Question

I cannot understand why a call to read after an lseek returns 0 number of bytes read.

//A function to find the next note for a given userID;
//returns -1 if at the end of file is reached;
//otherwise, it returns the length of the found note.
int find_user_note(int fd, int user_uid) {
    int note_uid = -1;
    unsigned char byte;
    int length;

    while(note_uid != user_uid) { // Loop until a note for user_uid is found.
        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.

            //printf("%x ", byte);
            length++;
        }
    }
    long cur_position = lseek(fd, length * -1, SEEK_CUR ); // Rewind file reading by length bytes.

    printf("cur_position: %i\n", cur_position);

    // this is debug
    byte = 0;
    int num_byte = read(fd, &byte, 1);

    printf("[DEBUG] found a %d byte note for user id %d\n", length, note_uid);
    return length;
}

The variable length value is 34 when it exist the outer while loop and the above code produces cur_position 5 (so there are definitely at least 34 bytes after the lseek function returns), but the variable num_byte returned from function read always returns 0 even though there are still more bytes to read.

Does anyone know the reason num_byte always return 0? If it is a mistake in my code, am not seeing what it is.

Just for information, the above code was run on the following machine

$ uname -srvpio
Linux 3.2.0-24-generic #39-Ubuntu SMP Mon May 21 16:52:17 UTC 2012 x86_64 x86_64 GNU/Linux

Update:

• I upload the full code here
• This is the content of file that I try to read

$ sudo hexdump -C /var/notes
00000000  e8 03 00 00 0a 74 68 69  73 20 69 73 20 61 20 74  |.....this is a t|
00000010  65 73 74 20 6f 66 20 6d  75 6c 74 69 75 73 65 72  |est of multiuser|
00000020  20 6e 6f 74 65 73 0a                              | notes.|
00000027

$

Answer 1

I finally found the issue!!! I have to put #include <unistd.h> in order to use the correct lseek(). However I’m not sure why without including unistd.h it was compile-able though resulting in unexpected behavior. I thought that without including the prototype of a function, it shouldn’t even compile-able.

The code was written in Hacking: The Art of Exploitation 2nd Edition by Jon Erickson and I have verified that in the book, there is no #include <unistd.h>.