Subject: Re: [PATCH SLOF v3 5/5] disk-label: add support for booting from GPT FAT partition

From: Nikunj A Dadhania <[email-masked]>
Date: Tue, 30 Jun 2015 17:54:14 +0530

Thomas Huth <[email-masked]> writes:

> Sorry, every time I look at this gpt stuff, my eyes stumble
> over something new ...

No worries :-)

> On Tue, 30 Jun 2015 16:31:21 +0530
> Nikunj A Dadhania <[email-masked]> wrote:
>
>> For a GPT+LVM combination disk, older bootloader that does not support
>> LVM, cannot load kernel from LVM.
>> 
>> The patch adds support to read from BASIC_DATA UUID partitions for the
>> case that the OS installer has installed the CHRP-BOOT config on a FAT
>> file system.
>> 
>> Makes GPT detection robust
>> * Check for Protective MBR Magic
>> * Check for valid GPT Signature
>> * Boundary check for allocated block size before reading into the
>>   buffer
>> 
>> Signed-off-by: Nikunj A Dadhania <[email-masked]>
>> ---
>>  slof/fs/packages/disk-label.fs | 99 +++++++++++++++++++++++++++++++++---------
>>  1 file changed, 79 insertions(+), 20 deletions(-)
>> 
>> diff --git a/slof/fs/packages/disk-label.fs b/slof/fs/packages/disk-label.fs
>> index 347dc5d..5267ddb 100644
>> --- a/slof/fs/packages/disk-label.fs
>> +++ b/slof/fs/packages/disk-label.fs
> ...
>> +\ The routine checks whether the protective MBR has GPT ID and then
>> +\ reads the gpt data from the sector. Also set the seek position and
>> +\ the partition size used in caller routines.
>> +
>> +: get-gpt-partition ( -- true|false )
>> +   no-gpt? IF false EXIT THEN
>> +   debug-disk-label? IF cr ." GPT partition found " cr  THEN
>> +   1 read-disk-buf
>> +   disk-buf gpt>part-entry-lba x@-le
>>     block-size * to seek-pos
>>     disk-buf gpt>part-entry-size l@-le to gpt-part-size
>> -   disk-buf gpt>num-part-entry l@-le dup 0= IF false EXIT THEN
>> +   gpt-part-size disk-buf-size > IF
>> +      cr ." GPT part size exceeds buffer allocated " cr
>> +      false exit
>> +   THEN
>> +   disk-buf gpt>signature x@ GPT-SIGNATURE =
>> +;
>> +
>> +: load-from-gpt-prep-partition ( addr -- size )
>> +   get-gpt-partition 0= IF false EXIT THEN
>> +   disk-buf gpt>num-part-entry l@-le dup 0= IF false exit THEN
>>     1+ 1 ?DO
>>        seek-pos 0 seek drop
>>        disk-buf gpt-part-size read drop gpt-prep-partition? IF
>> -         debug-disk-label? IF
>> -            ." GPT PReP partition found " cr
>> -         THEN
>> -         disk-buf gpt-part-entry>first-lba x@-le
>> -         disk-buf gpt-part-entry>last-lba x@-le
>> -         over - 1+                 ( addr offset len )
>> -         swap                      ( addr len offset )
>> -         block-size * to part-offset
>> -         0 0 seek drop             ( addr len )
>> -         block-size * read         ( size )
>> +         debug-disk-label? IF  ." GPT PReP partition found " cr THEN
>> +         disk-buf gpt-part-entry>first-lba x@-le ( addr first-lba )
>> +         disk-buf gpt-part-entry>last-lba x@-le  ( addr first-lba last-lba)
>> +         over - 1+                            ( addr first-lba blocks )
>> +         swap                                 ( addr blocks first-lba )
>> +         block-size * to part-offset          ( addr blocks )
>> +         0 0 seek drop                        ( addr blocks )
>> +         block-size * read                    ( size )
>> +         UNLOOP EXIT
>> +     THEN
>> +     seek-pos gpt-part-size i * + to seek-pos
>
> Is this the right way to update the seek pos?

Good catch !! its jumping ahead, should go in the order of 128byte
(gpt-part-size)

> Looks somewhat suspicious
> to me, shouldn't this rather be:
>
> 	seek-pos gpt-part-size + to seek-pos

Would use this. Updated patch attached.

