multiboot2: Add support for relocatable images

Currently multiboot2 protocol loads image exactly at address specified in
ELF or multiboot2 header. This solution works quite well on legacy BIOS
platforms. It is possible because memory regions are placed at predictable
addresses (though I was not able to find any spec which says that it is
strong requirement, so, it looks that it is just a goodwill of hardware
designers). However, EFI platforms are more volatile. Even if required
memory regions live at specific addresses then they are sometimes simply
not free (e.g. used by boot/runtime services on Dell PowerEdge R820 and
OVMF). This means that you are not able to just set up final image
destination on build time. You have to provide method to relocate image
contents to real load address which is usually different than load address
specified in ELF and multiboot2 headers.

This patch provides all needed machinery to do self relocation in image code.
First of all GRUB2 reads min_addr (min. load addr), max_addr (max. load addr),
align (required image alignment), preference (it says which memory regions are
preferred by image, e.g. none, low, high) from multiboot_header_tag_relocatable
header tag contained in binary (at this stage load addresses from multiboot2
and/or ELF headers are ignored). Later loader tries to fulfill request (not only
that one) and if it succeeds then it informs image about real load address via
multiboot_tag_load_base_addr tag. At this stage GRUB2 role is finished. Starting
from now executable must cope with relocations itself using whole static and
dynamic knowledge provided by boot loader.

This patch does not provide functionality which could do relocations using
ELF relocation data. However, I was asked by Konrad Rzeszutek Wilk and Vladimir
'phcoder' Serbinenko to investigate that thing. It looks that relevant machinery
could be added to existing code (including this patch) without huge effort.
Additionally, ELF relocation could live in parallel with self relocation provided
by this patch. However, during research I realized that first of all we should
establish the details how ELF relocatable image should look like and how it should
be build. At least to build proper test/example files.

So, this patch just provides support for self relocatable images. If ELF file
with relocs is loaded then GRUB2 complains loudly and ignores it. Support for
such files will be added later.

This patch was tested with Xen image which uses that functionality. However, this Xen
feature is still under development and new patchset will be released in about 2-3 weeks.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>

grub-core/loader/multiboot_mbi2.c

@@ -68,6 +68,7 @@ static grub_size_t elf_sec_num, elf_sec_entsize;
 static unsigned elf_sec_shstrndx;
 static void *elf_sections;
 static int keep_bs = 0;
+static grub_uint32_t load_base_addr;
 
 void
 grub_multiboot_add_elfsyms (grub_size_t num, grub_size_t entsize,
@@ -101,36 +102,40 @@ find_header (grub_properly_aligned_t *buffer, grub_ssize_t len)
 grub_err_t
 grub_multiboot_load (grub_file_t file, const char *filename)
 {
-  grub_properly_aligned_t *buffer;
   grub_ssize_t len;
   struct multiboot_header *header;
   grub_err_t err;
   struct multiboot_header_tag *tag;
   struct multiboot_header_tag_address *addr_tag = NULL;
+  struct multiboot_header_tag_relocatable *rel_tag;
   int entry_specified = 0, efi_entry_specified = 0;
   grub_addr_t entry = 0, efi_entry = 0;
   grub_uint32_t console_required = 0;
   struct multiboot_header_tag_framebuffer *fbtag = NULL;
   int accepted_consoles = GRUB_MULTIBOOT_CONSOLE_EGA_TEXT;
+  mbi_load_data_t mld;
 
-  buffer = grub_malloc (MULTIBOOT_SEARCH);
-  if (!buffer)
+  mld.mbi_ver = 2;
+  mld.relocatable = 0;
+
+  mld.buffer = grub_malloc (MULTIBOOT_SEARCH);
+  if (!mld.buffer)
     return grub_errno;
 
-  len = grub_file_read (file, buffer, MULTIBOOT_SEARCH);
+  len = grub_file_read (file, mld.buffer, MULTIBOOT_SEARCH);
   if (len < 32)
     {
-      grub_free (buffer);
+      grub_free (mld.buffer);
       return grub_error (GRUB_ERR_BAD_OS, N_("premature end of file %s"), filename);
     }
 
   COMPILE_TIME_ASSERT (MULTIBOOT_HEADER_ALIGN % 4 == 0);
 
-  header = find_header (buffer, len);
+  header = find_header (mld.buffer, len);
 
   if (header == 0)
     {
-      grub_free (buffer);
+      grub_free (mld.buffer);
       return grub_error (GRUB_ERR_BAD_ARGUMENT, "no multiboot header found");
     }
 
