8 ms·

The Meaning of 'su'

141 points by myko 14y ago

deleted 14y ago
[deleted]
tedchs 14y ago
TL;DR: the author doesn't know, and lists the standard possibilities: superuser, switch user, or subshell. From the article: "Well, I wish I had an answer, but I don’t."
- whatshisface 14y ago
  Better TL;DR: The author points out that the meaning has changed over time from the initial meaning of "superuser" to "switch user", due to feature creep.
  - deleted 14y ago
    [deleted]
  - joshguthrie 14y ago
    Funny as it's the opposite of the UNIX philosophy: "Write programs that do one thing and do it well"...
    - eurleif 14y ago
      Is switching to an arbitrary user really "more than one thing"? Would the UNIX philosophy be to have a separate switch program for every user on the system?
      - beatgammit 14y ago
        I think `joshguthrie` means something along the lines of this: http://harmful.cat-v.org/cat-v/ http://harmful.cat-v.org/cat-v/ I think `su` is pretty good as-is, but sudo has a bit of feature creep. For example, I don't think these belong in sudo (taken from Linux): -e: edit something -l: list allowed/forbidden commands -p: change prompt Ideally, sudo would just run as root and su would just switch users. Then instead of: sudo -u some_user cmd You'd do this instead: sudo su -c "cmd" some_user Su only does one thing (switch users, -c for one-off cmd) and sudo only does one thing (change to root). Currently, both "sudo -u some_user cmd" and "sudo su -c some_user cmd" accomplish basically the same thing, which is a sign of feature creep.
        eurleif 14y ago
        sudo has security features. It's possible to restrict the set of commands that can be executed, for example. What would be a Unixy way to do that? How can you decouple the thing that gives you root access from the thing that restricts what you can do with it?
        deleted 14y ago
        [deleted]
        morpher 14y ago
        "-e: edit something" (or 'sudoedit') is actually very useful. It makes a temporary copy of th file that is editable by your user, allowing the editor to be run without elevated privileges. After the editor quits, it moves the edited file back over the original. This could be a simple wrapper script around sudo, but that could require entering the password twice.
      - joshguthrie 14y ago
        There is a difference of "functionality" between "Become Super User" and "Switch User", the base intent is not the same.
- joshguthrie 14y ago
  TL;DR: Legacy code with gotos in it.
  - dwyer 14y ago
    Way to nitpick about goto while completely overlooking the obvious buffer overflow.
    - jrockway 14y ago
      This was all I could think about when reading that code.
    - jeltz 14y ago
      There are at least two buffer overflows in that code.
  - metaphorm 14y ago
    thats not an inappropriate use of gotos either. they're in the same function, just a few lines down, and are just used to improve the safety and efficiency of the code. in fact thats the textbook case of the only time when a goto is actually a good idea.
  - UnoriginalGuy 14y ago
    It is C, of course it is going to have goto's in it. How else are you going to handle error conditions? Copy and paste the error code a dozen times within a single function? (hint: C doesn't support exceptions) You could use longjmp/setjmp but those have a lot of the same issues that people claim goto has. In fact worse in several ways, since while goto makes it hard to follow the program flow, longjmp makes it /impossible/.
    - derleth 14y ago
      > since while goto makes it hard to follow the program flow, longjmp makes it /impossible/. Only if you don't use it tastefully. It can be a nice way to bail out of a recursive function with an error code, which is precisely how I've used it. Really, it looks exactly the same as a try ... catch block in that C++/Java/C# language everyone's on about now.
deleted 14y ago
[deleted]
ams6110 14y ago
$ man su NAME su -- substitute user identity
- sek 14y ago
  This. I thought it meant super user too, but then I put in that command and said "ah". Then I moved on with my life....
  - joeblau 14y ago
    It does stand for super user. The article cites the su.c code from 1975.
- nsmartt 14y ago
  $ man su NAME su - change user ID or become superuser It's not quite so simple. The meaning has changed over time.
- kunai 14y ago
  What coreutils are you using? I get: ~$ man su NAME su - change user ID or become superuser OS X uses the FreeBSD coreutils and not the GNU ones, so if you're on either OS X or FreeBSD, that would explain it.
