Re: changing embedded RPATH in existing executables.

[Geoff Keating <geoffk@ozemail.com.au>]

> From: peeterj@ca.ibm.com

> I would like to be able to change the embedded rpath in executable and
> shared libraries using libbfd.  The development platform in question here
> is linux/i386.

_Changing_ is a little tricky, but the attached program strips rpaths
from executables (I find it essential for debugging the binutils).
It's endian-dependent, if you want this for x86 you can just change
the occurrences of 'MSB' to 'LSB' and compile (I should really fix
that).

-- 
Geoffrey Keating <geoffk@ozemail.com.au>

===File ~/src/killrpath.c===================================
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <elf.h>
#include <stdlib.h>
/* Reads an ELF file, nukes all the RPATH entries. */

int
main(int argc, char **argv)
{
  int fd;
  Elf32_Ehdr ehdr;
  int i;
  Elf32_Phdr phdr;
  Elf32_Dyn *dyns;
  int dynpos;

  if (argc != 2)
  {
    printf ("Usage: %s objectfile\n", argv[0]);
    return 1;
  }

  fd = open(argv[1], O_RDWR);
  if (fd == -1)
  {
    perror ("open");
    return 1;
  }
  
  if (read(fd, &ehdr, sizeof(ehdr)) != sizeof(ehdr))
  {
    perror ("reading header");
    return 1;
  }

  if (*(unsigned *)ehdr.e_ident != *(const unsigned *)ELFMAG ||
      ehdr.e_ident[EI_CLASS] != ELFCLASS32 ||
      ehdr.e_ident[EI_DATA] != ELFDATA2MSB ||
      ehdr.e_ident[EI_VERSION] != EV_CURRENT)
  {
    fprintf(stderr, "`%s' probably isn't a 32-bit MSB-first ELF file.\n",
	    argv[1]);
    return 1;
  }

  if (ehdr.e_phentsize != sizeof(Elf32_Phdr))
  {
    fprintf(stderr, "section size was read as %d, not %d!\n",
	    ehdr.e_phentsize, sizeof(Elf32_Phdr));
    return 1;
  }

  if (lseek(fd, ehdr.e_phoff, SEEK_SET) == -1)
  {
    perror ("positioning for sections");
    return 1;
  }
  
  for (i = 0; i < ehdr.e_phnum; i++)
  {
    if (read(fd, &phdr, sizeof(phdr)) != sizeof(phdr))
    {
      perror ("reading section header");
      return 1;
    }
    if (phdr.p_type == PT_DYNAMIC)
      break;
  }
  if (i == ehdr.e_phnum)
    {
      fprintf (stderr, "No dynamic section found.\n");
      return 2;
    }

  dyns = malloc(phdr.p_memsz);
  if (dyns == NULL)
    {
      perror ("allocating memory for dynamic section");
      return 1;
    }
  memset(dyns, 0, phdr.p_memsz);
  if (lseek(fd, phdr.p_offset, SEEK_SET) == -1
      || read(fd, dyns, phdr.p_filesz) != phdr.p_filesz)
    {
      perror ("reading dynamic section");
      return 1;
    }

  dynpos = 0;
  for (i = 0; dyns[i].d_tag != DT_NULL; i++)
    {
      dyns[dynpos] = dyns[i];
      if (dyns[i].d_tag != DT_RPATH)
	dynpos++;
    }
  for (; dynpos < i; dynpos++)
    dyns[dynpos].d_tag = DT_NULL;

  if (lseek(fd, phdr.p_offset, SEEK_SET) == -1
      || write(fd, dyns, phdr.p_filesz) != phdr.p_filesz)
    {
      perror ("writing dynamic section");
      return 1;
    }
  return 0;
}
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