0026-gpt-prefer-disk-size-from-header-over-firmware.patch

From 81126be2c4ee13b747586e15797a9c5505c0ce0b Mon Sep 17 00:00:00 2001
From: Michael Marineau <michael.marineau@coreos.com>
Date: Tue, 23 Aug 2016 13:09:14 -0700
Subject: [PATCH] gpt: prefer disk size from header over firmware

The firmware and the OS may disagree on the disk configuration and size.
Although such a setup should be avoided users are unlikely to know about
the problem, assuming everything behaves like the OS. Tolerate this as
best we can and trust the reported on-disk location over the firmware
when looking for the backup GPT. If the location is inaccessible report
the error as best we can and move on.
---
 grub-core/lib/gpt.c   | 18 +++++++++++++-----
 tests/gpt_unit_test.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 5 deletions(-)

diff --git a/grub-core/lib/gpt.c b/grub-core/lib/gpt.c
index 205779192..f0c71bde1 100644
--- a/grub-core/lib/gpt.c
+++ b/grub-core/lib/gpt.c
@@ -394,13 +394,21 @@ grub_gpt_read_backup (grub_disk_t disk, grub_gpt_t gpt)
   grub_disk_addr_t addr;
 
   /* Assumes gpt->log_sector_size == disk->log_sector_size  */
-  if (grub_gpt_disk_size_valid(disk))
+  if (gpt->status & GRUB_GPT_PRIMARY_HEADER_VALID)
+    {
+      sector = grub_le_to_cpu64 (gpt->primary.alternate_lba);
+      if (grub_gpt_disk_size_valid (disk) && sector >= disk->total_sectors)
+	return grub_error (GRUB_ERR_OUT_OF_RANGE,
+			   "backup GPT located at 0x%llx, "
+			   "beyond last disk sector at 0x%llx",
+			   (unsigned long long) sector,
+			   (unsigned long long) disk->total_sectors - 1);
+    }
+  else if (grub_gpt_disk_size_valid (disk))
     sector = disk->total_sectors - 1;
-  else if (gpt->status & GRUB_GPT_PRIMARY_HEADER_VALID)
-    sector = grub_le_to_cpu64 (gpt->primary.alternate_lba);
   else
-    return grub_error (GRUB_ERR_NOT_IMPLEMENTED_YET,
-		       "Unable to locate backup GPT");
+    return grub_error (GRUB_ERR_OUT_OF_RANGE,
+		       "size of disk unknown, cannot locate backup GPT");
 
   grub_dprintf ("gpt", "reading backup GPT from sector 0x%llx\n",
 		(unsigned long long) sector);
diff --git a/tests/gpt_unit_test.c b/tests/gpt_unit_test.c
index 9cf3414c2..218b18697 100644
--- a/tests/gpt_unit_test.c
+++ b/tests/gpt_unit_test.c
@@ -544,6 +544,46 @@ repair_test (void)
   close_disk (&data);
 }
 
+/* Finding/reading/writing the backup GPT may be difficult if the OS and
+ * BIOS report different sizes for the same disk.  We need to gracefully
+ * recognize this and avoid causing trouble for the OS.  */
+static void
+weird_disk_size_test (void)
+{
+  struct test_data data;
+  grub_gpt_t gpt;
+
+  open_disk (&data);
+
+  /* Chop off 65536 bytes (128 512B sectors) which may happen when the
+   * BIOS thinks you are using a software RAID system that reserves that
+   * area for metadata when in fact you are not and using the bare disk.  */
+  grub_test_assert(data.dev->disk->total_sectors == DISK_SECTORS,
+		   "unexpected disk size: 0x%llx",
+		   (unsigned long long) data.dev->disk->total_sectors);
+  data.dev->disk->total_sectors -= 128;
+
+  gpt = read_disk (&data);
+  assert_error_stack_empty ();
+  /* Reading the alternate_lba should have been blocked and reading
+   * the (new) end of disk should have found no useful data.  */
+  grub_test_assert ((gpt->status & GRUB_GPT_BACKUP_HEADER_VALID) == 0,
+		    "unreported missing backup header");
+
+  /* We should be able to reconstruct the backup header and the location
+   * of the backup should remain unchanged, trusting the GPT data over
+   * what the BIOS is telling us.  Further changes are left to the OS.  */
+  grub_gpt_repair (data.dev->disk, gpt);
+  grub_test_assert (grub_errno == GRUB_ERR_NONE,
+		    "repair failed: %s", grub_errmsg);
+  grub_test_assert (memcmp (&gpt->primary, &example_primary,
+	                    sizeof (gpt->primary)) == 0,
+		    "repair corrupted primary header");
+
+  grub_gpt_free (gpt);
+  close_disk (&data);
+}
+
 static void
 iterate_partitions_test (void)
 {
@@ -774,6 +814,7 @@ grub_unit_test_init (void)
   grub_test_register ("gpt_iterate_partitions_test", iterate_partitions_test);
   grub_test_register ("gpt_large_partitions_test", large_partitions_test);
   grub_test_register ("gpt_invalid_partsize_test", invalid_partsize_test);
+  grub_test_register ("gpt_weird_disk_size_test", weird_disk_size_test);
   grub_test_register ("gpt_search_part_label_test", search_part_label_test);
   grub_test_register ("gpt_search_uuid_test", search_part_uuid_test);
   grub_test_register ("gpt_search_disk_uuid_test", search_disk_uuid_test);
@@ -791,6 +832,7 @@ grub_unit_test_fini (void)
   grub_test_unregister ("gpt_iterate_partitions_test");
   grub_test_unregister ("gpt_large_partitions_test");
   grub_test_unregister ("gpt_invalid_partsize_test");
+  grub_test_unregister ("gpt_weird_disk_size_test");
   grub_test_unregister ("gpt_search_part_label_test");
   grub_test_unregister ("gpt_search_part_uuid_test");
   grub_test_unregister ("gpt_search_disk_uuid_test");