deleted 14y ago
[deleted]
northisup 14y ago
$ man su SU(1) BSD General Commands Manual NAME su -- substitute user identity SYNOPSIS su [-] [-flm] [login [args]] ...
nu2ycombinator 14y ago
Copy pasting the best comment from the article. It basically covers the crux. ======================================================== "jonkx using on | May 31, 2010 at 10:22 am | Permalink I worked with the Unix OS from the early 1980′s (at first we ran early versions on a DEC PDP-11) until 1992. Starting in 1992, I did contract work on proprietary versions based on SVR4. It was just easier to say the initials “S U” or “super-user” than to say “switch user” or “substitute user”. To insist on correctness, whether authoritarian, historical or otherwise seems petty. Depending on the options used, su can be used to switch user, substitute user or become “super user”. Success depends on knowledge of the appropriate password. “sudo” on the other hand may allow one to become any other user (depending on the configuration of sudo and being a “sodoer”) knowing only the login password. On the systems I have used, a sudoer can become root (“super user”) using this shell command at a terminal: sudo su – root and responding to the prompt with the login password used to sign in the current user. I think it is important to point out that you cannot become “super user” or root from a shell with the su command alone unless there is a root password and that password is entered at the prompt. =========================================
- mrb 14y ago
  To all people who type "sudo su - root": the much simpler "sudo -i" accomplishes the same (-i launches an interactive shell).
  - Osiris 14y ago
    I thought "sudo -s" was the correct way to drop into a root user shell.
    - ralph 14y ago
      No, unless one understands the subtleties, it may be best to use -i. $ sudo -s $ pwd /home/ralph $ exit $ sudo -i root@orac:~# pwd /root root@orac:~# logout $ $ sudo strace -fe execve sudo -s execve("/usr/bin/sudo", ["sudo", "-s"], [/* 16 vars */]) = 0 execve("/bin/bash", ["/bin/bash"], [/* 16 vars */]) = 0 ... $ exit $ sudo strace -fe execve sudo -i execve("/usr/bin/sudo", ["sudo", "-i"], [/* 16 vars */]) = 0 execve("/bin/bash", ["-bash"], [/* 16 vars */]) = 0 ... root@orac:~# logout $
ppierald 14y ago
I would recommend against using sudo su -, esp if you have <user> ALL=(ALL) NOPASSWD: ALL or such constructs in your /etc/sudoers file that exempt password requirements from executing sudo commands. 1. su gives you a root shell. Principle of least privilege dictates you should executes the fewest commands as root as required, not all commands as root because you need one. 2. sudo su - loses auditability of who executed what/when. All we know is that someone got a sudo shell at some point. When the forensics guys come along post-breach, you won't have a lot of good info to give them. 3. Loss of a shell (via command injection on a web form) may allow remote Command-and-Control of your machine. If you need a cron or other headless process (daemons, etc.) then use NOPASSWD in conjunction with a whitelisted set of commands (not /bin/bash) to be executed. If you need an operator to execute a command as a specific user, then use sudo -u <user> <cmd>. Even if you allow ALL commands, at least your audit logs will know what/when the command was executed. With all powerful commands comes the responsibility of understanding how to use them safely and what the possible repercussions are.
- mverwijs 14y ago
  4. And if you need to do anything extensive as root on a any machine, you might want to consider doing it with a configuration management system (puppet, chef, ansible).
- rlpb 14y ago
  In production, yes. For debugging on a test machine, or during a post-mortem, it's just inconvenient not to. If the majority of your time is spent in the shell of a production machine, you're doing something wrong.
davidw 14y ago
In Italian it means "up".
- kunil 14y ago
  In Turkish, it is water
klrr 14y ago
I think it really stands for "super user", here's its man page from Unix v1 http://man.cat-v.org/unix-1st/1/su http://man.cat-v.org/unix-1st/1/su .
- deleted 14y ago
  [deleted]
- UnoriginalGuy 14y ago
  > su allows one to become the super--user, who has all sortsof marvelous powers. I enjoy how informal that man page is. All of today's man-pages would never get away with "fun."
acc00 14y ago
My beard is rather short as of yet, but I've always read 'su' as an acronym for 'setuid' (because that's what it does).
