A tutorial on setuid and sticky bits in Unix

Unix users are sometimes confused with setuid and sticky bit permissions on files and directories. Below is a small tutorial on the most common use of setuid and sticky bits.

There are certain files or devices that are writable only by root. Nevertheless, ordinary users often have to use root-owned programs that try to write to those files. Since these files are writable only by root, how would a non-root user run the program that writes into that file. The solution in Unix is setuid bit. When this bit is set on a root-owned program, the program gets the effective privileges of root even when run by non-root user. This happens only for setuid programs i.e. for programs that have setuid bit set by owner/root. Example is passwd program that modifies the password files that are writable only by root. Since passwd program is setuid id, any non-root user can run this program and modify his/her password. Another example is ping program that is also setuid since ordinary users also run the ping program that accesses network devices.

In the file permission listing found by command “ls –l” setuid programs have the s in place of x. This means the program is both executable by owner and setuid. Another possibility is S in place of x which means the program is setuid but NOT executable. The program permissions could look like

rwsr-xr-x ( executable by root and setuid; what matters for setuid bit is the third letter)

OR

rwSr-xr-x ( hmmm…does it make sense !?)

One can make a program setuid by

chmod u+s prog_name

OR

chmod 4755 prog_name

Although the explanation above was specific to root-owned files, it could apply to any owner. So any user can make a program owned by himself/herself setuid and let others modify some of the owner’s files by this program.

Another related special permission setting for a program is setgid bit which is similar to above, but is useful only to the owner’s group. The setuid and setgid bits have different meanings when applied to directories. setgid bit on directory "d" means that any file or directory created under it would get the group id of "d". Remember that normally the group id of any newly created directory is the group id of the user who created it.

Yet another special permission is the sticky bit. Now-a-days, it is mainly used for directories. Let us first understand what directory permissions mean. Some directories are writable by all users. That means all users can create files as well as delete files inside such directories. Execute permission for a directory means search permission into that directory. So, execute permission is necessary to descend into that directory.

An example of directory with sticky bit set is /tmp. This directory stores temporary data that is created by user programs. Since these directories are writable by all, any user can delete any file! Even those files that are owned by others! To fix this state, such directories have sticky bit set. Now only the owners can delete any file in such directories.

Directories with sticky bit set have t letter in the execute permission set for others, as seen by the output of “ls –l”.

rwxrwxrwt ( sticky bit is set and execute permission for others)

OR

rwxrwxrwT ( sticky bit set but no execute permission for others)

Capital T means that others don’t have execute permission for that directory, so they can’t search into it.

update: Added setgid directory explanation.

How Unix shell executes commands

Unix shell runs all the commands in a new forked child process. It means, the shell in which the command is invoked becomes the parent that creates a child process ( a shell process). The child in turn exec's the command by overlaying itself with the command's image. Why would the new process be created for executing the command? Can't the shell run the command in the same process. The answer is:
If the shell overlays itself with the command's process image, it would have nowhere to return when the command finishes executing. This would close the parent shell. Running "exec command_name" effectively does this i.e. runs the command that evetually closes the shell when the command execution is over. To avoid closing the invoking shell itself, all commands are run in a new child process by the shell.

We can use "strace" utility to track down the child process creation and execution system calls. Note that doing strace on the command will only trace the command after fork i.e. the output will show execve(...) followed by other syscalls ending with some exit call. This is so because we are tracing only the command which really is in the child process. To trace how the shell has created a new child process by fork() (clone() family in Linux), do strace on the current shell in a separate terminal and then run a command in the current shell.

PS: edited off the 'subshell' part as that is a different beast altogether...

Software engineering vs teaching

I faced a reality check last week. I always wondered why anyone, competent and educated in the field of computers, would pursue a career in teaching and not join a company as software engineer. It was perhaps the inability of the person to cope up with the demanding environment in the software field. That could be the reason anyone leaves a better paying job and settles for lowly-paid teaching career - or so I thought.

Last week I sat down and tried to list the pros and cons of the two careers.
Here's what I got from my "findings".

