dolohow/ctest.txt

## ctest.txt
> 1) Write a C (C99) program which reads a standard input and prints each line in reversed order. Assume '\n' as line separator. Do not assume limit of number of character per line. Example:
>
>       $ printf "123\nhello\n" | ./a.out
>       321
>       olleh

     #include <stdlib.h>
     #include <stdio.h>
     #include <string.h>

     void strrev(char *str, size_t n)
     {
             /* Because xor swap is less efficient on modern CPU */
             for(size_t i = 0; i < n/2; ++i) {
                     char tmp = str[i];
                     str[i] = str[n - i - 1];
                     str[n - i - 1] = tmp;
             }
     }

     int main()
     {
             FILE *stream = stdin;
             size_t bufflen = 128;
             char *buff = malloc(bufflen);

             while (fgets(buff, bufflen, stream)) {
                     size_t slen = strlen(buff);

                     /*
                      * When last element is not a new line and stream is not
                      * EOF that means we exhausted buffer
                      */
                     while (buff[slen - 1] != '\n' && !feof(stream)) {
                             /* Increase buffer twice the current length */
                             bufflen *= 2;
                             buff = realloc(buff, bufflen);

                             /* Continue where NULL terminator sits */
                             if (!fgets(&buff[slen], bufflen/2, stream))
                                     break;

                             slen = strlen(buff);
                     }

                     /*
                      * New line characters acts as a separator, we don't
                      * want it to be reversed and precede our string.
                      * So let's terminate string earlier.  \n will be
                      * added by puts at the end after all.
                      */
                     if (buff[slen - 1] == '\n')
                             buff[--slen] = '\0';

                     strrev(buff, slen);
                     puts(buff);
             }

             free(buff);

             return EXIT_SUCCESS;
     }

> 2) Write a command line to compile a program consisting of two .c files.

     gcc a.c b.c

> 3) Write short program in C to demonstrate stack overflow.

     #include <stdio.h>
     #include <sys/resource.h>

     int main()
     {
             struct rlimit lim;
             /* Get maximum stack size in bytes */
             getrlimit(RLIMIT_STACK, &lim);

             /* Allocate array of the maximum size of stack */
             char stack_overflow[lim.rlim_cur];

             /* Following call to printf will end up with SIGSEGV */
             printf("Hello world");
     }

> 4) Write a command line to link two object files and a math library (libm).

     ld -dynamic-linker /lib64/ld-linux-x86-64.so.2 /usr/lib/crt1.o \
       /usr/lib/crti.o /usr/lib/crtn.o --no-as-needed -lc -lm a.o b.o -o a.out

     or

     gcc a.o b.o -o a.out -lm

     Where `a.o`, `b.o` are those two object files and `a.out` is a elf
     executable output.

> 5) Name your 5 favourite C functions from standard C library.

     printf, malloc, memcpy, rand, strtok

> 6) Write short program in C to demonstrate buffer overflow.

     #include <string.h>
     #include <stdio.h>

     int main()
     {
             char src[] = "hello word, hello word";
             char a[] = "foo foo foo";
             char dest[2];
             /*
              * I intentionally do not copy null terminator so the result is
              * better visible in printf
              * */
             memcpy(dest, src, 22);

             /*
              * Will print "o wordo foo" instead of "foo" due to buffer
              * overflow * which will cause overwrite the place where
              * char *a is.
              * */
             printf("%s\n", a);

             /*
              * Due to alignment there is 16 bytes difference in their
              * addresses which means 6 first bytes of char * (22 - 16) will
              * be overwritten so the result is clear.
              *
              *                 foo foo foo
              * hello word, hello word
              *                 o wordo foo
              * */
             printf("dest: 0x%x\n", &dest);
             printf("a:    0x%x\n", &a);
     }

> 7) How to compile a C program with debug symbols?

     gcc a.c -g

> 8) How one can strip debug symbols from a binary?

     strip --strip-debug a.out

> 9) Write a makefile with targets to compile a program (e.g. from question 1) and do a clean up.

