[PATCH] Drivemap module

* [PATCH] Drivemap module
@ 2008-06-10 20:09 Javier Martín
  2008-06-10 20:23 ` Vesa Jääskeläinen
  2008-06-11 14:44 ` Marco Gerards
  0 siblings, 2 replies; 47+ messages in thread
From: Javier Martín @ 2008-06-10 20:09 UTC (permalink / raw)
  To: grub-devel


[-- Attachment #1.1: Type: text/plain, Size: 1956 bytes --]

Since the last posts in the "reimplementing the legacy map command"
thread seemingly died silently, I'm sending the improved version of the
patch here. If the death of the thread was intentional, please just tell
me (either nicely or rudely) and I'll be off.

Recapitulating the other thread, this is a new module for x86-pc that
provides functionality similar to the legacy "map" command, i.e. allows
BIOS drives to be remapped and have the second HD appear as 0x80 (the
first), which is necessary because some OSes refuse to boot otherwise,
ignoring the "boot drive" passed to their loaders in DL (Windows XP and
other NTs); or can only boot with limited functionality (FreeDOS). The
syntax of this new command is similar to the old "map", though the
second argument must be the target BIOS drive number instead of a GRUB
device, though a hack-patch parsing them would be simple.

Regarding the objections raised by Vesa, I followed most of the advices,
removing chatter in the source and completely separating drivemap from
the kernel. However, I've left the "abort_on_error" part of the new
preboot-hook interface intact, mostly because I don't know how to
replace it. As drivemap is the only module using it, the problem should
be harmless for now, but a solution should be devised - not by me,
though, finals are here (literally, I had one today).

I hope I didn't leave anything out this time. This code has been tested
on the following situations:
 * Emulators: Bochs & QEMU, booting FreeDOS in (hd1) with another fat
partition in (hd0) - works wonderfully, and even allows access to both
drives if they are crossmapped and a faux root=(hd0) is set.
 * Real hardware: an Athlon64 (939) CPU on a Gigabyte motherboard,
booting Windows XP Pro x64 in (hd1) with Ubuntu in (hd0) - works too,
with no hacks required at all (by the way, booting XP was my reason to
do this).

Well, here it is. Have a nice day!

Habbit

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Index: commands/i386/pc/drivemap.c
===================================================================
RCS file: commands/i386/pc/drivemap.c
diff -N commands/i386/pc/drivemap.c