Let's tackle the salary part first. Since a teaching career asks for a formal degree, preferably a post graduation, let's assume the person is a post graduate in Computers.

Now, if the person choses to join an IT career, he has to devote at least 40 hours a week for work; in reality it could be much more if one takes into account the unpaid "late-nights". Assuming a 30 days month, and a good INR 50,000 a month in salary, the person makes about INR 285/hr.

On the other hand, a teacher at an engineering institute has about 18 hrs a week of teaching load. For INR 40,000 a month, the teacher would be making about INR 500/hr on average - a full 215/hr more on average than his counterpart!

Other pros of a teaching job vs software engineering -

- no late-night unpaid overtime work that one is expected to do
- better quality of life, better family life as well; a teacher can spend much more time with family
- no continuous sitting-in-front-of-computer-screen until you visit your doc for back-pain; backache is known to be a common ailment with IT professionals
- less stress; no deadlines
- stability; none of those pink-slip days inspite of very good performance. It's not uncommon to find star-achievers, poster-boys being given a boot if the head honchos make company wide cost-cutting plans

Taking all the above into account, I wonder why anyone would prefer IT career over teaching if given a choice. Is it for that extra few thousands? Consider this! IT companies are located mostly in places where the extra salary of a software professional would be spent on rent and other costs. A teacher can take up consultancy work and could earn even more ...

C++ inheritance - public, private, protected

Inheritance in C++ could be public, private, or public. It could be confusing for a beginner to find out accessibility of members and objects of these classes. Though public inheritance is mostly used, especially if there are virtual functions in the base class, other two types of inheritance have their uses as well. Here's an example program to make it clear how the 3 of them work i.e. public inheritance, private inheritance, and protected inheritance.

#include

class B{
char a;

public:
char b;

protected:
char c;
};

// public inheritance

class D:public B{

// private members of B can't be accessed
//can access public and protected members of B

char d;

public:

void f(){
//can't do this
// d = a;

d = b; // works
d = c; // works
}

// class D has the following additional members inherited from B
// public: char b
// and
// protected: char c
// this means that any class derived from D can access both b and c
// b is accessible by objects of derived class, c is not (#1)
};

// private inheritance

class E:private B{

// private members of B can't be accessed...
// can access public and protected members of B

char d;

public:
void f(){
//can't do this
// d = a;
d = b; // works
d = c; // works
}


// class E has the following additional data members inherited from B
// private:char b
// and
// private: char c
// this means that any class derived from E can't access any of the above derived data
// members, nor are they accessible to objects of derived class, public functions
// though are accessible to objects of this class (#2)

};

// protected inheritance

class F:protected B{

// private members of B can't be accessed in this class as above, so
// can access public and protected members of B

char d;

public:

void f(){
//can't do this
// d = a;
d = b; // works
d = c; // works
}

// class F has following additional members inherited from B
// protected:char b
// and
// protected: char c
// this means that any class derived from F can access
// the above members, but they can't be accessed from
// derived class objects (#3)
};

class DF: public F{
char d;

public:
void g(){
d = b; // works from #3 above
d = c; // works from #3
}
};



int main(){

char i;

B b;
D d;
E e;
F f;

// these won't work by definition
// b.c = i;
// b.a = i;


// for object d of publicly derived class
// this works from (#1)
d.b = i;
// this won't work from (#1)
// d.c = i;


// for object e of privately derived class
// these won't work from (#2)
// e.b = i;
// e.c = i;


// for object f of protected-ly derived class
// these won't work from (#3)
// f.b = i;
// f.c = i;

return 0;
}

Small company or MNC

One of my friends has recently graduated and is in a dilemma on which among the two offers to accept. An offer he has is from a multi-billion, multi-thousand-employees product MNC where he would be using C language and Unix at work that is about debugging one of the company products. The other offer is from a small EDA company where he would be using C++ and Windows as a developer of EDA tools for engineers. The smaller company has less than 50 people on roll. The CEO himself interviewed my friend and has suggested that the friend would have more opportunities to learn and grow in a smaller company like his one.

