[PATCHv2] Unable to boot very old Linux kernels

[Piotr Krysiuk <piotras@gmail.com>]

GRUB cannot determine end of BSS section of compressed image when loading
very old Linux kernels.  Booting these kernels, grub_relocator32_start and
new Global Descriptor Table are placed in the area overlapping with the BSS
section of loaded image.  When Linux executes, it initializes BSS also
wiping Global Descriptor Table that is still active.  This leads to failure
on segment reload that follows.

Current HEAD of GRUB repository is affected, with "Clear BSS" code from
https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/arch/x86_64/boot/compressed/head.S?h=linux-2.6.17.y#n57
wiping GDT.

Old versions of GRUB, from before relocator was introduced, placed GDT close
to end of physical memory avoiding this issue.  This patch fixes regression
by moving GDT into conventional memory.

	* grub-core/lib/i386/relocator.c: Move GDT into conventional memory
	to avoid collision with BSS section of compressed Linux image for
	very old kernels.
---
 grub-core/lib/i386/relocator.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/grub-core/lib/i386/relocator.c b/grub-core/lib/i386/relocator.c
index d2a1b27..ffaf25f 100644
--- a/grub-core/lib/i386/relocator.c
+++ b/grub-core/lib/i386/relocator.c
@@ -81,10 +81,13 @@ grub_relocator32_boot (struct grub_relocator *rel,
   void *relst;
   grub_relocator_chunk_t ch;
 
-  err = grub_relocator_alloc_chunk_align (rel, &ch, 0,
-					  (0xffffffff - RELOCATOR_SIZEOF (32))
-					  + 1, RELOCATOR_SIZEOF (32), 16,
-					  GRUB_RELOCATOR_PREFERENCE_NONE,
+  /* Specific memory range due to Global Descriptor Table for use by payload
+     that we will store in returned chunk.  The address range and preference
+     are based on "THE LINUX/x86 BOOT PROTOCOL" specification.  */
+  err = grub_relocator_alloc_chunk_align (rel, &ch, 0x1000,
+					  0x9a000 - RELOCATOR_SIZEOF (32),
+					  RELOCATOR_SIZEOF (32), 16,
+					  GRUB_RELOCATOR_PREFERENCE_LOW,
 					  avoid_efi_bootservices);
   if (err)
     return err;
-- 
1.7.9.5