--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ commands/i386/pc/drivemap.c	10 Jun 2008 20:02:19 -0000
@@ -0,0 +1,348 @@
+/* drivemap.c - command to manage the BIOS drive mappings.  */
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2008  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <grub/normal.h>
+#include <grub/dl.h>
+#include <grub/mm.h>
+#include <grub/misc.h>
+#include <grub/disk.h>
+#include <grub/loader.h>
+#include <grub/machine/loader.h>
+#include <grub/machine/biosdisk.h>
+
+/* Uncomment the following line to enable debugging output */
+/* #define DRIVEMAP_DEBUG */
+
+#ifdef DRIVEMAP_DEBUG
+# define DBG_PRINTF(...) grub_printf(__VA_ARGS__)
+#else
+# define DBG_PRINTF(...)
+#endif
+
+static const struct grub_arg_option options[] =
+  {
+    {"list", 'l', 0, "show the current mappings", 0, 0},
+    {"reset", 'r', 0, "reset all mappings to the default values", 0, 0},
+    {0, 0, 0, 0, 0, 0}
+  };
+
+/* ASSEMBLY SYMBOLS/VARS/FUNCS - start */
+
+/* realmode far ptr = 2 * 16b */
+extern grub_uint32_t EXPORT_VAR(grub_drivemap_int13_oldhandler);
+/* Size of the section to be copied */
+extern grub_uint16_t EXPORT_VAR(grub_drivemap_int13_size);
+
+/* NOT a typo - just need the symbol's address with &symbol */
+typedef void grub_symbol_t;
+extern grub_symbol_t EXPORT_VAR(grub_drivemap_int13_handler_base);
+extern grub_symbol_t EXPORT_VAR(grub_drivemap_int13_mapstart);
+
+void EXPORT_FUNC(grub_drivemap_int13_handler)(void);
+
+/* ASSEMBLY SYMBOLS/VARS/FUNCS - end */
+
+static grub_preboot_hookid insthandler_hook = 0;
+
+typedef struct drivemap_node
+{
+  grub_uint8_t newdrive;
+  grub_uint8_t redirto;
+  struct drivemap_node *next;
+} drivemap_node_t;
+
+static drivemap_node_t *drivemap = 0;
+static grub_err_t drivemap_install_int13_handler(void);
+
+static grub_err_t
+drivemap_set (grub_uint8_t newdrive, grub_uint8_t redirto)
+  /* Puts the specified mapping into the table, replacing an existing mapping
+   * for newdrive or adding a new one if required. */
+{
+  drivemap_node_t *mapping = 0, *search = drivemap;
+  while (search)
+    {
+      if (search->newdrive == newdrive)
+        {
+          mapping = search;
+          break;
+        }
+      search = search->next;
+    }
+
+  if (mapping)  /* There was a mapping already in place, modify it */
+    mapping->redirto = redirto;
+  else  /* Create a new mapping and add it to the head of the list */
+    {
+      mapping = grub_malloc (sizeof (drivemap_node_t));
+      if (!mapping)
+        return grub_error (GRUB_ERR_OUT_OF_MEMORY, "cannot allocate map entry");
+      mapping->newdrive = newdrive;
+      mapping->redirto = redirto;
+      mapping->next = drivemap;
+      drivemap = mapping;
+    }
+  return GRUB_ERR_NONE;
+}
+
+static void
+drivemap_remove (grub_uint8_t newdrive)
+  /* Removes the mapping for newdrive from the table. If there is no mapping,
+   * then this function behaves like a no-op on the map. */
+{
+  drivemap_node_t *mapping = 0, *search = drivemap, *previous = 0;
+  while (search)
+    {
+      if (search->newdrive == newdrive)
+        {
+          mapping = search;
+          break;
+        }
+      previous = search;
+      search = search->next;
+    }
+  if (mapping) /* Found */
+    {
+      if (previous)
+        previous->next = mapping->next;
+      else drivemap = mapping->next; /* Entry was head of list */
+      grub_free (mapping);
+    }
+}
+
+static grub_err_t parse_biosdisk (const char *name, grub_uint8_t *disknum)
+{
+  if (!name) return GRUB_ERR_BAD_ARGUMENT;
+  if (name[0] == '(')
+    name++; /* Skip the first ( in (hd0) - disk_open wants just the name! */
+  grub_disk_t disk = grub_disk_open (name);
+  if (!disk)
+    return GRUB_ERR_UNKNOWN_DEVICE;
+  else
+    {
+      enum grub_disk_dev_id id = disk->dev->id;
+      if (disknum)
+        *disknum = disk->id;   /* Only valid, of course if it's a biosdisk */
+      grub_disk_close (disk);
+      return (GRUB_DISK_DEVICE_BIOSDISK_ID != id) ?
+          GRUB_ERR_BAD_DEVICE : GRUB_ERR_NONE;
+    }
+}
+
+static grub_err_t revparse_biosdisk(const grub_uint8_t dnum, const char **output)
+{
+  grub_err_t retval = GRUB_ERR_UNKNOWN_DEVICE;
+  auto int find (const char *name);
+  int find (const char *name)
+  {
+    grub_disk_t disk = grub_disk_open (name);
+    if (!disk)
+      return 0;
+    else
+      {
+        int found = 0;
+        if (dnum == disk->id && GRUB_DISK_DEVICE_BIOSDISK_ID == disk->dev->id)
+          {
+            found = 1;
+            *output = name;
+            retval = GRUB_ERR_NONE;
+          }
+        grub_disk_close (disk);
+        return found;
+      }
+  }
+
+  grub_disk_dev_iterate (&find);
+  return retval;
+}
+
+static grub_err_t
+grub_cmd_drivemap (struct grub_arg_list *state, int argc, char **args)
+{
+  if (state[0].set) /* Show: list mappings */
+    {
+      if (!drivemap)
+        grub_printf ("No drives have been remapped");
+      else
+        {
+          grub_printf ("Showing only remapped drives. Drives that have had "
+                       "their slot assigned to another one and have not been "
+                       "themselves remapped will become inaccessible through "
+                       "the BIOS routines to the booted OS.\n\n");
+          grub_printf ("Mapped\tGRUB\n");
+          drivemap_node_t *curnode = drivemap;
+          while (curnode)
+            {
+              const char *dname = 0;
+              grub_err_t err = revparse_biosdisk (curnode->redirto, &dname);
+              if (err != GRUB_ERR_NONE)
+                return grub_error (err, "invalid mapping: non-existent disk or"
+                                        "not managed by the BIOS");