asveikau 14y ago
A bit off topic, but man it really bugs me when people (like this author) say "command" because they don't know the right word. (A mistake made by this article) "I wonder why the creat() command doesn't have an 'e'." (Similar examples I've seen around the web) "I'm using the printf command." "I was doing assembly programming and I used the jump command." Syscall! Function! Instruction! Let's be precise people.
- wolfhumble 14y ago
  So to be even more precise ;-) are you treating Syscall, Function and Instruction as synonyms?
  - mdwrigh2 14y ago
    I don't think he is. He references creat, printf and jump, which happen to be a syscall, function and instruction, respectively.
- pbhjpbhj 14y ago
  Isn't "command" [at least conversationally] a supergroup of syscalls, functions and instructions?
nathell 14y ago
When perusing early Unix code, it's striking how concise and down-to-the-point it is. We've really accumulated a lot of cruft (mostly for the sake of compatibility and feeping creaturism) over the years.
4ad 14y ago
This is a great article and people quoting modern manual pages miss the point. The point is that the meaning of su(8) changed and that is conveyed through an nice anecdote where the author discovers exactly how it changed. In the research version of Tenth Edition Unix, su(8) is described as: su, setlog -- substitute userid temporarily, become super user Which is an interesting intermediate step from today's (or rather 2.9BSD's) su -- substitute user identity found in BSD derivatives. Btw, the Tenth Edition manual was written mostly by Douglas McIlroy, inventor of pipes. Even if the 1127 group imported outside code for the VAX versions of research Unix (v8-10), they liked a particular kind of documentation, which the imported code lacked, so they wrote their own. Dare I say the style of research Unix manuals closely matches the style of Go documentation today (unremarkable, considering the pedigree).
- shizcakes 14y ago
  This comment has it correct. For those of you on a Mac (and I specifically mention a Mac, due to its BSD userland utilities), you can see that this is still defined this way today: type `man su` and you'll see: su -- substitute user identity
bnegreve 14y ago
> “su” was written to only change to the root user on the system. It wasn’t designed to switch to any other user that has an account. “su” meant “super-user”. I need to sit down for a second. It is slightly over dramatized, isn't it ? :)
deleted 14y ago
[deleted]
zx2c4 14y ago
q = password; while((*q = getchar()) != '\n') if(*q++ == '\0') return; Notably, you don't actually need the password... Buffer overflow!
qompiler 14y ago
What a cute su.c file! You should see this beast in Solaris! http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/cmd/su/su.c http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/sr...
jonisalonen 14y ago
Oh, so it isn't an acronym for suit-up? </barney>
drucken 14y ago
To old hands, su has always and will always mean super user. The meaning does not "change" (even if the program does).
michaelfeathers 14y ago
It's funny how the article and most of the comments here assume that there is one meaning and it is "out there" for all of us to see. Maybe there isn't any one definite meaning: http://michaelfeathers.typepad.com/michael_feathers_blog/2013/02/the-fallacy-of-one-definite-meaning.html http://michaelfeathers.typepad.com/michael_feathers_blog/201...
jasonkostempski 14y ago
Taking 'super-user' to mean 'root' seems like a big assumption. Is there anywhere in the Unixverse where root is specifically referred to as 'super-user'? I sort of thought 'super-user' would refer to any user with higher privileges. Well, really just different privileges, but why would you switch to a user with lesser permission to preform the action you probably just got denied? I assume the presented code was just that way because it covered 99% of use cases and was good enough for then and that root is now the default user for the same reason.
- mturmon 14y ago
  I'm not 100% sure where you're going with all that, but there are legitimate reasons for the super-user to "move down" into a specific user's identity -- for example, to be sure that files/directories created during some maintenance are owned by that specific user, and have that user's umask applied. Another instance would be file cleanups when the super-user specifically didn't want to be able to delete arbitrary files. There are lots of other cases.

