TUHS

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6532 discussions

dmr streams & networking [was: Re: If not Linux, then what?]

by Paul Ruizendaal

> I find it hard to believe what you remember Dennis saying. The point of > dmr's streams was to support networking research in the lab and avoid the > myriad bugs of the mpx interface by stepping around them completely. > > Perhaps it's out of context. > > -rob > I could be wrong but that's my memory. What he told me was streams was > for line disciplines for tty drivers. That's what I know but you were > there, I was not. I'm pretty confused because what Dennis said to me > was that he did not think streams would work for networking, he thought > they made sense for a stream but not for a networking connection because > that had multiple connections coming up through a stream. There is some contemporary material that gives a bit of context. The quotes are a bit contradictory and perhaps reflect evolving views. [1] The original dmr paper (1984) on streams (http://cm.bell-labs.co/who/dmr/st.html) seems to support the no networking view, focussing on terminal handling in its discussion. Also, near the end it says: "Streams are linear connections; by themselves, they support no notion of multiplexing, fan-in or fan-out. [...] It seems likely that a general multiplexing mechanism could help in both cases, but again, I do not yet know how to design it.” This seems to exclude usage for networking, which is typically multiplexed. [2] However, now that the V8 sources are available it is clear that the streams mechanism was used (by dmr?) to implement TCP/IP networking. He explains how that tallies with the above quote on multiplexing in a 1985 usenet post: https://groups.google.com/forum/#!topicsearchin/net.unix-wizards/subject$3A… The config files in the surviving TUHS V8 source tree actually match with the setup that dmr described in the penultimate paragraph. If the post by dmr does not immediately appear, click on the 8-10-85 post by 'd...(a)dutoit.xn--uucp-y96a to make it fold out. For ease of reference, I’m including the message text below. <quote> Steven J. Langdon claimed that without multiplexing one couldn't do a proper kernel-resident version of TCP/IP in the V8 stream context. Here's how it's done. It is still true in our system that stream multiplexing does not occur, in the sense that every stream connection has (from the point of view of the formal data structures) exactly two ends, one at a user process, and the other at a device or another process. However, this has, in practice, not turned out to be a problem. Say you have a hardware device that hands you packets with a channel (or socket) number buried inside in some complicated format. The general scheme to handle the situation uses both a line discipline (stream filter module) and associated code that, to the system, looks like a stream device driver with several minor devices; these have entries in /dev. A watchdog process opens the underlying real device, and pushes the stream module. Arriving packets from the real device are passed to this module, where they are analyzed, and then given to the appropriate associated pseudo-device. Likewise, messages written on the pseudo-device are shunted over to the line discipline, where they are encoded appropriately and sent to the real device. This is where the multiplexing-demultiplexing occurs; formally, it is outside of the stream structure, because the data-passing doesn't follow the forward and backward links of the stream modules. However, the interfaces of the combined larger module obey stream rules. For example, IP works this way: The IP line discipline looks at the type field of data arriving from the device, and determines whether the packet is TCP or UDP or ARP or whatever, and shunts it off to the stream associated with /dev/ip6 or /dev/ip17 or whatever the numbers are. TCP, of course, is multiplexed as well. So there is a TCP line discipline, and a bunch of TCP devices; a watchdog process opens /dev/ip6, and pushes the TCP line discipline; then the TCP packets it gets are parcelled out to the appropriate /dev/tcpXX device. Each TCP device looks like the end of a stream, and may, of course, have other modules (e.g. tty processor) inserted in this stream. UDP sits on top of IP in the same way. This example is complicated, because (TCP,UDP)/IP is. However, it works well. In particular, the underlying real device can be either an ethernet or our own Datakit network; the software doesn't care. For example, from my machine, I can type "rlogin purdy" and connect to a Sequent machine running 4.2; the TCP connection goes over Datakit to machine "research" where it is gatewayed to a local ethernet that purdy is connected to. A further generalization (that we haven't made) is in principle easy: there can be protocol suites other than IP on an Ethernet cable. So there could be another layer to separate IP from XNS from Chaosnet, etc. Dennis Ritchie </quote> Maybe the subtle notion expressed as "formally, it is outside of the stream structure, because the data-passing doesn't follow the forward and backward links of the stream modules. However, the interfaces of the combined larger module obey stream rules” explains how dmr could talk about streams as being just suitable for line disciplines without meaning to say that they did not have good use in networking. Paul