> or maybe if you store the base value somewhere instead, something like:
>
> 	seek-pos-base gpt-part-size i * + to seek-pos
>
> ?
>
>> +   LOOP
>> +   false
>> +;
>> +
>> +: try-gpt-dos-partition ( -- true|false )
>> +   get-gpt-partition 0= IF false EXIT THEN
>> +   disk-buf gpt>num-part-entry l@-le dup 0= IF false EXIT THEN
>> +   1+ 1 ?DO
>> +      seek-pos 0 seek drop
>> +      disk-buf gpt-part-size read drop
>> +      gpt-basic-data-partition? IF
>> +         debug-disk-label? IF ." GPT BASIC DATA partition found " cr THEN
>> +         disk-buf gpt-part-entry>first-lba x@-le       ( first-lba )
>> +         dup to part-start                          ( first-lba )
>> +         disk-buf gpt-part-entry>last-lba x@-le        ( first-lba last-lba )
>> +         over - 1+                                  ( first-lba s1 )
>> +         block-size * to part-size                  ( first-lba )
>> +         block-size * to part-offset                ( )
>> +         0 0 seek drop
>> +         disk-buf block-size read drop
>> +         disk-buf fat-bootblock?                    ( true|false )
>>           UNLOOP EXIT
>>        THEN
>>        seek-pos gpt-part-size i * + to seek-pos
>
> dito (so this bug was likely there before?)

Seems was working because it happens to be the first partition. If not
would have failed.

    disk-label: add support for booting from GPT FAT partition
    
    For a GPT+LVM combination disk, older bootloader that does not support
    LVM, cannot load kernel from LVM.
    
    The patch adds support to read from BASIC_DATA UUID partitions for the
    case that the OS installer has installed the CHRP-BOOT config on a FAT
    file system.
    
    Makes GPT detection robust
    * Fix bug in seek-pos updation code
    * Check for Protective MBR Magic
    * Check for valid GPT Signature
    * Boundary check for allocated block size before reading into the
      buffer
    
    Signed-off-by: Nikunj A Dadhania <[email-masked]>

diff --git a/slof/fs/packages/disk-label.fs b/slof/fs/packages/disk-label.fs
index 347dc5d..fa5df27 100644
--- a/slof/fs/packages/disk-label.fs
+++ b/slof/fs/packages/disk-label.fs
@@ -179,7 +179,8 @@ CONSTANT /gpt-part-entry
 \ This word returns true if the currently loaded disk-buf has _NO_ GPT partition id
 : no-gpt? ( -- true|false )
    0 read-disk-buf
-   1 partition>part-entry part-entry>id c@ ee <>
+   1 partition>part-entry part-entry>id c@ ee <> IF true EXIT THEN
+   disk-buf mbr>magic w@-le aa55 <>
 ;
 
 : pc-extended-partition? ( part-entry-addr -- true|false )
@@ -267,7 +268,10 @@ CONSTANT /gpt-part-entry
 
 : try-dos-partition ( -- okay? )
    \ Read partition table and check magic.
-   no-mbr? IF cr ." No DOS disk-label found." cr false EXIT THEN
+   no-mbr? IF
+       debug-disk-label? IF cr ." No DOS disk-label found." cr THEN
+       false EXIT
+   THEN
 
    count-dos-logical-partitions TO dos-logical-partitions
 
@@ -377,31 +381,85 @@ AA268B49521E5A8B    CONSTANT GPT-PREP-PARTITION-4
        8 + x@    GPT-PREP-PARTITION-4 =
 ;
 