+              grub_printf("0x%02x\t%4s\n", curnode->newdrive, dname);
+              curnode = curnode->next;
+            }
+        }
+    }
+  else if (state[1].set) /* Reset: just delete all mappings */
+    {
+      if (drivemap)
+        {
+          drivemap_node_t *curnode = drivemap, *prevnode = 0;
+          while (curnode)
+            {
+              prevnode = curnode;
+              curnode = curnode->next;
+              grub_free (prevnode);
+            }
+          drivemap = 0;
+        }
+    }
+  else
+    {
+      if (argc != 2)
+        return grub_error (GRUB_ERR_BAD_ARGUMENT, "two arguments required");
+      grub_uint8_t mapfrom = 0;
+      grub_err_t err = parse_biosdisk (args[0], &mapfrom);
+      if (err != GRUB_ERR_NONE)
+        return grub_error (err, "invalid disk or not managed by the BIOS");
+
+      grub_errno = GRUB_ERR_NONE;
+      unsigned long mapto = grub_strtoul (args[1], 0, 0);
+      if (grub_errno != GRUB_ERR_NONE)
+        return grub_error (grub_errno,
+                          "BIOS disk number must be between 0 and 255");
+      else if (mapto == mapfrom)  /* Reset to default */      
+        {
+          DBG_PRINTF ("Removing the mapping for %s (%02x)", args[0], mapfrom);
+          drivemap_remove (mapfrom);
+        }
+      else  /* Map */
+        {
+          DBG_PRINTF ("Mapping %s (%02x) to %02x\n", args[0], mapfrom, mapto);
+          return drivemap_set ((grub_uint8_t)mapto, mapfrom);
+        }
+    }
+
+  return GRUB_ERR_NONE;
+}
+
+typedef struct __attribute__ ((packed)) int13map_node
+{
+  grub_uint8_t disknum;
+  grub_uint8_t mapto;
+} int13map_node_t;
+
+/* The min amount of mem that must remain free after installing the handler.
+ * 32 KiB is just above 0x7C00-0x7E00, where the bootsector is loaded.  */
+#define MIN_FREE_MEM_KB 32
+#define INT13H_OFFSET(x) ( ((grub_uint8_t*)(x)) - ((grub_uint8_t*)&grub_drivemap_int13_handler_base) )
+#define INT13H_REBASE(x) ( (void*) (((grub_uint8_t*)handler_base) + (x)) )
+#define INT13H_TONEWADDR(x) INT13H_REBASE( INT13H_OFFSET( x ) )
+/* Int13h handler installer - reserves conventional memory for the handler,
+ * copies it over and sets the IVT entry for int13h.
+ * This code rests on the assumption that GRUB does not activate any kind of
+ * memory mapping, since it accesses realmode structures by their absolute
+ * addresses, like the IVT at 0 or the BDA at 0x400 */
+static grub_err_t drivemap_install_int13_handler(void)
+{
+  grub_size_t entries = 0;
+  drivemap_node_t *curentry = drivemap;
+  while (curentry)  /* Count entries to prepare a contiguous map block */
+    {
+      entries++;
+      curentry = curentry->next;
+    }
+  if (0 == entries)
+    {
+      DBG_PRINTF ("drivemap: No drives remapped, int13h handler not installed");
+      return GRUB_ERR_NONE;  /* No need to install the int13h handler */
+    }
+  else
+    {
+      DBG_PRINTF ("drivemap: Installing int13h handler...\n");
+      grub_uint32_t *ivtslot = (grub_uint32_t*)0x0000004c;
+      
+      /* Save the pointer to the old int13h handler */    
+      grub_drivemap_int13_oldhandler = *ivtslot;
+      DBG_PRINTF ("Old int13 handler at %04x:%04x\n",
+                  (grub_drivemap_int13_oldhandler >> 16) & 0x0ffff,
+                  grub_drivemap_int13_oldhandler & 0x0ffff);
+
+      /* Reserve a section of conventional memory as "BIOS memory" for handler:
+       * BDA offset 0x13 contains the top of such memory */
+      grub_uint16_t *bpaMemInKb = (grub_uint16_t*)0x00000413;
+      DBG_PRINTF ("Top of conventional memory: %u KiB\n", *bpaMemInKb);
+      grub_size_t totalSize = grub_drivemap_int13_size
+                            + (entries + 1) * sizeof(int13map_node_t);
+      grub_uint16_t payloadSizeKb = (totalSize >> 10) +
+                                    (((totalSize % 1024) == 0) ? 0 : 1);
+      if ((*bpaMemInKb - payloadSizeKb) < MIN_FREE_MEM_KB)
+        return grub_error (GRUB_ERR_OUT_OF_MEMORY, "refusing to install int13 handler, not enough free memory after");
+      DBG_PRINTF ("Payload is %u b long, reserving %u Kb\n", totalSize, payloadSizeKb);
+      *bpaMemInKb -= payloadSizeKb;
+
+      /* Copy int13h handler chunk to reserved area */
+      grub_uint8_t *handler_base = (grub_uint8_t*)(*bpaMemInKb << 10);
+      DBG_PRINTF ("Copying int13 handler to: %p\n", handler_base);
+      grub_memcpy (handler_base, &grub_drivemap_int13_handler_base, grub_drivemap_int13_size);
+
+      /* Copy the mappings to the reserved area */
+      curentry = drivemap;
+      grub_size_t i;
+      int13map_node_t *handler_map = (int13map_node_t*) INT13H_TONEWADDR(&grub_drivemap_int13_mapstart);
+      DBG_PRINTF ("Target map at %p, copying mappings...\n", handler_map);
+      for (i = 0; i < entries && curentry; i++, curentry = curentry->next)
+        {
+          handler_map[i].disknum = curentry->newdrive;
+          handler_map[i].mapto = curentry->redirto;
+          DBG_PRINTF ("\t#%d: 0x%02x <- 0x%02x\n", i, handler_map[i].disknum, handler_map[i].mapto);
+        }
+      /* Signal end-of-map */
+      handler_map[i].disknum = 0;
+      handler_map[i].mapto = 0;
+      DBG_PRINTF ("\t#%d: 0x%02x <- 0x%02x (end)\n", i, handler_map[i].disknum, handler_map[i].mapto);
+
+      /* Install our function as the int13h handler in the IVT */
+      grub_uint32_t ivtentry = ((grub_uint32_t)handler_base) << 12; /* Segment address */
+      ivtentry |= (grub_uint16_t) INT13H_OFFSET(grub_drivemap_int13_handler);
+      DBG_PRINTF ("New int13 handler IVT pointer: %04x:%04x\n",
+                  (ivtentry >> 16) & 0x0ffff, ivtentry & 0x0ffff);
+      *ivtslot = ivtentry;
