On Tue, Feb 20, 2018 at 06:01:19PM +0100, Daniel Kiper wrote:
> On Mon, Feb 19, 2018 at 03:26:35PM +0100, C. Masloch wrote:
> > The definition of bpb's num_total_sectors_16 and num_total_sectors_32
> > is that either the 16-bit field is non-zero and is used (in which case
> > eg mkfs.fat sets the 32-bit field to zero), or it is zero and the
> > 32-bit field is used. Therefore, a BPB is invalid only if *both*
> > fields are zero; having one field as zero and the other as non-zero is
> > the case to be expected. (Indeed, according to Microsoft's specification
> > one of the fields *must* be zero, and the other non-zero.)
> >
> > This affects all users of grub_chainloader_patch_bpb which are in
> > chainloader.c, freedos.c, and ntldr.c
> >
> > Some descriptions of the semantics of these two fields:
> >
> > https://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html
> >
> > > The old 2-byte fields "total number of sectors" and "number of
> > > sectors per FAT" are now zero; this information is now found in
> > > the new 4-byte fields.
> >
> > (Here given in the FAT32 EBPB section but the total sectors 16/32 bit
> > fields semantic is true of FAT12 and FAT16 too.)
> >
> > https://wiki.osdev.org/FAT#BPB_.28BIOS_Parameter_Block.29
> >
> > > 19 | 2 | The total sectors in the logical volume. If this value is 0,
> > > it means there are more than 65535 sectors in the volume, and the actual
> > > count is stored in "Large Sectors (bytes 32-35).
> >
> > > 32 | 4 | Large amount of sector on media. This field is set if there
> > > are more than 65535 sectors in the volume.
> >
> > (Doesn't specify what the "large" field is set to when unused, but as
> > mentioned mkfs.fat sets it to zero then.)
> >
> > https://technet.microsoft.com/en-us/library/cc976796.aspx
> >
> > > 0x13 | WORD | 0x0000 |
> > > Small Sectors . The number of sectors on the volume represented in 16
> > > bits (< 65,536). For volumes larger than 65,536 sectors, this field
> > > has a value of zero and the Large Sectors field is used instead.
> >
> > > 0x20 | DWORD | 0x01F03E00 |
> > > Large Sectors . If the value of the Small Sectors field is zero, this
> > > field contains the total number of sectors in the FAT16 volume. If the
> > > value of the Small Sectors field is not zero, the value of this field
> > > is zero.
> >
> > https://staff.washington.edu/dittrich/misc/fatgen103.pdf page 10
> >
> > > BPB_TotSec16 | 19 | 2 |
> > > This field is the old 16-bit total count of sectors on the volume.
> > > This count includes the count of all sectors in all four regions of the
> > > volume. This field can be 0; if it is 0, then BPB_TotSec32 must be
> > > non-zero. For FAT32 volumes, this field must be 0. For FAT12 and
> > > FAT16 volumes, this field contains the sector count, and
> > > BPB_TotSec32 is 0 if the total sector count ???fits??? (is less than
> > > 0x10000).
> >
> > > BPB_TotSec32 | 32 | 4 |
> > > This field is the new 32-bit total count of sectors on the volume.
> > > This count includes the count of all sectors in all four regions of the
> > > volume. This field can be 0; if it is 0, then BPB_TotSec16 must be
> > > non-zero. For FAT32 volumes, this field must be non-zero. For
> > > FAT12/FAT16 volumes, this field contains the sector count if
> > > BPB_TotSec16 is 0 (count is greater than or equal to 0x10000).
> >
> > (This specifies that an unused BPB_TotSec32 field is set to zero.)
> >
> > Tested with lDebug booted in qemu via grub2's
> > FreeDOS direct loading support, refer to
> > https://bitbucket.org/ecm/ldosboot + https://bitbucket.org/ecm/ldebug
> >
> > Signed-off-by: C. Masloch <pus...@38.de>
>
> I will add to the commit message that by the way you are fixing comments in 
> include/grub/fat.h.
>
> Otherwise Reviewed-by: Daniel Kiper <daniel.ki...@oracle.com>

Applied!

FYI, this is last time when I sent commit confirmation.
If I send one in the future then this will be rather
exception than rule. So, if you receive my reviewed-by
and patch is not committed in a week or two then it means
that it fallen into abyss and you have to ping me.

Have a nice weekend,

Daniel

_______________________________________________
Grub-devel mailing list
Grub-devel@gnu.org
https://lists.gnu.org/mailman/listinfo/grub-devel

Reply via email to