5 years, 10 months

/dev/null

by Norman Wilson

John Steinhart: Just curious - am doing a cross-country road trip with my kid and saw a Wisconsin dev null license plate. Didn't get a look at the driver. ===== It's in ken/mem.c in Fourth Edition, dmr/mem.c in Fifth and Sixth. Norman (no sheds) Wilson Toronto ON

5 years, 10 months

/dev/null

by Jon Steinhart

Just curious - am doing a cross-country road trip with my kid and saw a Wisconsin dev null license plate. Didn't get a look at the driver. Does it belong to anyone on this list? Jon

5 years, 10 months

Shell builtin exec

by Adam Thornton

This is probably the place to ask: I understand why the shell builtin "exec" is the same as the syscall exec() in the sense of "replace this process with that one." But why is it also the way to redirect filehandles in the current shell? (That is, why isn't the redirection named something else?) Adam

5 years, 10 months

Re: [TUHS] Video of PDP-7 with Graphics-2?

by Warren Toomey

On Fri, Aug 02, 2019 at 06:49:19PM -0400, Jim Carpenter wrote: > On 8/2/19 4:14 PM, Warren Toomey wrote: > > Hi all, I'm chasing the Youtube video of the PDP-7 at Bell Labs where > > people are using it to draw circuit schematics. > > A Bell Labs video? The only Bell Labs video I remember seeing that had > someone doing circuit schematics had it being done on a PDP-5. The -7 was > shown later doing music stuff. (That's the -7 that I thought maybe Unix > started life on.) Thanks Jim, Is it this one? https://www.youtube.com/watch?v=iwVu2BWLZqA They mention a Graphics-1 device, so maybe I'm getting this confused with the PDP-7 and the Graphics-2. Cheers, Warren

5 years, 10 months

Re: [TUHS] Set-uid shell scripts

by Norman Wilson

Oops. Didn't think it through: the problem is argv[1], passed as the name of the script being executed, not argv[0]. Disregard my previous execl(...). A related problem is the inherent race condition: If you do ln -s /bin/setuidscript . ./setuidscript ./setuidscript is opened twice: once when the kernel reads it and finds #! as magic number and execs the interpreter, a second time when the interpreter opens ./setuidscript. If you meanwhile run something that swoops in in the background and replaces ./setuidscript with malicious instructions for the interpreter, you win. I remember managing to do this myself at one point in the latter part of the 1980s. That was when I fell out of love with setuid interpreter scripts. It looks like we didn't disable the danger in the Research kernel, though. I don't remember why not. Norman Wilson Toronto ON

