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Thoughts on MAXPATHLEN

Hi fellow shepherds :)

I recently started playing around with the Hurd again and one of the
first things I do is to patch my pet editor so it builds and works on
the Hurd. It's a MAXPATHLEN issue and I always start with a trivial
patch that just defines it to see if it is the only issue, but once I
got a hacked up Debian package I move on to something more

Anyhow, I recently played around with paths > MAXPATHLEN and
discovered the following:

1. Linux

I've got a Debian testing installation here:

% uname -a
Linux thinkbox 3.1.0-1-amd64 #1 SMP Tue Jan 10 05:01:58 UTC 2012 x86_64 GNU/Linux
% mount | grep thinkbox-var
/dev/mapper/thinkbox-var on /var type ext4 (rw,nosuid,nodev,noatime,errors=remount-ro,user_xattr,barrier=1,data=ordered)
% echo -e "#include <sys/param.h>\nMAXPATHLEN" | gcc -E -x c - | tail -n 1

Linux happily creates paths longer than MAXPATHLEN (see recipe at the
end) and if you try to manipulate it or start programs with cwd >
MAXPATHLEN, things go really bad, e.g.:

* nautilus crashes b/c of unhandled signal 8, arithmetic exception
* tar can create an archive containing such paths, but cannot untar it
* filelight just ignores the path
* gdb refuses to work

(off topic: bonus points for anyone who finds a MAXPATHLEN related
buffer overflow in a commonly installed suid binary or a daemon
running as root that traverses a region of the vfs that unprivileged
users control and be sure to buy me a beer if you succeed ;)

2. Hurd

I tried to do the same on one of my Hurd installations and ended up in
/ with mkdir complaining that I'm not allowed to write to /.

Not sure what's going on here, but iirc the MIG string type is

Some thoughts:

* Any software that does not dynamically allocate resources for path
  manipulations is broken, even on systems that define MAXPATHLEN.

* So this is a problem that exists on Linux too, but it is obscured by
  the fact that Linux just defines MAXPATHLEN to an arbitrary value.

* But the problem is more apparent on the Hurd since it chose not to
  define MAXPATHLEN, resulting in an enormous amount of manual and
  tedious work while porting packages.

* The Hurd community is way to small to carry the burden of fixing all
  those issues and there are more important issues that need to be

* It is sometimes difficult to get the patches upstream since Hurd
  might be seen as an toy OS.

* It is very easy to create paths longer than MAXPATHLEN on Linux and
  to crash / dos / evade programs using it and it might (can anyone
  confirm this?) not even be possible to create paths of arbitrary
  length on the Hurd b/c of a MIG limitation. But Hurd is the platform
  that chose not to define MAXPATHLEN and Linux just lies to it's
  users yet Hurd is perceived as the problematic one.

Based on this observations I'd vote to do the following:

1. File bugs for software that uses MAXPATHLEN and co, do so politely
   and explain the issues. Talk directly to upstream and *do not*
   mention the Hurd.

2. Define MAXPATHLEN and work on more important issues.

Well, flame on :)


~~~ recipe ~~~
to create such a path, do

% A=`python -c 'print 128 * "a"'`
% for i in `seq 40` ; do mkdir $A ; cd $A ; done
% pwd | wc --bytes

to reenter it, do

% while cd $A ; do true ; done

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