+      
+      return GRUB_ERR_NONE;
+    }
+}
+#undef INT13H_TONEWADDR
+#undef INT13H_REBASE
+#undef INT13H_OFFSET
+
+GRUB_MOD_INIT(drivemap)
+{
+  (void)mod;			/* To stop warning. */
+  grub_register_command ("drivemap", grub_cmd_drivemap, GRUB_COMMAND_FLAG_BOTH,
+			 "drivemap -s | -r | (hdX) newdrivenum", "Manage the BIOS drive mappings", options);
+  insthandler_hook = grub_loader_register_preboot (&drivemap_install_int13_handler, 1);
+}
+
+GRUB_MOD_FINI(drivemap)
+{
+  grub_loader_unregister_preboot (insthandler_hook);
+  insthandler_hook = 0;
+  grub_unregister_command ("drivemap");
+}
Index: commands/i386/pc/drivemap_int13h.S
===================================================================
RCS file: commands/i386/pc/drivemap_int13h.S
diff -N commands/i386/pc/drivemap_int13h.S
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ commands/i386/pc/drivemap_int13h.S	10 Jun 2008 20:02:19 -0000
@@ -0,0 +1,128 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2008 Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+
+/*
+ * Note: These functions defined in this file may be called from C.
+ *       Be careful of that you must not modify some registers. Quote
+ *       from gcc-2.95.2/gcc/config/i386/i386.h:
+
+   1 for registers not available across function calls.
+   These must include the FIXED_REGISTERS and also any
+   registers that can be used without being saved.
+   The latter must include the registers where values are returned
+   and the register where structure-value addresses are passed.
+   Aside from that, you can include as many other registers as you like.
+
+  ax,dx,cx,bx,si,di,bp,sp,st,st1,st2,st3,st4,st5,st6,st7,arg
+{  1, 1, 1, 0, 0, 0, 0, 1, 1,  1,  1,  1,  1,  1,  1,  1,  1 }
+ */
+
+/*
+ * Note: GRUB is compiled with the options -mrtd and -mregparm=3.
+ *       So the first three arguments are passed in %eax, %edx, and %ecx,
+ *       respectively, and if a function has a fixed number of arguments
+ *       and the number if greater than three, the function must return
+ *       with "ret $N" where N is ((the number of arguments) - 3) * 4.
+ */
+
+/*
+ *  This is the area for all of the special variables.
+ */
+
+#include <grub/symbol.h>
+
+#define GRUB_DRIVEMAP_INT13H_OFFSET(x) ((x) - grub_drivemap_int13_handler_base)
+
+/* Copy starts here. When deployed, this label must be segment-aligned */
+VARIABLE(grub_drivemap_int13_handler_base)  
+
+VARIABLE(grub_drivemap_int13_oldhandler)
+  .word 0xdead, 0xbeef
+/* Drivemap module - INT 13h handler - BIOS HD map */
+/* We need to use relative addressing, and with CS to top it all, since we
+ * must make as few changes to the registers as possible. Pity we're not on
+ * amd64: rIP-relative addressing would make life easier here.
+ */
+.code16
+FUNCTION(grub_drivemap_int13_handler)
+  push %bp
+  mov %sp, %bp
+  push %ax  /* We'll need it later to determine the used BIOS function */
+
+  /* Map the drive number (always in DL?) */
+  push %ax
+  push %bx
+  push %si
+  mov $GRUB_DRIVEMAP_INT13H_OFFSET(grub_drivemap_int13_mapstart), %bx
+  xor %si, %si
+1:movw %cs:(%bx,%si), %ax
+  cmp %ah, %al
+  jz 3f /* DRV=DST => map end - drive not remapped, leave DL as-is */
+  cmp %dl, %al
+  jz 2f /* Found - drive remapped, modify DL */
+  add $2, %si
+  jmp 1b /* Not found, but more remaining, loop  */
+2:mov %ah, %dl
+3:pop %si
+  pop %bx
+  xchgw %ax, -4(%bp) /* Recover the old AX and save the map entry for later */
+  
+  push %bp  
+  /* Simulate interrupt call: push flags and do a far call in order to set
+   * the stack the way the old handler expects it so that its iret works */
+  push 6(%bp)
+  movw (%bp), %bp  /* Restore the caller BP (is this needed and/or sensible?) */
+  lcall *%cs:GRUB_DRIVEMAP_INT13H_OFFSET(grub_drivemap_int13_oldhandler)
+  pop %bp /* The pushed flags were removed by iret */
+  /* Set the saved flags to what the int13h handler returned */
+  push %ax
+  pushf
+  pop %ax
+  movw %ax, 6(%bp)
+  pop %ax
+
+  /* Reverse map any returned drive number if the data returned includes it.
+   * The only func that does this seems to be origAH = 0x08, but many BIOS
+   * refs say retDL = # of drives connected. However, the GRUB Legacy code
+   * treats this as the _drive number_ and "undoes" the remapping. Thus,
+   * this section has been disabled for testing if it's required */
+#  cmpb $0x08, -1(%bp) /* Caller's AH */
+#  jne 4f
+#  push %es
+#  pushal
+#  mov $0, %di
+#  mov $0xb800, %bx
+#  mov %bx, %es
+#  xchgw %ax, -4(%bp) /* Map entry used */
+#  cmp %ah, %al  /* DRV=DST => drive not remapped */
+#  je 4f
+#  mov %ah, %dl  /* Undo remap */
+
+4:mov %bp, %sp
+  pop %bp
+  iret
+/* This label MUST be at the end of the copied block, since the installer code
+ * reserves additional space for mappings at runtime and copies them over it */
+.align 2
+VARIABLE(grub_drivemap_int13_mapstart)
+  .space 0
+.code32 /* Copy stops here */
+VARIABLE(grub_drivemap_int13_size)
+  .word GRUB_DRIVEMAP_INT13H_OFFSET(grub_drivemap_int13_size)
+#undef GRUB_DRIVEMAP_INT13H_OFFSET
Index: conf/i386-pc.rmk
===================================================================
RCS file: /sources/grub/grub2/conf/i386-pc.rmk,v
retrieving revision 1.119
diff -u -p -r1.119 i386-pc.rmk
--- conf/i386-pc.rmk	30 May 2008 04:20:47 -0000	1.119
+++ conf/i386-pc.rmk	10 Jun 2008 20:02:26 -0000
@@ -152,7 +152,7 @@ pkglib_MODULES = biosdisk.mod _chain.mod
 	vbe.mod vbetest.mod vbeinfo.mod video.mod gfxterm.mod \
 	videotest.mod play.mod bitmap.mod tga.mod cpuid.mod serial.mod	\
 	ata.mod vga.mod memdisk.mod jpeg.mod png.mod pci.mod lspci.mod \
-	aout.mod _bsd.mod bsd.mod
+	aout.mod _bsd.mod bsd.mod drivemap.mod
 