     SRCS = main.c
     EXECUTABLES = main
     OBJS=$(SRCS:.c=.o)

     CC=gcc
     CFLAGS=-Wall -std=c99

     all: $(EXECUTABLES)

     $(EXECUTABLES): $(OBJS)
      $(CC) $(CFLAGS) $(OBJS) -o $@

     .c.o:
      $(CC) $(CFLAGS) -c $< -o $@

     clean:
     	rm -rf $(EXECUTABLES) $(OBJS)

     Note: I had all those programs in one file so I could just create a
           hardcoded target, but I suppose it would not be that
           interesting seeing something like that.

           main:
            gcc -std=c99 main.c -o main

## shelltest.txt
The following questions/tasks focus on your ability to run the appropriate
command from the command line (running /bin/sh or compatible [ksh, bash, ...]).
The answers should be one-liners if possible.

Note that it is not required to answer all the questions for the successful
application for the job.

Example:
	Q: List all files in the current directory.
	A: ls -a

1	Run the program "test.sh" and discard any output (standard and error)
	of that script.

  sh test.sh &> /dev/null

2	Copy a directory tree (with files) from /opt/test/ to your home
	directory using tar.

  tar cf - /opt/test | (cd ~/ && tar xf -)


3	Rename all .htm files in /tmp/test/ (but not subdirectories), so that
	they have .html extension instead of .htm. Ignore hidden files.

  find /tmp/test -maxdepth 1 -type f -not -path '*/\.*' -exec rename 's/\.htm$/\.html/' {} \;

  Note: I am aware that rename programs differs, for example default
        provider of rename on my Arch box comes from `util-linux`, but
        for the purpose of solving this test I am using virtual machine
        on Ubuntu and it comes from `perl`.


4	Print contents of all files in /tmp/test/ (but not subdirectories),
	including hidden files. Ensure that the filesystem object is a regular
	file.

  find /tmp/test -maxdepth 1 -type f -exec cat {} +


5	List all files (their full paths) in /tmp/test/ and subdirectories,
	that contain the left bracket "["

  find /tmp/test -type f | grep "\["


6	You have a file "/tmp/test/IMPORTANT" and a new version of it in your
	home directory named "IMPORTANT.new". Write a sequence of commands
	that would replace the old version of the file with the new one,
	and keep the backup named "/tmp/test/IMPORTANT.backup". There must
	be no race condition (time slice when the file is not there, or when
	there is just part of it).

  ln /tmp/test/IMPORTANT /tmp/test/IMPORTANT.backup
  mv ~/IMPORTANT.new /tmp/test/IMPORTANT

  Note: I did some additional investigation concerning race condition.
        I used `strace` while doing those two commands, first it calls
        `stat` and `lstat` and then
          linkat(AT_FDCWD, "/tmp/test/IMPORTANT", AT_FDCWD, "/tmp/test/IMPORTANT.backup", 0) = 0

        which is atomic. `mv` is atomic too, because it calls rename
        along with `lstat` and `stat`.
          rename("/home/vagrant/IMPORTANT.new", "/tmp/test/IMPORTANT") = 0

        The `rename(2)` says that this operation is atomic.


7	Transfer directory "/tmp/test/" (and everything below) to a machine
	named "orion" to directory "/var/tmp/test". Use ssh.

  scp -r /tmp/test orion:/var/tmp/test


8	Force apache to reload its configuration by sending a signal to it;
	"apache.pid" file is located in "/var/apache/".

  kill -USR1 `cat /var/apache/apache.pid`


9	In "/tmp/test" and subdirs there are files with ".dat" extension (among
	others). The files contain numbers separated by 0x0A (LF).
	Print the smallest number from each of the file.

   find -type f -regex ".*dat$" -exec sort -n {} \; | head -n1


10	There is a filesystem mounted on /usr containing new version of
	binaries. Create a hard link "/bin/perl" for file "/usr/bin/perl".

  ln /usr/bin/perl /bin/perl

  Note: If /usr reside on a different volume/partition etc. than /bin
        it is not possible to create hard link, only symbolic.


