PROGRAMMING - Hacking: The Art of Exploitation - page 95

if(read(fd, &note_uid, 4) != 4) // Read the uid data.
return -1; // If 4 bytes aren't read, return end of file code.
if(read(fd, &byte, 1) != 1) // Read the newline separator.
return -1;
byte = length = 0;
while(byte != '\n') { // Figure out how many bytes to the end of line.
if(read(fd, &byte, 1) != 1) // Read a single byte.
return -1; // If byte isn't read, return end of file code.
length++;
}
}
lseek(fd, length * -1, SEEK_CUR); // Rewind file reading by length bytes.
printf("[DEBUG] found a %d byte note for user id %d\n", length, note_uid);
return length;
}
// A function to search a note for a given keyword;
// returns 1 if a match is found, 0 if there is no match.
int search_note(char *note, char *keyword) {
int i, keyword_length, match=0;
keyword_length = strlen(keyword);
if(keyword_length == 0) // If there is no search string,
return 1; // always "match".
for(i=0; i < strlen(note); i++) { // Iterate over bytes in note.
if(note[i] == keyword[match]) // If byte matches keyword,
match++; // get ready to check the next byte;
else { // otherwise,
if(note[i] == keyword[0]) // if that byte matches first keyword byte,
match = 1; // start the match count at 1.
else
match = 0; // Otherwise it is zero.
}
if(match == keyword_length) // If there is a full match,
return 1; // return matched.
}
return 0; // Return not matched.
}
Most of this code should make sense, but there are some new concepts.
The filename is defined at the top instead of using heap memory. Also, the
function lseek() is used to rewind the read position in the file. The function
call of lseek(fd, length * -1, SEEK_CUR); tells the program to move the read
position forward from the current position in the file by length * -1 bytes.
Since this turns out to be a negative number, the position is moved backward
by length bytes.
reader@hacking:~/booksrc $ gcc -o notesearch notesearch.c
reader@hacking:~/booksrc $ sudo chown root:root ./notesearch
reader@hacking:~/booksrc $ sudo chmod u+s ./notesearch
reader@hacking:~/booksrc $ ./notesearch