libdwfl/elf-from-memory.c

/* Reconstruct an ELF file by reading the segments out of remote memory.
   Copyright (C) 2005 Red Hat, Inc.

   This program is Open Source software; you can redistribute it and/or
   modify it under the terms of the Open Software License version 1.0 as
   published by the Open Source Initiative.

   You should have received a copy of the Open Software License along
   with this program; if not, you may obtain a copy of the Open Software
   License version 1.0 from http://www.opensource.org/licenses/osl.php or
   by writing the Open Source Initiative c/o Lawrence Rosen, Esq.,
   3001 King Ranch Road, Ukiah, CA 95482.   */

#include <config.h>
#include "../libelf/libelfP.h"
#undef _

#include "libdwflP.h"

#include <gelf.h>
#include <sys/types.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

/* Reconstruct an ELF file by reading the segments out of remote memory
   based on the ELF file header at EHDR_VMA and the ELF program headers it
   points to.  If not null, *LOADBASEP is filled in with the difference
   between the addresses from which the segments were read, and the
   addresses the file headers put them at.

   The function READ_MEMORY is called to copy at least MINREAD and at most
   MAXREAD bytes from the remote memory at target address ADDRESS into the
   local buffer at DATA; it should return -1 for errors (with code in
   `errno'), 0 if it failed to read at least MINREAD bytes due to EOF, or
   the number of bytes read if >= MINREAD.  ARG is passed through.  */

Elf *
elf_from_remote_memory (GElf_Addr ehdr_vma,
			GElf_Addr *loadbasep,
			ssize_t (*read_memory) (void *arg, void *data,
						GElf_Addr address,
						size_t minread,
						size_t maxread),
			void *arg)
{
  /* First read in the file header and check its sanity.  */

  const size_t initial_bufsize = 256;
  unsigned char *buffer = malloc (initial_bufsize);
  if (buffer == NULL)
    {
    no_memory:
      __libdwfl_seterrno (DWFL_E_NOMEM);
      return NULL;
    }

  ssize_t nread = (*read_memory) (arg, buffer, ehdr_vma,
				  sizeof (Elf32_Ehdr), initial_bufsize);
  if (nread <= 0)
    {
    read_error:
      free (buffer);
      __libdwfl_seterrno (nread < 0 ? DWFL_E_ERRNO : DWFL_E_TRUNCATED);
      return NULL;
    }

  if (memcmp (buffer, ELFMAG, SELFMAG) != 0)
    {
    bad_elf:
      __libdwfl_seterrno (DWFL_E_BADELF);
      return NULL;
    }

  /* Extract the information we need from the file header.  */

  union
  {
    Elf32_Ehdr e32;
    Elf64_Ehdr e64;
  } ehdr;
  Elf_Data xlatefrom =
    {
      .d_type = ELF_T_EHDR,
      .d_buf = buffer,
      .d_version = EV_CURRENT,
    };
  Elf_Data xlateto =
    {
      .d_type = ELF_T_EHDR,
      .d_buf = &ehdr,
      .d_size = sizeof ehdr,
      .d_version = EV_CURRENT,
    };

  GElf_Off phoff;
  uint_fast16_t phnum;
  uint_fast16_t phentsize;
  GElf_Off shdrs_end;

  switch (buffer[EI_CLASS])
    {
    case ELFCLASS32:
      xlatefrom.d_size = sizeof (Elf32_Ehdr);
      if (elf32_xlatetom (&xlateto, &xlatefrom, buffer[EI_DATA]) == NULL)
	{
	libelf_error:
	  __libdwfl_seterrno (DWFL_E_LIBELF);
	  return NULL;
	}
      phoff = ehdr.e32.e_phoff;
      phnum = ehdr.e32.e_phnum;
      phentsize = ehdr.e32.e_phentsize;
      if (phentsize != sizeof (Elf32_Phdr) || phnum == 0)
	goto bad_elf;
      shdrs_end = ehdr.e32.e_shoff + ehdr.e32.e_shnum * ehdr.e32.e_shentsize;
      break;