141 points by myko 14y ago

deleted 14y ago

[deleted]

TL;DR: the author doesn't know, and lists the standard possibilities: superuser, switch user, or subshell. From the article: "Well, I wish I had an answer, but I don’t."

whatshisface 14y ago
Better TL;DR: The author points out that the meaning has changed over time from the initial meaning of "superuser" to "switch user", due to feature creep.
- deleted 14y ago
  [deleted]
- joshguthrie 14y ago
  Funny as it's the opposite of the UNIX philosophy: "Write programs that do one thing and do it well"...
  - eurleif 14y ago
    Is switching to an arbitrary user really "more than one thing"? Would the UNIX philosophy be to have a separate switch program for every user on the system?
    - beatgammit 14y ago
      I think `joshguthrie` means something along the lines of this: http://harmful.cat-v.org/cat-v/ http://harmful.cat-v.org/cat-v/ I think `su` is pretty good as-is, but sudo has a bit of feature creep. For example, I don't think these belong in sudo (taken from Linux): -e: edit something -l: list allowed/forbidden commands -p: change prompt Ideally, sudo would just run as root and su would just switch users. Then instead of: sudo -u some_user cmd You'd do this instead: sudo su -c "cmd" some_user Su only does one thing (switch users, -c for one-off cmd) and sudo only does one thing (change to root). Currently, both "sudo -u some_user cmd" and "sudo su -c some_user cmd" accomplish basically the same thing, which is a sign of feature creep.
      - eurleif 14y ago
        sudo has security features. It's possible to restrict the set of commands that can be executed, for example. What would be a Unixy way to do that? How can you decouple the thing that gives you root access from the thing that restricts what you can do with it?
      - deleted 14y ago
        [deleted]
      - morpher 14y ago
        "-e: edit something" (or 'sudoedit') is actually very useful. It makes a temporary copy of th file that is editable by your user, allowing the editor to be run without elevated privileges. After the editor quits, it moves the edited file back over the original. This could be a simple wrapper script around sudo, but that could require entering the password twice.
    - joshguthrie 14y ago
      There is a difference of "functionality" between "Become Super User" and "Switch User", the base intent is not the same.
joshguthrie 14y ago
TL;DR: Legacy code with gotos in it.
- dwyer 14y ago
  Way to nitpick about goto while completely overlooking the obvious buffer overflow.
  - jrockway 14y ago
    This was all I could think about when reading that code.
  - jeltz 14y ago
    There are at least two buffer overflows in that code.
- metaphorm 14y ago
  thats not an inappropriate use of gotos either. they're in the same function, just a few lines down, and are just used to improve the safety and efficiency of the code. in fact thats the textbook case of the only time when a goto is actually a good idea.
- UnoriginalGuy 14y ago
  It is C, of course it is going to have goto's in it. How else are you going to handle error conditions? Copy and paste the error code a dozen times within a single function? (hint: C doesn't support exceptions) You could use longjmp/setjmp but those have a lot of the same issues that people claim goto has. In fact worse in several ways, since while goto makes it hard to follow the program flow, longjmp makes it /impossible/.
  - derleth 14y ago
    > since while goto makes it hard to follow the program flow, longjmp makes it /impossible/. Only if you don't use it tastefully. It can be a nice way to bail out of a recursive function with an error code, which is precisely how I've used it. Really, it looks exactly the same as a try ... catch block in that C++/Java/C# language everyone's on about now.

deleted 14y ago

[deleted]

ams6110 14y ago

$ man su NAME su -- substitute user identity

sek 14y ago
This. I thought it meant super user too, but then I put in that command and said "ah". Then I moved on with my life....
- joeblau 14y ago
  It does stand for super user. The article cites the su.c code from 1975.
nsmartt 14y ago
$ man su NAME su - change user ID or become superuser It's not quite so simple. The meaning has changed over time.
kunai 14y ago
What coreutils are you using? I get: ~$ man su NAME su - change user ID or become superuser OS X uses the FreeBSD coreutils and not the GNU ones, so if you're on either OS X or FreeBSD, that would explain it.

