Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!cbosgd!cbdkc1!pmd
From: pmd@cbdkc1.ATT.COM (Paul Dubuc)
Newsgroups: comp.lang.c
Subject: Re: system (and mkdir)
Message-ID: <2771@cbdkc1.ATT.COM>
Date: 20 Jan 88 20:26:48 GMT
References: <127@dcrbg1.UUCP> <9472@ccicpg.UUCP>
Reply-To: pmd@cbdkc1.UUCP (Paul Dubuc)
Organization: AT&T Bell Laboratories; Columbus, Ohio
Lines: 50
Keywords: c program


In article <9472@ccicpg.UUCP> miket@ccicpg.UUCP (Mike Tracy) writes:
}In article <127@dcrbg1.UUCP> bcf2303@dcrbg1.UUCP (Wing Chow) writes:
}>
}>can someone give me an example of how to use 'system' in a c program?
}
}The best example I have is the mkdir command.  Since a normal
}user can not create a directory. The mkdir (under Unix) is a SUID root
}program. That is, when the program is envoked its effective user id is
}that of the super user (stupid user :-)) root.
}
}To make a directory from you C program (unless your program is
}SUID root) you must use the system call (with the appropriate checks
}for success).
}
}	system( "mkdir mydir" );

No, you don't have to you system() for this. You can use fork()/exec().
See my previous article on this subject.  For the reasons I gave in
that article, system() is best reserved for running command strings that
need the facilities of the shell (pipelines, redirection, metacharacter
translation, etc.).

However, running mkdir from a C program either way brings up an interesting
example of another problem.  Suppose I (pmd) have a program that runs set-uid
to user 'joe'.  This program exec's mkdir to create a directory.  If it
then tries to change the mode, owner or group on the directory it just
created, it can't.  Why?  Because mkdir runs set-uid to root, creates
the directory and then sets the owner to the real user ID of the process
(pmd in my case).  My program is running as 'joe' and can't change
the mode, owner or group on any directory it creates because it is not
the owner (and not 'root' either).  This is a problem because it's good
practice for programs that create directories to be selective about the
permissions of those directories and the lack of a solution means your
program may be "broken" by merely setting the set-uid bit*.  It's also a
needless security risk to make the program set-uid to root just to avoid
this problem.

I know how much readers of this group like puzzles, so I won't detail the
solution (it's fairly simple, anyway).  For my solution, I implemented a
mkdir() subroutine that runs the mkdir command (with fork() and exec(),
not system()) and takes mode, owner and group specifications as arguments,
so the whole proceedure is isolated to one routine.  This allows a safe,
easy way to create directories independent of the real or effective user ID
of the calling process.

--
* The set-uid bit is patented by Dennis Ritchie.
-- 

Paul Dubuc	{ihnp4,cbosgd}!cbdkc1!pmd