@@ -174,10 +179,11 @@ grub_multiboot_load (grub_file_t file, const char *filename)
 	      case MULTIBOOT_TAG_TYPE_EFI_BS:
 	      case MULTIBOOT_TAG_TYPE_EFI32_IH:
 	      case MULTIBOOT_TAG_TYPE_EFI64_IH:
+	      case MULTIBOOT_TAG_TYPE_LOAD_BASE_ADDR:
 		break;
 
 	      default:
-		grub_free (buffer);
+		grub_free (mld.buffer);
 		return grub_error (GRUB_ERR_UNKNOWN_OS,
 				   "unsupported information tag: 0x%x",
 				   request_tag->requests[i]);
@@ -215,6 +221,27 @@ grub_multiboot_load (grub_file_t file, const char *filename)
 	accepted_consoles |= GRUB_MULTIBOOT_CONSOLE_FRAMEBUFFER;
 	break;
 
+      case MULTIBOOT_HEADER_TAG_RELOCATABLE:
+	mld.relocatable = 1;
+	rel_tag = (struct multiboot_header_tag_relocatable *) tag;
+	mld.min_addr = rel_tag->min_addr;
+	mld.max_addr = rel_tag->max_addr;
+	mld.align = rel_tag->align;
+	switch (rel_tag->preference)
+	  {
+	  case MULTIBOOT_LOAD_PREFERENCE_LOW:
+	    mld.preference = GRUB_RELOCATOR_PREFERENCE_LOW;
+	    break;
+
+	  case MULTIBOOT_LOAD_PREFERENCE_HIGH:
+	    mld.preference = GRUB_RELOCATOR_PREFERENCE_HIGH;
+	    break;
+
+	  default:
+	    mld.preference = GRUB_RELOCATOR_PREFERENCE_NONE;
+	  }
+	break;
+
 	/* GRUB always page-aligns modules.  */
       case MULTIBOOT_HEADER_TAG_MODULE_ALIGN:
 	break;
@@ -228,7 +255,7 @@ grub_multiboot_load (grub_file_t file, const char *filename)
       default:
 	if (! (tag->flags & MULTIBOOT_HEADER_TAG_OPTIONAL))
 	  {
-	    grub_free (buffer);
+	    grub_free (mld.buffer);
 	    return grub_error (GRUB_ERR_UNKNOWN_OS,
 			       "unsupported tag: 0x%x", tag->type);
 	  }
@@ -237,7 +264,7 @@ grub_multiboot_load (grub_file_t file, const char *filename)
 
   if (addr_tag && !entry_specified && !(keep_bs && efi_entry_specified))
     {
-      grub_free (buffer);
+      grub_free (mld.buffer);
       return grub_error (GRUB_ERR_UNKNOWN_OS,
 			 "load address tag without entry address tag");
     }
@@ -246,8 +273,8 @@ grub_multiboot_load (grub_file_t file, const char *filename)
     {
       grub_uint64_t load_addr = (addr_tag->load_addr + 1)
 	? addr_tag->load_addr : (addr_tag->header_addr
-				 - ((char *) header - (char *) buffer));
-      int offset = ((char *) header - (char *) buffer -
+				 - ((char *) header - (char *) mld.buffer));
+      int offset = ((char *) header - (char *) mld.buffer -
 	   (addr_tag->header_addr - load_addr));
       int load_size = ((addr_tag->load_end_addr == 0) ? file->size - offset :
 		       addr_tag->load_end_addr - addr_tag->load_addr);
@@ -260,27 +287,50 @@ grub_multiboot_load (grub_file_t file, const char *filename)
       else
 	code_size = load_size;
 
-      err = grub_relocator_alloc_chunk_addr (grub_multiboot_relocator, 
-					     &ch, load_addr,
-					     code_size);
+      if (mld.relocatable)
+	{
+	  if (code_size > mld.max_addr || mld.min_addr > mld.max_addr - code_size)
+	    {
+	      grub_free (mld.buffer);
+	      return grub_error (GRUB_ERR_BAD_OS, "invalid min/max address and/or load size");
+	    }
+
+	  err = grub_relocator_alloc_chunk_align (grub_multiboot_relocator, &ch,
+						  mld.min_addr, mld.max_addr - code_size,
+						  code_size, mld.align ? mld.align : 1,
+						  mld.preference, keep_bs);
+	}
+      else
+	err = grub_relocator_alloc_chunk_addr (grub_multiboot_relocator,
+					       &ch, load_addr, code_size);
       if (err)
 	{
 	  grub_dprintf ("multiboot_loader", "Error loading aout kludge\n");
-	  grub_free (buffer);
+	  grub_free (mld.buffer);
 	  return err;
 	}
+      mld.link_base_addr = load_addr;
+      mld.load_base_addr = get_physical_target_address (ch);
       source = get_virtual_current_address (ch);
 
