Subject: Re: [PATCH SLOF v2 5/5] disk-label: add support for booting from GPT FAT partition

From: Nikunj A Dadhania <[email-masked]>
Date: Mon, 29 Jun 2015 16:43:38 +0530

Thomas Huth <[email-masked]> writes:

> On Thu, 25 Jun 2015 12:15:29 +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 | 96 +++++++++++++++++++++++++++++++++---------
>>  1 file changed, 76 insertions(+), 20 deletions(-)
>> 
>> diff --git a/slof/fs/packages/disk-label.fs b/slof/fs/packages/disk-label.fs
>> index 7ed5526..e5759a3 100644
>> --- a/slof/fs/packages/disk-label.fs
>> +++ b/slof/fs/packages/disk-label.fs
> ...
>> @@ -378,29 +382,80 @@ AA268B49521E5A8B    CONSTANT GPT-PREP-PARTITION-4
>>     true
>>  ;
>>  
>> -: 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 <> IF false EXIT THEN
>> +   true
>> +;
>
> You could remove the "<> IF false EXIT THEN true" at the end and
> replace it with a simple "=".

Sure, will update the prep code as well.

>
>> +\
>> +\ GPT Signature
>> +\ ("EFI PART", 45h 46h 49h 20h 50h 41h 52h 54h)
>> +\
>> +4546492050415254 CONSTANT GPT-SIGNATURE
>> +
>> +: verify-gpt-partition ( -- true | false )
>
> I'd prefer if you could write "true|false" without spaces around the
> "|" so that it is clear at a glance that there is only one item on the
> stack.

Sure.

> Could you maybe also add a comment above the function that it sets up
> gpt-part-size with the size of partition entry and seek-pos with the
> position of the partition entry? ...since these are non-obvious
> side-effect of this function... All in all, maybe you should also name
> the function differently, since it does more than just verifying.

Yes, I was thinking to have this out of verify-gpt-partition, but then
it would be duplication, will rename and add comments accordingly.

>> +   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 )
>> +   verify-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 )
>
> The stack comment looks wrong here, should this be:
>
>  ( addr blocks first-lba )
>
> ?

Yes, i missed first-lap at top :-(

>
>> +         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
>> +   LOOP
>> +   false
>> +;
>> +
>> +: try-gpt-dos-partition ( -- true | false )
>
> "true | false" --> "true|false" ?

OK

>
>> +   verify-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 LINUX DATA partition found " cr THEN
>
> I think that string should maybe rather talk about "basic data
> partition" instead of "LINUX data partition" ?

Ok

>
>> +         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? 0= IF false UNLOOP EXIT THEN
>> +         true UNLOOP EXIT
>
> You could simplify the above two lines to:
>
>             disk-buf fat-bootblock?
>             UNLOOP EXIT

Right, as we exit in both cases.

>
>>        THEN
>>        seek-pos gpt-part-size i * + to seek-pos
>>     LOOP
>> @@ -492,7 +547,7 @@ 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
>> @@ -592,6 +647,7 @@ 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
>
>  Thomas

Thanks
Nikunj