 # For biosdisk.mod.
 biosdisk_mod_SOURCES = disk/i386/pc/biosdisk.c
@@ -319,4 +319,11 @@ bsd_mod_SOURCES = loader/i386/bsd_normal
 bsd_mod_CFLAGS = $(COMMON_CFLAGS)
 bsd_mod_LDFLAGS = $(COMMON_LDFLAGS)
 
+# For drivemap.mod.
+drivemap_mod_SOURCES = commands/i386/pc/drivemap.c \
+                       commands/i386/pc/drivemap_int13h.S
+drivemap_mod_ASFLAGS = $(COMMON_ASFLAGS)
+drivemap_mod_CFLAGS = $(COMMON_CFLAGS)
+drivemap_mod_LDFLAGS = $(COMMON_LDFLAGS)
+
 include $(srcdir)/conf/common.mk
Index: include/grub/loader.h
===================================================================
RCS file: /sources/grub/grub2/include/grub/loader.h,v
retrieving revision 1.9
diff -u -p -r1.9 loader.h
--- include/grub/loader.h	21 Jul 2007 23:32:22 -0000	1.9
+++ include/grub/loader.h	10 Jun 2008 20:02:26 -0000
@@ -37,6 +37,19 @@ void EXPORT_FUNC(grub_loader_set) (grub_
 /* Unset current loader, if any.  */
 void EXPORT_FUNC(grub_loader_unset) (void);
 
