Question

I have a sample application that I have created to understand an experiment with ELF binary format.

When I run it, it crashes after receiving SIGSEGV.
After attaching it with gdb and then running, I see that it crashes at the following line

(gdb) x/i 0x08054697
=> 0x8054697:   mov    %edx,0x80f8f5c

But, the destination address of this instruction is a valid address and this memory is mapped as writable.

(gdb) p/x *0x80f8f5c
$3 = 0x0
(gdb) si

Program received signal SIGSEGV, Segmentation fault.
0x08054697 in ?? ()

I am trying to understand why does this process receive SIGSEGV? What other things should I look for in order to figure out the reason.

Here is the output of readelf showing regions of virtual memory that are mapped.

Elf file type is EXEC (Executable file)
Entry point 0x8048e08
There are 13 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x08047034 0x08047034 0x002a4 0x002a4 R E 0x1
  INTERP         0x0001d4 0x080471d4 0x080471d4 0x00013 0x00013 R   0x1
      [Requesting program interpreter: /lib/ld-linux.so.2]
  DYNAMIC        0x0001e7 0x080471e7 0x080471e7 0x00060 0x00060 RW  0x1
  LOAD           0x000000 0x08047000 0x08047000 0x01000 0x01000 R E 0x1
  LOAD           0x001000 0x08048000 0x08048000 0xae948 0xae948 R E 0x1000
  LOAD           0x0b06dc 0x080f86dc 0x080f86dc 0x015f8 0x07730 RW  0x1000
  LOAD           0x0c52b8 0x081002b8 0x081002b8 0x00400 0x00400 R E 0x1
  LOAD           0x0c56b8 0x081006b8 0x081006b8 0x00400 0x00400 R E 0x1
  LOAD           0x0c5ab8 0x08100ab8 0x08100ab8 0x00400 0x00400 R E 0x1
  NOTE           0x0010f4 0x080480f4 0x080480f4 0x00044 0x00044 R   0x4
  TLS            0x0b06dc 0x080f86dc 0x080f86dc 0x00010 0x00030 R   0x4
  GNU_STACK      0x001000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4
  GNU_RELRO      0x0b06dc 0x080f86dc 0x080f86dc 0x00924 0x00924 R   0x1

Relevant instructions from the binary are

0x805467d:  mov    0x64(%esp),%edx
0x8054681:  mov    0x68(%esp),%ecx
0x8054685:  mov    %eax,0x80f9a44
0x805468a:  lea    0x4(%ecx,%edx,4),%eax
0x805468e:  mov    0x78(%esp),%edx
0x8054692:  mov    %eax,0x80ff1c8
==> 0x8054697:  mov    %edx,0x80f8f5c
0x805469d:  lea    0x0(%esi),%esi

Is there a way in gdb to figure out if the address is mapped as readonly or not?

What could be the reason for this Segmentation fault?

C Code

---

/*

ECHOSERV.C
==========
(c) Paul Griffiths, 1999
Email: [e-mail]

Simple TCP/IP echo server.

*/


#include <sys/socket.h>       /*  socket definitions        */
#include <sys/types.h>        /*  socket types              */
#include <arpa/inet.h>        /*  inet (3) functions         */
#include <unistd.h>           /*  misc. UNIX functions      */

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

#include <sys/stat.h>
#include <fcntl.h>

#include "helper.h"

#define LOG_FILE "test_disk.txt"

/*  Global constants  */

#define ECHO_PORT          (20002)
#define MAX_LINE           (1000)


int main(int argc, char *argv[]) {
    int       list_s;                /*  listening socket          */
    int       conn_s;                /*  connection socket         */
    short int port;                  /*  port number               */
    struct    sockaddr_in servaddr;  /*  socket address structure  */
    char      buffer[MAX_LINE];      /*  character buffer          */
    char     *endptr;                /*  for strtol()              */
    int file_fd = open(LOG_FILE, O_WRONLY|O_CREAT);

    /*  Get port number from the command line, and
        set to default port if no arguments were supplied  */
    if ( argc == 2 ) {
        port = strtol(argv[1], &endptr, 0);
        if ( *endptr ) {
            fprintf(stderr, "ECHOSERV: Invalid port number.\n");
            exit(EXIT_FAILURE);
        }
    }
    else if ( argc < 2 ) {
        port = ECHO_PORT;
    }
    else {
        fprintf(stderr, "ECHOSERV: Invalid arguments.\n");
        exit(EXIT_FAILURE);
    }


