Patch for mmap problem in KConfigBackEnd, Bug #72586

[Dr. Juergen Pfennig]

Hello whoever,

I send you a patch to avoid that Konqueror hangs and Syslogd quickly fills 
your harddisk with error messages. The problem occurs with mmap() and files 
that are accessed via smb (or nfs) and are not accessible due to ownership 
and protection on the server side. See the remark concerning nfs in 'man 2 
access'. Typically the problem shows up with konqueror and the .directory 
file. Lots of lines containing just the work "SIGBUS" are written to stderr 
and konqueror hangs.

About the current implementation in kconfigbackend.cpp: mmap() is used always  
to read conf files. Interrestingly the author was aware of a SIGBUS problem 
and tried to recover from the SIGBUS. BUT THIS DOES NOT WORK. Maybe it never 
worked? Or maybe it's a compiler optimization problem.

About the patch: as mmap() may return unaccessible memory, the use of mmap is 
restricted now to large files. In any case a classical read() syscall is used 
either to read an entiere small conf file, or to probe if the (large) file is 
readable. If the read succeeds for a large file, mmap is used as before. mmap 
is not trivial for the kernel, maybe this patch also gives a little speed 
improvement. The patch is against today's HEAD (3.1.94 plus updates).

I also attach a little test program that can be used to play with access() and 
mmap() if you don't trust me. My kernel is a 2.4.21 from Mantel/SuSE. I am 
using a P4 and the problem occurred also in older Versions of KDE.

Yours Jürgen
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