jbayer@ispi.UUCP (Jonathan Bayer) (08/19/89)
I have a need to be able to link a directory to another directory. I know that it normally is not possible, but I was wondering if there was a way to fool the OS into linking a directory to another? The problem is here because I have a program that for safety's sake must run chroot'ed in a directory, but also has to have access to the standard /etc directory, and has to be able to create files in /etc that will be there later. Any ideas? Thanks JB -- Jonathan Bayer Intelligent Software Products, Inc. (201) 245-5922 500 Oakwood Ave. jbayer@ispi.UUCP Roselle Park, NJ 07204
cpcahil@virtech.UUCP (Conor P. Cahill) (08/20/89)
In article <1114@ispi.UUCP>, jbayer@ispi.UUCP (Jonathan Bayer) writes: > > I have a need to be able to link a directory to another directory. I > know that it normally is not possible, but I was wondering if there was > a way to fool the OS into linking a directory to another? Normally the super-user can link directories (note that the .. in the current directory is a link to the . in the parent directory), so that should solve your problem. (The /etc/mvdir shell that is usefull only for root uses this feature). A side issue is that having linked directories that are not in the normal child/parent relationship can wreck havoc on your backups if the backup utilities are not designed to handle this situation. The find(1) utility (which is part of lots of backup utilities) does not correctly handle this situation because of the way it traverses the directory structure. As it processes sub-directories, find changes to the child directory and when finished changes to "..". If you link a directory to another place where it has a different parent directory, when find changes to ".." it will go to the true parent. For example: you link /etc to /usr/rootdir/etc if you cd to /usr/rootdir/etc and cd .. you will be in / if you cd to /usr/rootdir/etc/.. you will also be in / > is here because I have a program that for safety's sake must run > chroot'ed in a directory, but also has to have access to the standard > /etc directory, and has to be able to create files in /etc that will be > there later. Another solution would be to copy all modified /etc files into the local etc directory, run the chrooted program, and copy back out all modified etc files. Inefficient, but it works (unless, of course, you need the modified files to be placed into /etc immediately).
davidsen@sungod.crd.ge.com (ody) (08/22/89)
You can do this as root using the link(S) system call. Of course if
you mistrust the program to the extent of making it run chroot then *I*
wouldn't let it touch the space where my passwd and group live. I
strongly encourage doing something like copying the modified files
(perhaps after checking WHICH files they are) into the live /etc.
Another note (I found out the hard way), most backup programs will
fail in some way trying to back up such a link. Be *very* careful, as
some of the failure modes I found involved trashing the filesystems
completely.
bill davidsen (davidsen@crdos1.crd.GE.COM)
{uunet | philabs}!crdgw1!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -meclewis@eci386.uucp (Chris Lewis) (08/22/89)
In article <1114@ispi.UUCP> jbayer@ispi.UUCP (Jonathan Bayer) writes: >I have a need to be able to link a directory to another directory. I >know that it normally is not possible, but I was wondering if there was >a way to fool the OS into linking a directory to another? The problem >is here because I have a program that for safety's sake must run >chroot'ed in a directory, but also has to have access to the standard >/etc directory, and has to be able to create files in /etc that will be >there later. > >Any ideas? Getting the files in is easy: ln /etc/* /etc/rootdir/etc Getting them out ain't, though as long as these files aren't deleted, you could "touch" 'em in /etc and link 'em to /etc/rootdir/etc. Or simply copy the files back after your special application runs. Frankly, if you don't trust the program to run without chrooting, I don't know why you'd trust it enough to have /etc writeable. Are you concerned about integrity or security? You could hard "link" (rather than ln) the directory as root, but a hostile program might be able to break the chroot by chdir("..")'ing. -- Chris Lewis, R.H. Lathwell & Associates: Elegant Communications Inc. UUCP: {uunet!mnetor, utcsri!utzoo}!lsuc!eci386!clewis Phone: (416)-595-5425
debra@alice.UUCP (Paul De Bra) (08/23/89)
