[PATCH v4] slof/fs/packages/disk-label.fs: improve checking for DOS boot partitions

While testing with a qcow2 with a DOS boot partition it was found that
when we set the logical_block_size in the guest XML to >512 then the
boot would fail in the following interminable loop:
<SNIP>
Trying to load:  from: /pci@800000020000000/scsi@3 ...
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
</SNIP>

Change the "read-sector" Forth subroutine to throw an exception whenever
it fails to read a full block-size length of sector from the disk.
Also change the "open" method to initiate CATCH exception handling for the calls to
try-partitions/try-files which will also call read-sector which could potentially
now throw this new exception.

After making the above changes, it fails properly with the correct error
message as follows:
<SNIP>
Trying to load:  from: /pci@800000020000000/scsi@3 ...
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!
virtioblk_transfer: Access beyond end of device!

E3404: Not a bootable device!

E3407: Load failed

  Type 'boot' and press return to continue booting the system.
  Type 'reset-all' and press return to reboot the system.

Ready!
0 >
</SNIP>

Signed-off-by: Kautuk Consul <[email protected]>
---
 slof/fs/packages/disk-label.fs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/slof/fs/packages/disk-label.fs b/slof/fs/packages/disk-label.fs
index 661c6b0..a6fb231 100644
--- a/slof/fs/packages/disk-label.fs
+++ b/slof/fs/packages/disk-label.fs
@@ -136,7 +136,8 @@ CONSTANT /gpt-part-entry
 : read-sector ( sector-number -- )
    \ block-size is 0x200 on disks, 0x800 on cdrom drives
    block-size * 0 seek drop      \ seek to sector
-   block block-size read drop    \ read sector
+   block block-size read         \ read sector
+   block-size < IF throw THEN    \ if we read less than the block-size then throw an exception
 ;
 
 : (.part-entry) ( part-entry )
@@ -723,10 +724,15 @@ CREATE GPT-LINUX-PARTITION 10 allot
    THEN
 
    partition IF
-       try-partitions
+       ['] try-partitions
    ELSE
-       try-files
+       ['] try-files
    THEN
+
+   \ Catch any exception that might happen due to read-sector failing to read
+   \ block-size number of bytes from any sector of the disk.
+   CATCH IF false THEN
+
    dup 0= IF debug-disk-label? IF ." not found." cr THEN close THEN \ free memory again
 ;
 
-- 
2.31.1