11	Set ownership to "root" group "sys" for directory "/tmp/test" and
	everything below.

  chown -R root:sys /tmp/test


12	Set rights for "/tmp/test/" tree (both files and directories), so that
	the owner can read and write files (but not execute), group can only
	read, and others are denied access altogether. The directories must
	be accessible for owner and group only.

  find /tmp/test -type d -exec chmod 750 {} \;
  find /tmp/test -type f -exec chmod 640 {} \;


13	Write a shell glob matching all files (and other objects) in the
	current directory, including hidden files, but not matching
	the "." or ".."

  !(..|.)


14	Set permissions for binary "/usr/local/bin/my_program", so that only
	"root" and members of the "adm" group can execute it; and when
	executed, the program runs under "daemon" UID.

  chown daemon:adm /usr/local/bin/my_program
  chmod a-x,u+x,g+x /usr/local/bin/my_program
  chmod u+s /usr/local/bin/my_program


15	Set permissions for "/tmp/test/", so that root cannot delete files in
	this directory.

  chattr -R +i test

  Note: This is more like a prevent deleting by accident. Nothing stops
        root from unsetting this attribute.  In addition one can have
        filesystem which does not support this.  If /tmp/test could be
        mounted read-only then this is another option, but again,
        nothing stops root.  Another option would be to experiment with
        SELinux policies and allow modifying them after reboot.


16	List lines in file "/tmp/test/pkgs" that start with SUNW then a
	hexadecimal number followed by a space, and end with an asterisk.

  grep -P "^SUNW[0-9a-fA-F]+ \*$" /tmp/test/pkgs

  Note: I assumed that hexadecimal number looks like 1AF or 1af, not
        0x1af.


17	Copy file "/tmp/test/pkgs" to your home directory, so that the new name
	is pkgs-YYYYMMDD (where YYYYMMDD is the current date).

  cp /tmp/test/pkgs ~/pkgs-$(date +"%Y%m%d")


18	List user names from /etc/passwd that have UID > 500

  awk -F: '{if ($3 > 500) print $1}' /etc/passwd
	> 1) Write a C (C99) program which reads a standard input and prints each line in reversed order. Assume '\n' as line separator. Do not assume limit of number of character per line. Example:
	>
	> $ printf "123\nhello\n" \| ./a.out
	> 321
	> olleh

	#include <stdlib.h>
	#include <stdio.h>
	#include <string.h>

	void strrev(char *str, size_t n)
	{
	/* Because xor swap is less efficient on modern CPU */
	for(size_t i = 0; i < n/2; ++i) {
	char tmp = str[i];
	str[i] = str[n - i - 1];
	str[n - i - 1] = tmp;
	}
	}

	int main()
	{
	FILE *stream = stdin;
	size_t bufflen = 128;
	char *buff = malloc(bufflen);

	while (fgets(buff, bufflen, stream)) {
	size_t slen = strlen(buff);

	/*
	* When last element is not a new line and stream is not
	* EOF that means we exhausted buffer
	*/
	while (buff[slen - 1] != '\n' && !feof(stream)) {
	/* Increase buffer twice the current length */
	bufflen *= 2;
	buff = realloc(buff, bufflen);

	/* Continue where NULL terminator sits */
	if (!fgets(&buff[slen], bufflen/2, stream))
	break;

	slen = strlen(buff);
	}

	/*
	* New line characters acts as a separator, we don't
	* want it to be reversed and precede our string.
	* So let's terminate string earlier. \n will be
	* added by puts at the end after all.
	*/
	if (buff[slen - 1] == '\n')
	buff[--slen] = '\0';

	strrev(buff, slen);
	puts(buff);
	}

	free(buff);

	return EXIT_SUCCESS;
	}

	> 2) Write a command line to compile a program consisting of two .c files.

	gcc a.c b.c

	> 3) Write short program in C to demonstrate stack overflow.