    case ELFCLASS64:
      xlatefrom.d_size = sizeof (Elf64_Ehdr);
      if (elf32_xlatetom (&xlateto, &xlatefrom, buffer[EI_DATA]) == NULL)
	goto libelf_error;
      phoff = ehdr.e64.e_phoff;
      phnum = ehdr.e64.e_phnum;
      phentsize = ehdr.e64.e_phentsize;
      if (phentsize != sizeof (Elf64_Phdr) || phnum == 0)
	goto bad_elf;
      shdrs_end = ehdr.e64.e_shoff + ehdr.e64.e_shnum * ehdr.e64.e_shentsize;
      break;

    default:
      goto bad_elf;
    }


  /* The file header tells where to find the program headers.
     These are what we use to actually choose what to read.  */

  xlatefrom.d_type = xlateto.d_type = ELF_T_PHDR;
  xlatefrom.d_size = phnum * phentsize;

  if ((size_t) nread >= phoff + phnum * phentsize)
    /* We already have all the phdrs from the initial read.  */
    xlatefrom.d_buf = buffer + phoff;
  else
    {
      /* Read in the program headers.  */

      if (initial_bufsize < phnum * phentsize)
	{
	  unsigned char *newbuf = realloc (buffer, phnum * phentsize);
	  if (newbuf == NULL)
	    {
	      free (buffer);
	      goto no_memory;
	    }
	  buffer = newbuf;
	}
      nread = (*read_memory) (arg, buffer, ehdr_vma + phoff,
			      phnum * phentsize, phnum * phentsize);
      if (nread <= 0)
	goto read_error;

      xlatefrom.d_buf = buffer;
    }

  union
  {
    Elf32_Phdr p32[phnum];
    Elf64_Phdr p64[phnum];
  } phdrs;

  xlateto.d_buf = &phdrs;
  xlateto.d_size = sizeof phdrs;

  /* Scan for PT_LOAD segments to find the total size of the file image.  */
  size_t contents_size = 0;
  GElf_Off segments_end = 0;
  GElf_Addr loadbase = ehdr_vma;
  switch (ehdr.e32.e_ident[EI_CLASS])
    {
      inline void handle_segment (GElf_Addr vaddr, GElf_Off offset,
				  GElf_Xword filesz, GElf_Xword align)
	{
	  GElf_Off segment_end = ((offset + filesz + align - 1) & -align);

	  if (segment_end > (GElf_Off) contents_size)
	    contents_size = segment_end;

	  if ((offset & -align) == 0 && loadbase == ehdr_vma)
	    loadbase = ehdr_vma - (vaddr & -align);

	  segments_end = offset + filesz;
	}

    case ELFCLASS32:
      if (elf32_xlatetom (&xlateto, &xlatefrom,
			  ehdr.e32.e_ident[EI_DATA]) == NULL)
	goto libelf_error;
      for (uint_fast16_t i = 0; i < phnum; ++i)
	if (phdrs.p32[i].p_type == PT_LOAD)
	  handle_segment (phdrs.p32[i].p_vaddr, phdrs.p32[i].p_offset,
			  phdrs.p32[i].p_filesz, phdrs.p32[i].p_align);
      break;

    case ELFCLASS64:
      if (elf32_xlatetom (&xlateto, &xlatefrom,
			  ehdr.e32.e_ident[EI_DATA]) == NULL)
	goto libelf_error;
      for (uint_fast16_t i = 0; i < phnum; ++i)
	if (phdrs.p32[i].p_type == PT_LOAD)
	  handle_segment (phdrs.p64[i].p_vaddr, phdrs.p64[i].p_offset,
			  phdrs.p64[i].p_filesz, phdrs.p64[i].p_align);
      break;

    default:
      abort ();
      break;
    }

  /* Trim the last segment so we don't bother with zeros in the last page
     that are off the end of the file.  However, if the extra bit in that
     page includes the section headers, keep them.  */
  if ((GElf_Off) contents_size > segments_end
      && (GElf_Off) contents_size >= shdrs_end)
    {
      contents_size = segments_end;
      if ((GElf_Off) contents_size < shdrs_end)
	contents_size = shdrs_end;
    }
  else
    contents_size = segments_end;

  free (buffer);