+typedef struct hooklist_node *grub_preboot_hookid;
+
+/* Register a function to be called before the boot hook. Returns an id that
+   can be later used to unregister the preboot (i.e. if module unloaded). If
+   abort_on_error is set, the boot sequence will abort if any of the registered
+   functions return anything else than GRUB_ERR_NONE */
+grub_preboot_hookid EXPORT_FUNC(grub_loader_register_preboot)
+           (grub_err_t (*hook) (void), int abort_on_error); 
+
+/* Unregister a preboot hook by the id returned by loader_register_preboot.
+   This functions becomes a no-op if no such function is registered */
+void EXPORT_FUNC(grub_loader_unregister_preboot) (grub_preboot_hookid id);
+
 /* Call the boot hook in current loader. This may or may not return,
    depending on the setting by grub_loader_set.  */
 grub_err_t EXPORT_FUNC(grub_loader_boot) (void);
Index: kern/loader.c
===================================================================
RCS file: /sources/grub/grub2/kern/loader.c,v
retrieving revision 1.9
diff -u -p -r1.9 loader.c
--- kern/loader.c	21 Jul 2007 23:32:26 -0000	1.9
+++ kern/loader.c	10 Jun 2008 20:02:27 -0000
@@ -61,11 +61,78 @@ grub_loader_unset(void)
   grub_loader_loaded = 0;
 }
 
