134 lines
5.2 KiB
Diff
134 lines
5.2 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Kiper <daniel.kiper@oracle.com>
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Date: Fri, 23 Feb 2018 22:32:55 +0100
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Subject: [PATCH] chainloader: Fix wrong break condition (must be AND not, OR)
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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The definition of bpb's num_total_sectors_16 and num_total_sectors_32
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is that either the 16-bit field is non-zero and is used (in which case
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eg mkfs.fat sets the 32-bit field to zero), or it is zero and the
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32-bit field is used. Therefore, a BPB is invalid only if *both*
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fields are zero; having one field as zero and the other as non-zero is
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the case to be expected. (Indeed, according to Microsoft's specification
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one of the fields *must* be zero, and the other non-zero.)
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This affects all users of grub_chainloader_patch_bpb which are in
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chainloader.c, freedos.c, and ntldr.c
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Some descriptions of the semantics of these two fields:
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https://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html
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The old 2-byte fields "total number of sectors" and "number of
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sectors per FAT" are now zero; this information is now found in
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the new 4-byte fields.
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(Here given in the FAT32 EBPB section but the total sectors 16/32 bit
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fields semantic is true of FAT12 and FAT16 too.)
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https://wiki.osdev.org/FAT#BPB_.28BIOS_Parameter_Block.29
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19 | 2 | The total sectors in the logical volume. If this value is 0,
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it means there are more than 65535 sectors in the volume, and the actual
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count is stored in "Large Sectors (bytes 32-35).
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32 | 4 | Large amount of sector on media. This field is set if there
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are more than 65535 sectors in the volume.
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(Doesn't specify what the "large" field is set to when unused, but as
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mentioned mkfs.fat sets it to zero then.)
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https://technet.microsoft.com/en-us/library/cc976796.aspx
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0x13 | WORD | 0x0000 |
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Small Sectors . The number of sectors on the volume represented in 16
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bits (< 65,536). For volumes larger than 65,536 sectors, this field
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has a value of zero and the Large Sectors field is used instead.
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0x20 | DWORD | 0x01F03E00 |
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Large Sectors . If the value of the Small Sectors field is zero, this
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field contains the total number of sectors in the FAT16 volume. If the
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value of the Small Sectors field is not zero, the value of this field
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is zero.
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https://staff.washington.edu/dittrich/misc/fatgen103.pdf page 10
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BPB_TotSec16 | 19 | 2 |
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This field is the old 16-bit total count of sectors on the volume.
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This count includes the count of all sectors in all four regions of the
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volume. This field can be 0; if it is 0, then BPB_TotSec32 must be
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non-zero. For FAT32 volumes, this field must be 0. For FAT12 and
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FAT16 volumes, this field contains the sector count, and
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BPB_TotSec32 is 0 if the total sector count “fits” (is less than
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0x10000).
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BPB_TotSec32 | 32 | 4 |
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This field is the new 32-bit total count of sectors on the volume.
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This count includes the count of all sectors in all four regions of the
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volume. This field can be 0; if it is 0, then BPB_TotSec16 must be
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non-zero. For FAT32 volumes, this field must be non-zero. For
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FAT12/FAT16 volumes, this field contains the sector count if
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BPB_TotSec16 is 0 (count is greater than or equal to 0x10000).
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(This specifies that an unused BPB_TotSec32 field is set to zero.)
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By the way fix offsets in include/grub/fat.h.
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Tested with lDebug booted in qemu via grub2's
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FreeDOS direct loading support, refer to
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https://bitbucket.org/ecm/ldosboot + https://bitbucket.org/ecm/ldebug
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Signed-off-by: C. Masloch <pushbx@38.de>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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---
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grub-core/loader/i386/pc/chainloader.c | 2 +-
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include/grub/fat.h | 17 ++++++-----------
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2 files changed, 7 insertions(+), 12 deletions(-)
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diff --git a/grub-core/loader/i386/pc/chainloader.c b/grub-core/loader/i386/pc/chainloader.c
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index c79c4fe0fc9..18220b7aaab 100644
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--- a/grub-core/loader/i386/pc/chainloader.c
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+++ b/grub-core/loader/i386/pc/chainloader.c
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@@ -117,7 +117,7 @@ grub_chainloader_patch_bpb (void *bs, grub_device_t dev, grub_uint8_t dl)
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if (bpb->num_reserved_sectors == 0)
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break;
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- if (bpb->num_total_sectors_16 == 0 || bpb->num_total_sectors_32 == 0)
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+ if (bpb->num_total_sectors_16 == 0 && bpb->num_total_sectors_32 == 0)
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break;
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if (bpb->num_fats == 0)
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diff --git a/include/grub/fat.h b/include/grub/fat.h
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index 4a5aab79346..8d7e4a1e54d 100644
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--- a/include/grub/fat.h
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+++ b/include/grub/fat.h
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@@ -28,20 +28,15 @@ struct grub_fat_bpb
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grub_uint16_t bytes_per_sector;
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grub_uint8_t sectors_per_cluster;
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grub_uint16_t num_reserved_sectors;
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- grub_uint8_t num_fats;
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- /* 0x10 */
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+ grub_uint8_t num_fats; /* 0x10 */
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grub_uint16_t num_root_entries;
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grub_uint16_t num_total_sectors_16;
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- grub_uint8_t media;
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- /*0 x15 */
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+ grub_uint8_t media; /* 0x15 */
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grub_uint16_t sectors_per_fat_16;
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- grub_uint16_t sectors_per_track;
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- /*0 x19 */
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- grub_uint16_t num_heads;
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- /*0 x1b */
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- grub_uint32_t num_hidden_sectors;
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- /* 0x1f */
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- grub_uint32_t num_total_sectors_32;
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+ grub_uint16_t sectors_per_track; /* 0x18 */
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+ grub_uint16_t num_heads; /* 0x1A */
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+ grub_uint32_t num_hidden_sectors; /* 0x1C */
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+ grub_uint32_t num_total_sectors_32; /* 0x20 */
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union
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{
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struct
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