	#include <stdio.h>
	#include <sys/resource.h>

	int main()
	{
	struct rlimit lim;
	/* Get maximum stack size in bytes */
	getrlimit(RLIMIT_STACK, &lim);

	/* Allocate array of the maximum size of stack */
	char stack_overflow[lim.rlim_cur];

	/* Following call to printf will end up with SIGSEGV */
	printf("Hello world");
	}

	> 4) Write a command line to link two object files and a math library (libm).

	ld -dynamic-linker /lib64/ld-linux-x86-64.so.2 /usr/lib/crt1.o \
	/usr/lib/crti.o /usr/lib/crtn.o --no-as-needed -lc -lm a.o b.o -o a.out

	or

	gcc a.o b.o -o a.out -lm

	Where `a.o`, `b.o` are those two object files and `a.out` is a elf
	executable output.

	> 5) Name your 5 favourite C functions from standard C library.

	printf, malloc, memcpy, rand, strtok

	> 6) Write short program in C to demonstrate buffer overflow.

	#include <string.h>
	#include <stdio.h>

	int main()
	{
	char src[] = "hello word, hello word";
	char a[] = "foo foo foo";
	char dest[2];
	/*
	* I intentionally do not copy null terminator so the result is
	* better visible in printf
	* */
	memcpy(dest, src, 22);

	/*
	* Will print "o wordo foo" instead of "foo" due to buffer
	* overflow * which will cause overwrite the place where
	* char *a is.
	* */
	printf("%s\n", a);

	/*
	* Due to alignment there is 16 bytes difference in their
	* addresses which means 6 first bytes of char * (22 - 16) will
	* be overwritten so the result is clear.
	*
	* foo foo foo
	* hello word, hello word
	* o wordo foo
	* */
	printf("dest: 0x%x\n", &dest);
	printf("a: 0x%x\n", &a);
	}

	> 7) How to compile a C program with debug symbols?

	gcc a.c -g

	> 8) How one can strip debug symbols from a binary?

	strip --strip-debug a.out

	> 9) Write a makefile with targets to compile a program (e.g. from question 1) and do a clean up.

	SRCS = main.c
	EXECUTABLES = main
	OBJS=$(SRCS:.c=.o)

	CC=gcc
	CFLAGS=-Wall -std=c99

	all: $(EXECUTABLES)

	$(EXECUTABLES): $(OBJS)
	$(CC) $(CFLAGS) $(OBJS) -o $@

	.c.o:
	$(CC) $(CFLAGS) -c $< -o $@

	clean:
	rm -rf $(EXECUTABLES) $(OBJS)

	Note: I had all those programs in one file so I could just create a
	hardcoded target, but I suppose it would not be that
	interesting seeing something like that.

	main:
	gcc -std=c99 main.c -o main
	The following questions/tasks focus on your ability to run the appropriate
	command from the command line (running /bin/sh or compatible [ksh, bash, ...]).
	The answers should be one-liners if possible.

	Note that it is not required to answer all the questions for the successful
	application for the job.

	Example:
	Q: List all files in the current directory.
	A: ls -a

	1 Run the program "test.sh" and discard any output (standard and error)
	of that script.

	sh test.sh &> /dev/null

	2 Copy a directory tree (with files) from /opt/test/ to your home
	directory using tar.

	tar cf - /opt/test \| (cd ~/ && tar xf -)


	3 Rename all .htm files in /tmp/test/ (but not subdirectories), so that
	they have .html extension instead of .htm. Ignore hidden files.

	find /tmp/test -maxdepth 1 -type f -not -path '/\.' -exec rename 's/\.htm$/\.html/' {} \;

	Note: I am aware that rename programs differs, for example default
	provider of rename on my Arch box comes from `util-linux`, but
	for the purpose of solving this test I am using virtual machine
	on Ubuntu and it comes from `perl`.


