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

From: Thomas Huth <[email-masked]>
Date: Tue, 30 Jun 2015 13:28:34 +0200

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

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? Looks somewhat suspicious
to me, shouldn't this rather be:

	seek-pos gpt-part-size + to seek-pos

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?)

 Thomas