-: load-from-gpt-prep-partition ( addr -- size )
-   no-gpt? IF drop false EXIT THEN
-   debug-disk-label? IF
-      cr ." GPT partition found " cr
-   THEN
-   1 read-disk-buf disk-buf gpt>part-entry-lba l@-le
+\ Check for GPT MSFT BASIC DATA GUID - fat based
+EBD0A0A2            CONSTANT GPT-BASIC-DATA-PARTITION-1
+B9E5                CONSTANT GPT-BASIC-DATA-PARTITION-2
+4433                CONSTANT GPT-BASIC-DATA-PARTITION-3
+87C068B6B72699C7    CONSTANT GPT-BASIC-DATA-PARTITION-4
+
+: gpt-basic-data-partition? ( -- true|false )
+   disk-buf gpt-part-entry>part-type-guid
+   dup l@-le     GPT-BASIC-DATA-PARTITION-1 <> IF drop false EXIT THEN
+   dup 4 + w@-le GPT-BASIC-DATA-PARTITION-2 <> IF drop false EXIT THEN
+   dup 6 + w@-le GPT-BASIC-DATA-PARTITION-3 <> IF drop false EXIT THEN
+       8 + x@    GPT-BASIC-DATA-PARTITION-4 =
+;
+
+\
+\ GPT Signature
+\ ("EFI PART", 45h 46h 49h 20h 50h 41h 52h 54h)
+\
+4546492050415254 CONSTANT GPT-SIGNATURE
+
+\ The routine checks whether the protective MBR has GPT ID and then
+\ reads the gpt data from the sector. Also set the seek position and
+\ the partition size used in caller routines.
+
+: get-gpt-partition ( -- true|false )
+   no-gpt? IF false EXIT THEN
+   debug-disk-label? IF cr ." GPT partition found " cr  THEN
+   1 read-disk-buf
+   disk-buf gpt>part-entry-lba x@-le
    block-size * to seek-pos
    disk-buf gpt>part-entry-size l@-le to gpt-part-size
-   disk-buf gpt>num-part-entry l@-le dup 0= IF false EXIT THEN
+   gpt-part-size disk-buf-size > IF
+      cr ." GPT part size exceeds buffer allocated " cr
+      false exit
+   THEN
+   disk-buf gpt>signature x@ GPT-SIGNATURE =
+;
+
+: load-from-gpt-prep-partition ( addr -- size )
+   get-gpt-partition 0= IF false EXIT THEN
+   disk-buf gpt>num-part-entry l@-le dup 0= IF false exit THEN
    1+ 1 ?DO
       seek-pos 0 seek drop
       disk-buf gpt-part-size read drop gpt-prep-partition? IF
-         debug-disk-label? IF
-            ." GPT PReP partition found " cr
-         THEN
-         disk-buf gpt-part-entry>first-lba x@-le
-         disk-buf gpt-part-entry>last-lba x@-le
-         over - 1+                 ( addr offset len )
-         swap                      ( addr len offset )
-         block-size * to part-offset
-         0 0 seek drop             ( addr len )
-         block-size * read         ( size )
+         debug-disk-label? IF  ." GPT PReP partition found " cr THEN
+         disk-buf gpt-part-entry>first-lba x@-le ( addr first-lba )
+         disk-buf gpt-part-entry>last-lba x@-le  ( addr first-lba last-lba)
+         over - 1+                            ( addr first-lba blocks )
+         swap                                 ( addr blocks first-lba )
+         block-size * to part-offset          ( addr blocks )
+         0 0 seek drop                        ( addr blocks )
+         block-size * read                    ( size )
+         UNLOOP EXIT
+     THEN
+     seek-pos gpt-part-size + to seek-pos
+   LOOP
+   false
+;
+
+: try-gpt-dos-partition ( -- true|false )
+   get-gpt-partition 0= IF false EXIT THEN
+   disk-buf gpt>num-part-entry l@-le dup 0= IF false EXIT THEN
+   1+ 1 ?DO
+      seek-pos 0 seek drop
+      disk-buf gpt-part-size read drop
+      gpt-basic-data-partition? IF
+         debug-disk-label? IF ." GPT BASIC DATA partition found " cr THEN
+         disk-buf gpt-part-entry>first-lba x@-le       ( first-lba )
+         dup to part-start                          ( first-lba )
+         disk-buf gpt-part-entry>last-lba x@-le        ( first-lba last-lba )
+         over - 1+                                  ( first-lba s1 )
+         block-size * to part-size                  ( first-lba )
+         block-size * to part-offset                ( )
+         0 0 seek drop
+         disk-buf block-size read drop
+         disk-buf fat-bootblock?                    ( true|false )
          UNLOOP EXIT
       THEN
-      seek-pos gpt-part-size i * + to seek-pos
+      seek-pos gpt-part-size + to seek-pos
    LOOP
    false
 ;
@@ -491,7 +549,7 @@ AA268B49521E5A8B    CONSTANT GPT-PREP-PARTITION-4
 
    debug-disk-label? IF ." Trying CHRP boot " .s cr THEN
    1 disk-chrp-boot !
-   dup load-chrp-boot-file ?dup 0 <> IF .s cr nip EXIT THEN
+   dup load-chrp-boot-file ?dup 0 <> IF nip EXIT THEN
    0 disk-chrp-boot !
 
    debug-disk-label? IF ." Trying GPT boot " .s cr THEN
@@ -591,6 +649,7 @@ AA268B49521E5A8B    CONSTANT GPT-PREP-PARTITION-4
 
 : try-partitions ( -- found? )
    try-dos-partition IF try-files EXIT THEN
+   try-gpt-dos-partition IF try-files EXIT THEN
    \ try-iso9660-partition IF try-files EXIT THEN
    \ ... more partition types here...
    false