+      grub_dprintf ("multiboot_loader", "link_base_addr=0x%x, load_base_addr=0x%x, "
+		    "load_size=0x%lx, relocatable=%d\n", mld.link_base_addr,
+		    mld.load_base_addr, (long) code_size, mld.relocatable);
+
+      if (mld.relocatable)
+	grub_dprintf ("multiboot_loader", "align=0x%lx, preference=0x%x, avoid_efi_boot_services=%d\n",
+		      (long) mld.align, mld.preference, keep_bs);
+
       if ((grub_file_seek (file, offset)) == (grub_off_t) -1)
 	{
-	  grub_free (buffer);
+	  grub_free (mld.buffer);
 	  return grub_errno;
 	}
 
       grub_file_read (file, source, load_size);
       if (grub_errno)
 	{
-	  grub_free (buffer);
+	  grub_free (mld.buffer);
 	  return grub_errno;
 	}
 
@@ -290,19 +340,41 @@ grub_multiboot_load (grub_file_t file, const char *filename)
     }
   else
     {
-      err = grub_multiboot_load_elf (file, filename, buffer);
+      mld.file = file;
+      mld.filename = filename;
+      mld.avoid_efi_boot_services = keep_bs;
+      err = grub_multiboot_load_elf (&mld);
       if (err)
 	{
-	  grub_free (buffer);
+	  grub_free (mld.buffer);
 	  return err;
 	}
     }
 
+  load_base_addr = mld.load_base_addr;
+
   if (keep_bs && efi_entry_specified)
     grub_multiboot_payload_eip = efi_entry;
   else if (entry_specified)
     grub_multiboot_payload_eip = entry;
 
+  if (mld.relocatable)
+    {
+      /*
+       * Both branches are mathematically equivalent. However, it looks
+       * that real life (C?) is more complicated. I am trying to avoid
+       * wrap around here if mld.load_base_addr < mld.link_base_addr.
+       * If you look at C operator precedence then everything should work.
+       * However, I am not 100% sure that a given compiler will not
+       * optimize/break this stuff. So, maybe we should use signed
+       * 64-bit int here.
+       */
+      if (mld.load_base_addr >= mld.link_base_addr)
+	grub_multiboot_payload_eip += mld.load_base_addr - mld.link_base_addr;
+      else
+	grub_multiboot_payload_eip -= mld.link_base_addr - mld.load_base_addr;
+    }
+
   if (fbtag)
     err = grub_multiboot_set_console (GRUB_MULTIBOOT_CONSOLE_FRAMEBUFFER,
 				      accepted_consoles,
@@ -411,6 +483,7 @@ grub_multiboot_get_mbi_size (void)
     + ALIGN_UP (sizeof (struct multiboot_tag_framebuffer), MULTIBOOT_TAG_ALIGN)
     + ALIGN_UP (sizeof (struct multiboot_tag_old_acpi)
 		+ sizeof (struct grub_acpi_rsdp_v10), MULTIBOOT_TAG_ALIGN)
+    + ALIGN_UP (sizeof (struct multiboot_tag_load_base_addr), MULTIBOOT_TAG_ALIGN)
     + acpiv2_size ()
     + net_size ()
 #ifdef GRUB_MACHINE_EFI
@@ -693,6 +766,15 @@ grub_multiboot_make_mbi (grub_uint32_t *target)
 		       % sizeof (grub_properly_aligned_t) == 0);
   ptrorig += (2 * sizeof (grub_uint32_t)) / sizeof (grub_properly_aligned_t);
 
+  {
+    struct multiboot_tag_load_base_addr *tag = (struct multiboot_tag_load_base_addr *) ptrorig;
+    tag->type = MULTIBOOT_TAG_TYPE_LOAD_BASE_ADDR;
+    tag->size = sizeof (struct multiboot_tag_load_base_addr);
+    tag->load_base_addr = load_base_addr;
+    ptrorig += ALIGN_UP (tag->size, MULTIBOOT_TAG_ALIGN)
+       / sizeof (grub_properly_aligned_t);
+  }
+
   {
     struct multiboot_tag_string *tag = (struct multiboot_tag_string *) ptrorig;
     tag->type = MULTIBOOT_TAG_TYPE_CMDLINE;