	4 Print contents of all files in /tmp/test/ (but not subdirectories),
	including hidden files. Ensure that the filesystem object is a regular
	file.

	find /tmp/test -maxdepth 1 -type f -exec cat {} +


	5 List all files (their full paths) in /tmp/test/ and subdirectories,
	that contain the left bracket "["

	find /tmp/test -type f \| grep "\["


	6 You have a file "/tmp/test/IMPORTANT" and a new version of it in your
	home directory named "IMPORTANT.new". Write a sequence of commands
	that would replace the old version of the file with the new one,
	and keep the backup named "/tmp/test/IMPORTANT.backup". There must
	be no race condition (time slice when the file is not there, or when
	there is just part of it).

	ln /tmp/test/IMPORTANT /tmp/test/IMPORTANT.backup
	mv ~/IMPORTANT.new /tmp/test/IMPORTANT

	Note: I did some additional investigation concerning race condition.
	I used `strace` while doing those two commands, first it calls
	`stat` and `lstat` and then
	linkat(AT_FDCWD, "/tmp/test/IMPORTANT", AT_FDCWD, "/tmp/test/IMPORTANT.backup", 0) = 0

	which is atomic. `mv` is atomic too, because it calls rename
	along with `lstat` and `stat`.
	rename("/home/vagrant/IMPORTANT.new", "/tmp/test/IMPORTANT") = 0

	The `rename(2)` says that this operation is atomic.


	7 Transfer directory "/tmp/test/" (and everything below) to a machine
	named "orion" to directory "/var/tmp/test". Use ssh.

	scp -r /tmp/test orion:/var/tmp/test


	8 Force apache to reload its configuration by sending a signal to it;
	"apache.pid" file is located in "/var/apache/".

	kill -USR1 `cat /var/apache/apache.pid`


	9 In "/tmp/test" and subdirs there are files with ".dat" extension (among
	others). The files contain numbers separated by 0x0A (LF).
	Print the smallest number from each of the file.

	find -type f -regex ".*dat$" -exec sort -n {} \; \| head -n1


	10 There is a filesystem mounted on /usr containing new version of
	binaries. Create a hard link "/bin/perl" for file "/usr/bin/perl".

	ln /usr/bin/perl /bin/perl

	Note: If /usr reside on a different volume/partition etc. than /bin
	it is not possible to create hard link, only symbolic.


	11 Set ownership to "root" group "sys" for directory "/tmp/test" and
	everything below.

	chown -R root:sys /tmp/test


	12 Set rights for "/tmp/test/" tree (both files and directories), so that
	the owner can read and write files (but not execute), group can only
	read, and others are denied access altogether. The directories must
	be accessible for owner and group only.

	find /tmp/test -type d -exec chmod 750 {} \;
	find /tmp/test -type f -exec chmod 640 {} \;


	13 Write a shell glob matching all files (and other objects) in the
	current directory, including hidden files, but not matching
	the "." or ".."

	!(..\|.)


	14 Set permissions for binary "/usr/local/bin/my_program", so that only
	"root" and members of the "adm" group can execute it; and when
	executed, the program runs under "daemon" UID.

	chown daemon:adm /usr/local/bin/my_program
	chmod a-x,u+x,g+x /usr/local/bin/my_program
	chmod u+s /usr/local/bin/my_program


	15 Set permissions for "/tmp/test/", so that root cannot delete files in
	this directory.

	chattr -R +i test

	Note: This is more like a prevent deleting by accident. Nothing stops
	root from unsetting this attribute. In addition one can have
	filesystem which does not support this. If /tmp/test could be
	mounted read-only then this is another option, but again,
	nothing stops root. Another option would be to experiment with
	SELinux policies and allow modifying them after reboot.


	16 List lines in file "/tmp/test/pkgs" that start with SUNW then a
	hexadecimal number followed by a space, and end with an asterisk.

	grep -P "^SUNW[0-9a-fA-F]+ \*$" /tmp/test/pkgs

	Note: I assumed that hexadecimal number looks like 1AF or 1af, not
	0x1af.


	17 Copy file "/tmp/test/pkgs" to your home directory, so that the new name
	is pkgs-YYYYMMDD (where YYYYMMDD is the current date).

	cp /tmp/test/pkgs ~/pkgs-$(date +"%Y%m%d")


	18 List user names from /etc/passwd that have UID > 500

	awk -F: '{if ($3 > 500) print $1}' /etc/passwd