1. Static Linking. CS3214 Spring 2012 Exercise 5

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1 Due: Friday, Mar 16, :59pm (no extensions). What to submit: Upload a tar archive that contains a text file answers.txt with your answers for the questions not requiring code, as well as individual files for those that do, as listed below. This exercise is intended to reinforce the content of the lectures related to linking using small examples. As some answers may be specified to our current environment, you must again do this exercise on our rlogin cluster or the lab machines. 1. Static Linking To effectively work with object files and use libraries, you should be familiar with a number of tools. 1. Familiarize yourselves with the nm command (use info nm, for instance), nm displays the values of symbols contained in object files and libraries along with their types. Create a file symbols.c that, when compiled, contains the following symbols: $ gcc -c symbols.c $ nm symbols.o C C D D T T U U b b d d r r t t (The first column need not match.) Include symbols.c in your tarball! 2. What does the command strip do? 3. What does the command size do? What is the total amount of compiled machine code contained in all object modules of /usr/lib/libc.a? Provide the number as a decimal counted in bytes! 4. The directory /home/courses/cs3214/linking contains a subdirectory coreutils build which contains a number of object files and libraries. Unfortunately, the Makefile containing the build instructions is missing. Your task is to build the program cat whose main() function is defined in src/cat.o Start by issuing these commands: 1

2 cp -R cs3214/linking/coreutils-8.13-build. cd coreutils-8.13-build/src gcc -o cat cat.o As you encounter undefined symbols, determine which object files and/or libraries you need to add to the command line until your executable links successfully. Find out which.o files and/or libraries you need to include to build cat successfully! Provide the command line you issued in your answer. You must not include any unnecessary libraries or object files. Hint: Use nm to identify those object files and/or libraries that contain the symbols upon which cat.o depends. 5. Linker map. Learning how to read linker maps is a useful skill that can help debugging linking problems. Add -Wl,-Map -Wl,linkermap to the command line obtained above. Examine the resulting file linkermap and answer the following questions: (a) Which module provides the symbol fdadvise? (b) Which undefined symbol prompted the linker to include the object module mbchar.o? 2. Pointer Puzzle Consider the following program: /* * sort_pointers.c * * Relating pointers to link/runtime layout. * * You should implement the body of weave_pointers in a file * sort_pointers_solution.c, which is to be #included in this * file. * * CS 3214, Spring */ #include <stdio.h> #include <stdlib.h> #include <assert.h> struct list { struct list *next; ; struct list x; #define N 10 static struct list z[n]; 2

3 const struct list c = {.next = 0 ; struct list y = {.next = (struct list *)&c ; static void weave_pointers(struct list *lp, struct list *sp) { /* Implement weave_pointers such that the program prints "Success" * You may not define any static variables in this function, nor * are you allowed to make use of dynamic memory allocation. */ #include "sort_pointers_solution.c" int main(void) { static struct list s; struct list l, *p; int i; int success = 1; weave_pointers(&l, &s); for (i = 0, p = &l; p && i < 20; i++, p = p->next) { if (p < p->next) { printf("pointers out of order: %p < %p\n", p, p->next); success = 0; if (i!= 15) { printf("list has only %d elements, should be 15\n", i); abort(); assert(success!!!"at least one element was out of order"); printf("success.\n"); return 0; which arranges a single-linked list that connects various variables defined in the program. Implement sort pointers solutions.c such that this program outputs Success! Note: You may not use dynamic memory allocation or define static variables in weave pointers. 3. Dynamic Linking 1. Observing Dynamic Linking. When a program starts, the dynamic linker ld-linux.so resolves the program s dynamic dependencies. On Linux, the function of the dynamic linker can be controlled using various environment variables, as described in ld.so(8). The shell script ldd displays an executable s shared library dependencies 3

4 by performing a dry-run that resolves the executable s dynamic dependencies without actually running the program. The environment variable LD DEBUG provides information about what the dynamic linker is doing. Run the following program and answer the following questions env LD_DEBUG=all id (a) The first shared library that is needed is libselinux.so.1. In which directories does the linker look for this file? (b) Give the full path of the actual file being used! 2. Building a Shared Library. The directory /home/courses/cs3214/linking/whoami contains an dynamically linked executable whoami. This executable does not run because a shared library is missing. > /home/courses/cs3214/linking/whoami/whoami /home/courses/cs3214/linking/whoami/whoami: error while loading shared libraries: libwhoami.so: cannot open shared object file: No such file or directory Implement, compile, and link a shared library that implements the functions getunixname() and getrealname() defined in /home/courses/cs3214/linking/whoami/- whoami.h The output should be your Unix id, followed by your real name. For instance, when I run the sample solution after placing it in the current directory (as./libwhoami.so), it would show: > env LD_LIBRARY_PATH=. /home/courses/cs3214/linking/whoami/whoami gback (Godmar Back) Hints: Make sure to invoke whoami using its full path so that you do not accidentally run the system program /usr/bin/whoami by the same name. Make sure to set LD LIBRARY PATH to include the directory in which libwhoami.so is contained! Use the function getuid() and getpwuid() to implement the required functionality. You may not recompile or relink whoami.c; you must use the executable in /home/courses/cs3214/linking/whoami/whoami. Submit your libwhoami.c file and a script makelibwhoami.sh which, when run by the TA, will produce the necessary.so file. 4

5 3. Hiding Files. A rootkit is a set of programs, libraries, and scripts designed to hide the fact that a system was compromised. Some rootkits use a system s dynamic loading facilities to hide their presence. For instance, an attacker may wish to hide the presence of certain files in a directory. The LD PRELOAD environment variable instructs the dynamic linker to load one or more libraries before loading the shared libraries referenced in an executable. Thus, if a symbol is defined in such a library, the program s reference will be resolved against the preloaded symbol, allowing it to intercept calls. In this exercise, create a shared library invisible.so which, when added to LD PRELOAD, will hide the existence of files whose name starts with the prefix invisible. Although a real rootkit would need to make sure that no call reveal the presence of this file, for the purposes of this exercise, you need to intercept only the readdir function that is used, for instance, by /bin/ls. Consult the manpage for readdir(3). The following file provides a skeleton you should fill in. #include <string.h> #include <stdio.h> #define USE_GNU 1 #define USE_LARGEFILE64 1 #include <dirent.h> #include <dlfcn.h> /* Skip files with this prefix */ #define INVISIBLE "invisible_" /* Define function pointer type that matches the signature of readdir */ typedef struct dirent * (*readdir_t)(dir *dir); /* Intercept readdir. If the call would return a directory entry * whose name starts with INVISIBLE, return the next entry instead. * Otherwise, return the original directory entry. */ struct dirent *readdir(dir *dir) { #include "readdirsolution.c" Submit readdirsolution.c! To obtain the value to which readdir would have resolved without interposition, use the dlsym() function, passing the special handle RTLD NEXT. To following sample session shows how invisible.so should work $ mkdir readdir-test $ touch readdir-test/a_file $ ls readdir-test a_file 5

6 $ touch readdir-test/invisible_file $ ls readdir-test a_file invisible_file $ env LD_PRELOAD=./invisible.so ls readdir-test a_file Submit readdirsolution.c and a file makeinvisible.sh that builds it. 6