deleted 14y ago

[deleted]

northisup 14y ago

$ man su SU(1) BSD General Commands Manual NAME su -- substitute user identity SYNOPSIS su [-] [-flm] [login [args]] ...

nu2ycombinator 14y ago

Copy pasting the best comment from the article. It basically covers the crux. ======================================================== "jonkx using on | May 31, 2010 at 10:22 am | Permalink I worked with the Unix OS from the early 1980′s (at first we ran early versions on a DEC PDP-11) until 1992. Starting in 1992, I did contract work on proprietary versions based on SVR4. It was just easier to say the initials “S U” or “super-user” than to say “switch user” or “substitute user”. To insist on correctness, whether authoritarian, historical or otherwise seems petty. Depending on the options used, su can be used to switch user, substitute user or become “super user”. Success depends on knowledge of the appropriate password. “sudo” on the other hand may allow one to become any other user (depending on the configuration of sudo and being a “sodoer”) knowing only the login password. On the systems I have used, a sudoer can become root (“super user”) using this shell command at a terminal: sudo su – root and responding to the prompt with the login password used to sign in the current user. I think it is important to point out that you cannot become “super user” or root from a shell with the su command alone unless there is a root password and that password is entered at the prompt. =========================================

mrb 14y ago
To all people who type "sudo su - root": the much simpler "sudo -i" accomplishes the same (-i launches an interactive shell).
- Osiris 14y ago
  I thought "sudo -s" was the correct way to drop into a root user shell.
  - ralph 14y ago
    No, unless one understands the subtleties, it may be best to use -i. $ sudo -s $ pwd /home/ralph $ exit $ sudo -i root@orac:~# pwd /root root@orac:~# logout $ $ sudo strace -fe execve sudo -s execve("/usr/bin/sudo", ["sudo", "-s"], [/* 16 vars */]) = 0 execve("/bin/bash", ["/bin/bash"], [/* 16 vars */]) = 0 ... $ exit $ sudo strace -fe execve sudo -i execve("/usr/bin/sudo", ["sudo", "-i"], [/* 16 vars */]) = 0 execve("/bin/bash", ["-bash"], [/* 16 vars */]) = 0 ... root@orac:~# logout $

ppierald 14y ago

I would recommend against using sudo su -, esp if you have <user> ALL=(ALL) NOPASSWD: ALL or such constructs in your /etc/sudoers file that exempt password requirements from executing sudo commands. 1. su gives you a root shell. Principle of least privilege dictates you should executes the fewest commands as root as required, not all commands as root because you need one. 2. sudo su - loses auditability of who executed what/when. All we know is that someone got a sudo shell at some point. When the forensics guys come along post-breach, you won't have a lot of good info to give them. 3. Loss of a shell (via command injection on a web form) may allow remote Command-and-Control of your machine. If you need a cron or other headless process (daemons, etc.) then use NOPASSWD in conjunction with a whitelisted set of commands (not /bin/bash) to be executed. If you need an operator to execute a command as a specific user, then use sudo -u <user> <cmd>. Even if you allow ALL commands, at least your audit logs will know what/when the command was executed. With all powerful commands comes the responsibility of understanding how to use them safely and what the possible repercussions are.

mverwijs 14y ago
4. And if you need to do anything extensive as root on a any machine, you might want to consider doing it with a configuration management system (puppet, chef, ansible).
rlpb 14y ago
In production, yes. For debugging on a test machine, or during a post-mortem, it's just inconvenient not to. If the majority of your time is spent in the shell of a production machine, you're doing something wrong.

davidw 14y ago

In Italian it means "up".

kunil 14y ago
In Turkish, it is water

klrr 14y ago

I think it really stands for "super user", here's its man page from Unix v1 http://man.cat-v.org/unix-1st/1/su http://man.cat-v.org/unix-1st/1/su .

deleted 14y ago
[deleted]
UnoriginalGuy 14y ago
> su allows one to become the super--user, who has all sortsof marvelous powers. I enjoy how informal that man page is. All of today's man-pages would never get away with "fun."