  /* Now we know the size of the whole image we want read in.  */
  buffer = calloc (1, contents_size);
  if (buffer == NULL)
    goto no_memory;

  switch (ehdr.e32.e_ident[EI_CLASS])
    {
      inline bool handle_segment (GElf_Addr vaddr, GElf_Off offset,
				  GElf_Xword filesz, GElf_Xword align)
	{
	  GElf_Off start = offset & -align;
	  GElf_Off end = (offset + filesz + align - 1) & -align;
	  if (end > (GElf_Off) contents_size)
	    end = contents_size;
	  nread = (*read_memory) (arg, buffer + start,
				  (loadbase + vaddr) & -align,
				  end - start, end - start);
	  return nread <= 0;
	}

    case ELFCLASS32:
      for (uint_fast16_t i = 0; i < phnum; ++i)
	if (phdrs.p32[i].p_type == PT_LOAD)
	  if (handle_segment (phdrs.p32[i].p_vaddr, phdrs.p32[i].p_offset,
			      phdrs.p32[i].p_filesz, phdrs.p32[i].p_align))
	    goto read_error;

      /* If the segments visible in memory didn't include the section
	 headers, then clear them from the file header.  */
      if (contents_size < shdrs_end)
	{
	  ehdr.e32.e_shoff = 0;
	  ehdr.e32.e_shnum = 0;
	  ehdr.e32.e_shstrndx = 0;
	}

      /* This will normally have been in the first PT_LOAD segment.  But it
	 conceivably could be missing, and we might have just changed it.  */
      xlatefrom.d_type = xlateto.d_type = ELF_T_EHDR;
      xlatefrom.d_size = xlateto.d_size = sizeof ehdr.e32;
      xlatefrom.d_buf = &ehdr.e32;
      xlateto.d_buf = buffer;
      if (elf32_xlatetof (&xlateto, &xlatefrom,
			  ehdr.e32.e_ident[EI_DATA]) == NULL)
	goto libelf_error;
      break;

    case ELFCLASS64:
      for (uint_fast16_t i = 0; i < phnum; ++i)
	if (phdrs.p32[i].p_type == PT_LOAD)
	  if (handle_segment (phdrs.p64[i].p_vaddr, phdrs.p64[i].p_offset,
			      phdrs.p64[i].p_filesz, phdrs.p64[i].p_align))
	    goto read_error;

      /* If the segments visible in memory didn't include the section
	 headers, then clear them from the file header.  */
      if (contents_size < shdrs_end)
	{
	  ehdr.e64.e_shoff = 0;
	  ehdr.e64.e_shnum = 0;
	  ehdr.e64.e_shstrndx = 0;
	}

      /* This will normally have been in the first PT_LOAD segment.  But it
	 conceivably could be missing, and we might have just changed it.  */
      xlatefrom.d_type = xlateto.d_type = ELF_T_EHDR;
      xlatefrom.d_size = xlateto.d_size = sizeof ehdr.e64;
      xlatefrom.d_buf = &ehdr.e64;
      xlateto.d_buf = buffer;
      if (elf32_xlatetof (&xlateto, &xlatefrom,
			  ehdr.e64.e_ident[EI_DATA]) == NULL)
	goto libelf_error;
      break;

    default:
      abort ();
      break;
    }

  /* Now we have the image.  Open libelf on it.  */

  Elf *elf = elf_memory ((char *) buffer, contents_size);
  if (elf == NULL)
    {
      free (buffer);
      return NULL;
    }

  elf->flags |= ELF_F_MALLOCED;
  if (loadbasep != NULL)
    *loadbasep = loadbase;
  return elf;
}