    /*  Create the listening socket  */

    if ( (list_s = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) {
        fprintf(stderr, "ECHOSERV: Error creating listening socket.\n");
        exit(EXIT_FAILURE);
    }


    /*  Set all bytes in socket address structure to
        zero, and fill in the relevant data members   */

    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family      = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port        = htons(port);


    /*  Bind our socket addresss to the 
        listening socket, and call listen()  */

    if ( bind(list_s, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0 ) {
        fprintf(stderr, "ECHOSERV: Error calling bind()\n");
        exit(EXIT_FAILURE);
    }

    if ( listen(list_s, LISTENQ) < 0 ) {
        fprintf(stderr, "ECHOSERV: Error calling listen()\n");
        exit(EXIT_FAILURE);
    }


    /*  Enter an infinite loop to respond
        to client requests and echo input  */

    while ( 1 ) {

        /*  Wait for a connection, then accept() it  */

        if ( (conn_s = accept(list_s, NULL, NULL) ) < 0 ) {
            fprintf(stderr, "ECHOSERV: Error calling accept()\n");
            exit(EXIT_FAILURE);
        }


        /*  Retrieve an input line from the connected socket
            then simply write it back to the same socket.     */

        Readline(conn_s, buffer, MAX_LINE-1);
        Writeline(conn_s, buffer, strlen(buffer));
        Writeline(file_fd, buffer, strlen(buffer));
        printf("%s\n", buffer);

        /*  Close the connected socket  */

        if ( close(conn_s) < 0 ) {
            fprintf(stderr, "ECHOSERV: Error calling close()\n");
            exit(EXIT_FAILURE);
        }
    }
}


/*

HELPER.C
========
(c) Paul Griffiths, 1999
Email: [e-mail]

Implementation of sockets helper functions.

Many of these functions are adapted from, inspired by, or 
otherwise shamelessly plagiarised from "Unix Network 
Programming", W Richard Stevens (Prentice Hall).

*/

#include "helper.h"
#include <sys/socket.h>
#include <unistd.h>
#include <errno.h>


/*  Read a line from a socket  */

ssize_t Readline(int sockd, void *vptr, size_t maxlen) {
    ssize_t n, rc;
    char    c, *buffer;

    buffer = (char *)vptr;

    for ( n = 1; n < maxlen; n++ ) {

        if ( (rc = read(sockd, &c, 1)) == 1 ) {
            *buffer++ = c;
            if ( c == '\n' )
                break;
        }
        else if ( rc == 0 ) {
            if ( n == 1 )
                return 0;
            else
                break;
        }
        else {
            if ( errno == EINTR )
                continue;
            return -1;
        }
    }

    *buffer = 0;
    return n;
}


/*  Write a line to a socket  */

ssize_t Writeline(int sockd, const void *vptr, size_t n) {
    size_t      nleft;
    ssize_t     nwritten;
    const char *buffer;

    buffer = (const char *)vptr;
    nleft  = n;

    while ( nleft > 0 ) {
        if ( (nwritten = write(sockd, buffer, nleft)) <= 0 ) {
            if ( errno == EINTR )
                nwritten = 0;
            else
                return -1;
        }
        nleft  -= nwritten;
        buffer += nwritten;
    }

    return n;
}

Answer 1

But, the destination address of this instruction is a valid address and this memory is mapped as writable.

Not it’s not (or the instruction wouldn’t have caused a SIGSEGV).

The destination 0x80f8f5c is “covered” by this LOAD segment:

LOAD           0x0b06dc 0x080f86dc 0x080f86dc 0x015f8 0x07730 RW  0x1000

but also by this:

GNU_RELRO      0x0b06dc 0x080f86dc 0x080f86dc 0x00924 0x00924 R   0x1

the GNU_RELRO asks the runtime loader to make this part of address space read-only after the loader has performed the relocation (which is exactly what it did, and what triggered your crash).

Is there a way in gdb to figure out if the address is mapped as readonly or not?

You can ask gdb with info proc map, or just look in /proc/<pid>/maps. Either way you’ll discover that the memory is mapped read-only.