Studying a Few Tiny Programs

1. The dir ../StudyPrograms has several tiny files of C src code. ls -lg

-rw-r--r-- 1 pmateti   494 Aug 29 19:52 arrays.c
-rw-r--r-- 1 pmateti   228 Aug 29 19:52 example1.c
-rw-r--r-- 1 pmateti   270 Aug 29 19:52 example2.c
-rw-r--r-- 1 pmateti   159 Aug 29 19:52 execsh.c
-rw-r--r-- 1 pmateti    59 Aug 29 19:52 exit.c
-rw-r--r-- 1 pmateti 17775 Aug 29 20:42 index.html
-rw-r--r-- 1 pmateti  5137 Aug 29 20:42 index.org
-rw-r--r-- 1 pmateti   403 Aug 29 19:52 Makefile
-rw-r--r-- 1 pmateti   450 Aug 29 19:52 recurse.c
-rw-r--r-- 1 pmateti  1817 Aug 29 20:08 recurse.s
-rw-r--r-- 1 pmateti 16848 Aug 29 20:32 syslib
-rw-r--r-- 1 pmateti   415 Aug 29 19:52 syslib.c
-rw-r--r-- 1 pmateti   387 Aug 29 19:52 testsc.c
-rw-r--r-- 1 pmateti   776 Aug 29 19:52 yasc.c

1. Study the source to see what syscalls and library calls they make, and how the programs are to be built, and the numerical relationships between the addresses of variables.
2. What is the max amount of virtual memory can a Linux process have?
3. All these programs are expected to be invoked as non-root.

1. make will compile and link one program with no error messages. It is using the Makefile as-is, and CFLAGS = -g -Wall -ansi -pedantic Explain what these flags cause gcc to do. Expand the Makefile to become complete, so that all included programs in the directory are built properly.

1. Understand how arguments are passed to parameters.
2. E.g., how does function(1, 2, 3); in main pass arguments to procedure named function?

1. Lookup the man page of strcpy.
2. In C, a string is an array of chars ending in '\0' (ASCII NUL).
3. Spot sloppy programming.

1. Learn to use execve
2. name is an array of pointers to strings.
3. What is volume or section 2 of man pages?

1. How many system calls does this tiny program have?
2. Tradition is that Unix/Linux processes return, as they terminate, (i) a zero on "success", (ii) a negative integer on error(s).

1. gcc generates code for functions such that the return value is in the "accumulator" register A (RAX/ EAX/ AX/ …).
2. Try gcc -s recurse.c. Figure out what it does.
3. What are "segment registers"? What is a "Segmentation Fault"?
4. get_sp() is written in Gnu Assembly. It copies the stack pointer register (SP ) to the A register.
5. On my Linux PC, invoking it as in ; ./recurse prints

x 0 sp fea65670
x 1 sp fe965650
x 2 sp fe865630
x 3 sp fe765610
x 4 sp fe6655f0
x 5 sp fe5655d0
x 6 sp fe4655b0
Segmentation fault (core dumped)

before dying. My prompt string PS1 is ; .

1. Improve [whatever] so that it recurses more deeply. Explain what you did.
2. Must leave the array a the same.

1. Study its output.
2. Learn
   1. sizeof
   2. address computations.
   3. the syntax of printf format string.
3. If we were to exchange the two lines char b[5]; char c[3];, predict what the output will be [without re-compiling/ running].

1. This may appear tricky, but this kind of code is quite common in systems programming.
2. The bytes stored in scb[] are the x86 machine code manually produced by compiling the so-called "shell code", which invokes the shell through a system call.
3. rap stands for return-address-ptr.
4. First, experience the code by compiling and linking the code as in, gcc testsc.c -o testsc, and invoking it.
5. Invoking testsc as produced above, as-is, is going to produce *** stack smashing detected ***: <unknown> terminated; Aborted (core dumped)"
6. Re-compilining as in gcc -fno-stack-protector -z execstack testsc.c -o testsc, and invoking will terminate "silently". What did it do? We explore this topic in a few weeks under the topic of "stack smashing."

1. Similar to the above.
2. Explains the bytes stored in sc[] in the comments.
3. The ptr fp stores the address of a void method that takes no arguments.
4. In (void *) sc the (void *) is doing free type coercion. "Free" because this is happening at compile time, not run time. The type of sc, which is char [] is being coerced into a void *. [Are parens necessary here? [Advice: When in doubt, parenthesize!]]

1. Library Calls v. SysCalls
2. Build syslib. Explain the output.

1. Wikibooks, https://en.wikibooks.org/wiki/X86_Assembly/X86_Architecture, 2019.