoroughly.

However, by July 2005 a new mode was added to UML in Linux 2.6.13 called skas0 (which, for some reason, isn’t explained in the above page). This new mode is very close to skas3: it provides the same security model and most of its speed gains. The main difference is that you don’t need to patch the host kernel, so you can use a skas-enabled UML in your Linux system without having to mess with the host kernel. The patch is explained in the 2.6.13 changelog or in this article.

A skas0-enabled kernel boots like this:

Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Checking advanced syscall emulation patch for ptrace...OK
Checking for tmpfs mount on /dev/shm...OK
Checking PROT_EXEC mmap in /dev/shm/...OK
Checking for the skas3 patch in the host:
  - /proc/mm...not found  
  - PTRACE_FAULTINFO...not found
  - PTRACE_LDT...not found
UML running in SKAS0 mode