5 years, 10 months

Re: [TUHS] Additional groups and additional directory permissions

by Kirk McKusick

> Date: Fri, 2 Aug 2019 09:28:18 -0400 > From: Clem Cole <clemc(a)ccc.com> > To: Aharon Robbins <arnold(a)skeeve.com>, Doug McIlroy <doug(a)cs.dartmouth.edu> > Cc: The Eunuchs Hysterical Society <tuhs(a)tuhs.org> > Subject: Re: [TUHS] Additional groups and additional directory permissions > > The best I can tell/remember is that groups went through 4 phases: > 1.) No groups (earliest UNIX) [ I personally never used this except in the > V0 retrocomputing] > 2.) First group implementation (Thompson) [My first UNIX introduction was > with this implementation] > 3.) PWB 1.0 (Mashey version) [then saw this post PWB] > 4.) BSD 4.2 (wnj version) [and lived this transistion] > > Each was a little different in semantics. > > As Doug mentioned, many sites (like Research) really did not need much and > groups were really not used that widely. Thompson added something like > the Project number of TOPS and some earlier systems. Truth is, it did not > help much IMO. It was useful for grouping things like the binaries and > keeping some more privileged programs from having to be setuid root. > > Mashey added features in PWB, primarily because of the RJE/Front end to the > Mainframes and the need to have better protections/collections of certain > items. But they still were much more like the DEC PPN, were you were > running as a single group (i.e. the tuple UID/GID). This lasted a pretty > long time, as it worked reasonably well for larger academic systems, where > you had a user and were assigned a group, say for a course or class, you > might be talking. If you looked at big 4.1 BSN Vaxen like at Purdue/Penn > State, *etc.*, that how they were admin'd. But as Doug said, if you were > still a small site, the use of groups was still pretty shallow. > > But, as part of the CSRG support for DARPA, there was a push from the > community to have a list of groups that a user could be a part and you > carried that list around in a more general manner. The big sites, in > particular, were pushing for this because they were using groups as a major > feature. wnj implemented same and it would go out widely in 4.2, although > >>by memory<< that was in 4.1B or 4.1C first. It's possible Robert Elz > may have brought that to Bill with his quota changes, but frankly I've > forgotten. There was a lot of work being done to the FS at that point, > much less Kirk's rewrite. > > But as UNIX went back to workstations, the need for a more general group > system dropped away until the advent widely used distributed file systems > like CMU's AFS and Sun's NFS. Then the concept of a user being in more > than one group became much more de rigeur even on a small machine. > > Clem Late to answer... As far as I remember, Clem's description is correct. The filesystem itself stores only one owner and one group ID. When checking access to the file, the file owner is checked to see if the user ID matches. If so, then the owner permissions are applied. If not then the group array associated with the user is used to decide if the group of the file matches one of the groups of which the user is a member and if so the group permissions apply. Otherwise the other permissions are used. In BSD, the group assigned to the file is assigned from the group of the directory in which it is created. The setgid flag can be set only if that group is a member of the user's group array. The user can only change the group ID to one that appears in their group array. Until multiple group sets were added to System V, the group of the file was taken from the gid assigned to the user at login. Kirk McKusick

5 years, 10 months

Pre-init initialization

by Grant Taylor

Do, or did, anything other than Linux use a concept of an initramfs / initrd to create a pre-(main)-init initialization environment to prepare the system to execute the (main)-init process? -- Grant. . . . unix || die

5 years, 10 months

MERT aka UNIX/RT

by Warner Losh

Greetings, I was wondering if there were any early versions of MERT available? Reading different sources, it appears that MERT was a real time kernel that used EMT traps to implement unix system calls (from around V4 or V5 given the timelines) on top of a real time executive (though some sources seem to imply it was a derivative of V4, most disagree). I see this in our archives https://wiki.tuhs.org/doku.php?id=misc:snippets:mert1 which is quite handy for discover its (and other early) unix lineages for a talk I'm doing in about a month. Now that we have sources, I go back and double check the recollections of things like this to see if version numbers were right, etc. But I can't do that with MERT at all. I can find the Bell Systems Technical Journal for Unix that has a brief article on it, but no sources to double check. So I thought I'd ask here if we have any MERT artifacts I can look at that have escaped my casual browsing of the archive. So far I've just found an email from Kevin Bowling on the topic from last month with no replies. And a similar thread from 2002, plus pleading from time to time (I can't tell if Warren or Noel wants it more :). I guess the same could be said for CB-UNIX and UNIX/TS, though I see a USDL/CB_Unix directory in the archive I could look at :). Warner

5 years, 10 months

Re: [TUHS] Set-uid shell scripts

by Dave Horsfall

On Tue, 6 Aug 2019, Lyndon Nerenberg wrote: >> Just to extend this thread a bit more, when did the set[ug]id bit start >> getting turned off if the file was overwritten? > > I'm pretty sure that's been the case since the dawn of time. Hmmm... I have this vague memory of V5 (which I only used for a couple of months before we got V6) not clearing that bit, but after all these years my memory is starting to fail me :-( > It was certainly the case in every System V (release 0 and beyond) I > worked with, along with many BSDs derivatives (SunOS 3+, Ultrix, etc). > (And Xenix, which had it's own insanity that I now think selinux is > trying to inflict on me.) I've always thought that Xenix was insane to start with... Then again, my first experience with it was on a 286... Now, when porting Unify, should I use large memory model here or small memory model? Crazy. > This has been documented in chown(2) for as long as I can remember, so > that's a good place to start if you want to dig back through the various > source trees. I don't have access to the sources right now, but I'll take your word for it; it was just a passing thought. -- Dave

5 years, 10 months

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