+struct hooklist_node
+{
+  grub_err_t (*hook) (void);
+  int abort_on_error;
+  struct hooklist_node *next;
+};
+
+static struct hooklist_node *preboot_hooks = 0;
+
+grub_preboot_hookid
+grub_loader_register_preboot(grub_err_t (*hook) (void), int abort_on_error)
+{
+  if (0 == hook)
+    {
+      grub_error (GRUB_ERR_BAD_ARGUMENT, "preboot hoook must not be null");
+      return 0;
+    }
+  grub_preboot_hookid newentry = grub_malloc (sizeof (struct hooklist_node));
+  if (!newentry)
+    {
+      grub_error (GRUB_ERR_OUT_OF_MEMORY, "cannot alloc a hookinfo structure");
+      return 0;
+    }
+  else
+    {
+      newentry->hook = hook;
+      newentry->abort_on_error = abort_on_error;
+      newentry->next = preboot_hooks;
+      preboot_hooks = newentry;
+      return newentry;
+    }
+}
+
+void
+grub_loader_unregister_preboot(grub_preboot_hookid id)
+{
+  if (0 == id)
+    return;
+  grub_preboot_hookid entry = 0, search = preboot_hooks, previous = 0;
+  while (search)
+    {
+      if (search == id)
+        {
+          entry = search;
+          break;
+        }
+      previous = search;
+      search = search->next;
+    }
+  if (entry) /* Found */
+    {
+      if (previous)
+        previous->next = entry->next;
+      else preboot_hooks = entry->next; /* Entry was head of list */
+      grub_free (entry);
+    }
+}
+
 grub_err_t
 grub_loader_boot (void)
 {
   if (! grub_loader_loaded)
     return grub_error (GRUB_ERR_NO_KERNEL, "no loaded kernel");
+  
+  grub_preboot_hookid entry = preboot_hooks;
+  while (entry)
+    {
+      grub_err_t retVal = entry->hook();
+      if (retVal != GRUB_ERR_NONE && entry->abort_on_error)
+        return retVal;
+      entry = entry->next;
+    }
 
   if (grub_loader_noreturn)
     grub_machine_fini ();

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