On the other hand, the big MNC's salary offer is more than twice that of the smaller company's.

Friend is in a dilemma and is seeking my advice. Which company should he join?

pointer pitfall

Some C pointer fundamentals manage to fox even seasoned programmers.
For example, what is wrong with the following piece of code:

func() {

char *c;
c = "this is a string.";
*c = 'T';
printf("%s", c);
}


The code tries to capitalize the first letter of the string. It looks correct at first glance, but is not. It might even compile without any error, but will throw out an error when run.

Fun with JavaScript

This is a JavaScript I found while stumbling.

It works like this. Just search for an image in Google. Once it shows up images as the result of your search, enter the script (without quotes) in the address bar of your browser. It will rotate the images around. If you keep pressing enter again and again, the speed of rotation keeps increasing as well.

UltraSparc T2 : Mainframe-on-a-chip?

In my previous post, I wondered whether Niagara 2 (ULtraSparc T2) could run 64 different OS instances or were they just Solaris Zones (containers). It is confirmed that it could indeed run 64 OS instances using LDom built-in technology in T2. I found this Sun blog post that demonstrates 64 instances of Solaris running in 64 T2 threads. In another post it goes further and shows Solaris and Ubuntu running simultaneously. That makes T2 one helluva multicore chip!

Niagara 2 i.e. UltraSparc T2

It's very sad that most of the processor news in the world is confined to x86 only. When I commented on processors last year, I mentioned Niagara chip ie UltraSparc T1. Now Sun has come out with the second version of the energy efficient Niagara 2, officially named UltraSparc T2. With 8 cores in the chip and 8 threads per core it will have a total of 64 hardware threads. The 8 threads in each core would run in an I/O multiplexed way i.e. at any time only 1 thread can run and another thread will switch in if the running thread enters I/O cycle. That means at any point of time, 8 threads will be running simultaneously in the chip. UltraSparc T1 was similar but had 4 threads per core, for a total of 32 threads.

From the webcast on Sun's page, I found the following interesting information on T2 chip:

One major difference between T2 and T1 is that T2 has a floating point unit on each core. T1 had one such unit for the entire chip that made it practically useless for floating-point intensive tasks. T2 thus seems to have taken care of that limitation. T2 threads run at 1.4 GHz.

For crypto intensive tasks there is a cryptographic processor unit on each core. Also, there are two PCI express I/O ports on the chip as well as integrated 10 Gb Ethernet.

Sun aims to sell this chip to other vendors as well if they want to use it in their servers. This is a departure from its earlier policy where it used its chips only in its servers and sold those servers.

This chip seems great for multi-threaded applications written in languages such as Java.

One thing that was funny in the webcast was that T2 was getting marketed as the fastest processor at 89.6 GHz. It was simply calculated by multiplying 64 by 1.4 GHz.
It was like saying a train with 10 coaches running at 100 miles/hr the fastest vehicle at 1000 km/hr!

A few times during the webcast it was mentioned that 64 Operating systems could run simultaneously in 64 threads due to LDom technology. I think they were talking about zones, and not really different OS's. Or is it possible to really run different OS's in that way? I think not but correct me if I am wrong.

UltraSparc T2 sure is a great chip with energy efficiency and should give good throughput for well written applications, at a very low form-factor in a data center.

Preventing Gmail cookie stealing

There has been a news of a vulnerability from the use of cookies by email sites like Gmail at Wi-Fi hotspots. Cookies can be stolen by using sniffing softwares and entire session can be hijacked to do malicious things on the target accounts. A simple method to stop such attacks is to use SSL for the entire session, not just for login that gmail does by default. A nice add-on from CustomizeGoogle can be used for making sessions use SSL. In addition, there are many other cool features we get on installing this add-on to Firefox browser. These features can be selected from Tools menu of Firefox and includes options such as making ads invisible in gmail and google search results. Also, links to search results from Yahoo and other popular search engines can be added for the same search string in Google search.