acc00 14y ago

My beard is rather short as of yet, but I've always read 'su' as an acronym for 'setuid' (because that's what it does).

asveikau 14y ago

A bit off topic, but man it really bugs me when people (like this author) say "command" because they don't know the right word. (A mistake made by this article) "I wonder why the creat() command doesn't have an 'e'." (Similar examples I've seen around the web) "I'm using the printf command." "I was doing assembly programming and I used the jump command." Syscall! Function! Instruction! Let's be precise people.

wolfhumble 14y ago
So to be even more precise ;-) are you treating Syscall, Function and Instruction as synonyms?
- mdwrigh2 14y ago
  I don't think he is. He references creat, printf and jump, which happen to be a syscall, function and instruction, respectively.
pbhjpbhj 14y ago
Isn't "command" [at least conversationally] a supergroup of syscalls, functions and instructions?

nathell 14y ago

When perusing early Unix code, it's striking how concise and down-to-the-point it is. We've really accumulated a lot of cruft (mostly for the sake of compatibility and feeping creaturism) over the years.

4ad 14y ago

This is a great article and people quoting modern manual pages miss the point. The point is that the meaning of su(8) changed and that is conveyed through an nice anecdote where the author discovers exactly how it changed. In the research version of Tenth Edition Unix, su(8) is described as: su, setlog -- substitute userid temporarily, become super user Which is an interesting intermediate step from today's (or rather 2.9BSD's) su -- substitute user identity found in BSD derivatives. Btw, the Tenth Edition manual was written mostly by Douglas McIlroy, inventor of pipes. Even if the 1127 group imported outside code for the VAX versions of research Unix (v8-10), they liked a particular kind of documentation, which the imported code lacked, so they wrote their own. Dare I say the style of research Unix manuals closely matches the style of Go documentation today (unremarkable, considering the pedigree).

shizcakes 14y ago
This comment has it correct. For those of you on a Mac (and I specifically mention a Mac, due to its BSD userland utilities), you can see that this is still defined this way today: type `man su` and you'll see: su -- substitute user identity

bnegreve 14y ago

> “su” was written to only change to the root user on the system. It wasn’t designed to switch to any other user that has an account. “su” meant “super-user”. I need to sit down for a second. It is slightly over dramatized, isn't it ? :)

deleted 14y ago

[deleted]

zx2c4 14y ago

q = password; while((*q = getchar()) != '\n') if(*q++ == '\0') return; Notably, you don't actually need the password... Buffer overflow!

qompiler 14y ago

What a cute su.c file! You should see this beast in Solaris! http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/cmd/su/su.c http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/sr...

jonisalonen 14y ago

Oh, so it isn't an acronym for suit-up? </barney>

drucken 14y ago

To old hands, su has always and will always mean super user. The meaning does not "change" (even if the program does).

michaelfeathers 14y ago

It's funny how the article and most of the comments here assume that there is one meaning and it is "out there" for all of us to see. Maybe there isn't any one definite meaning: http://michaelfeathers.typepad.com/michael_feathers_blog/2013/02/the-fallacy-of-one-definite-meaning.html http://michaelfeathers.typepad.com/michael_feathers_blog/201...

jasonkostempski 14y ago

Taking 'super-user' to mean 'root' seems like a big assumption. Is there anywhere in the Unixverse where root is specifically referred to as 'super-user'? I sort of thought 'super-user' would refer to any user with higher privileges. Well, really just different privileges, but why would you switch to a user with lesser permission to preform the action you probably just got denied? I assume the presented code was just that way because it covered 99% of use cases and was good enough for then and that root is now the default user for the same reason.

mturmon 14y ago
I'm not 100% sure where you're going with all that, but there are legitimate reasons for the super-user to "move down" into a specific user's identity -- for example, to be sure that files/directories created during some maintenance are owned by that specific user, and have that user's umask applied. Another instance would be file cleanups when the super-user specifically didn't want to be able to delete arbitrary files. There are lots of other cases.