In article <1797@crdgw1.crd.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes: > You can do this as root using the link(S) system call... But fsck will bark every time you run it, because it doesn't like directories with more than one link. Paul. -- ------------------------------------------------------ |debra@research.att.com | uunet!research!debra | ------------------------------------------------------
thurm@shorty.CS.WISC.EDU (Matthew Thurmaier) (08/24/89)
In article <1114@ispi.uucp> Jonathan Bayer writes: > I have a need to be able to link a directory to another directory. I > know that it normally is not possible, but ... Jonathan, This is somewhat of a KLUDGE, but see if you can get it to work. I have not had time to try it myself, so there are NO PROMISSES, just hunches: 1.) We know that a directory is really just a way of maping file names to i-nodes. 2.) We know that directories have i-node #s, just like any other file. 3.) We know that directories entries may only be made by root. 4.) We know that the structure of a directory entry is listed in <sys/dir.h> SO, Write a program which you execute as root, which includes <sys/dir.h>, opens the directory that you want to link to, gets it's i-node number and other vital information (note that if you open that directory, the "." entry represents the directory itself), then open the directory in which you want to make the new link entry and create an entry with the same information, only with the name changed to whatever you want, instead of ".". Let me know how it turns out. Matthew. make sure that Snail Mail: E Mail: Matthew J. Thurmaier ...!{allegra,harvard,seismo}!shorty!matt The Computer Classroom matt@shorty.wisc.edu 6701 Seybold Road, Ste. 122 Madison, WI 53719 (608) 271-2171 "why am I ALWAYS going somewhere?" >>-matt-->
clewis@eci386.uucp (Chris Lewis) (08/26/89)
In article <8233@spool.cs.wisc.edu> thurm@shorty.cs.wisc.edu (Matthew Thurmaier) writes: >In article <1114@ispi.uucp> Jonathan Bayer writes: >> I have a need to be able to link a directory to another directory. I >> know that it normally is not possible, but ... > This is somewhat of a KLUDGE, but see if you can get it to work. I have >not had time to try it myself, so there are NO PROMISSES, just hunches: >3.) We know that directories entries may only be made by root. Nope. *Only* by the kernel. There's no way of writing into a directory file from a user program except by openning the special file for the file system, traversing the hierarchy yourself, and manually zapping a block in the directory. And, if you have to allocate a new block? Well, forget it. The "link" system call will do what you want if you're root, unfortunately, fsck will probably bitch. -- Chris Lewis, R.H. Lathwell & Associates: Elegant Communications Inc. UUCP: {uunet!mnetor, utcsri!utzoo}!lsuc!eci386!clewis Phone: (416)-595-5425
marks@mgse.UUCP (Mark Seiffert) (08/28/89)
In article <9807@alice.UUCP> debra@alice.UUCP () writes: >In article <1797@crdgw1.crd.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes: >> You can do this as root using the link(S) system call... > >But fsck will bark every time you run it, because it doesn't like >directories with more than one link. I have not ran fsck with a directory link yet. I have noticed that when you cd into a linked directory, and then cd .. out of it, you are in the parent directory of the linked file (is that the way to explain it?), it is aggravating. I have noticed the following problem, i would call it a bug, you decide for yourself. When the following script is ran it creates a directory in /usr/tmp and then links the directory to a second name. If you rm the second directory name, and then cd into the first, you can cd back out. It looks like rmdir does not check to see if a directory has links before it removes the . directory entry. I also noticed there is no online manual page for link. ln will not work with directories, as stated in the man page. link does not give a diagnostic message if it can not find the first directory named on the command line for the link command. ------------------ cut here ------------------ set -x cd /usr/tmp mkdir test1 link test1 test2 rmdir test2 cd test1 l cd /usr/tmp rmdir test1 ------------------ cut here ------------------ Any comments? Is this a known bug, i decided to drop SCO's softcare support. > >Paul. >-- >------------------------------------------------------ >|debra@research.att.com | uunet!research!debra | >------------------------------------------------------ -- Mark Seiffert, Metairie, LA. uucp: rex!mgse!marks bitnet: marks%mgse@REX.CS.TULANE.EDU internet: marks%mgse@rex.cs.tulane.edu