Libguestfs - Mar 2021 - Consider to replace isoinfo by own code rather than by xorriso

Hi,

thank you for flying xorriso. :))

But as its author and looking at

  https://github.com/libguestfs/libguestfs/blob/master/daemon/isoinfo.ml

i feel obliged to point out that using it as replacement for isoinfo
will increase the consumption of RAM and CPU cycles substantially.

xorriso command -indev loads the whole directory tree of the ISO filesystem
with names and metadata. Depending on that tree this can be dozens of
megabytes and lots of insertion operations in a not much optimized tree
model.
On the other hand it is quite uncomplicated to implement an own ISO 9660
PVD reader and to become independent of any external program for that
purpose.

The Primary Volume Descriptor begins at LBA 16 (= byte offset 32768).
Its layout is described in ECMA-119 8.4 (note: BP = byte offset + 1)
 
https://www.ecma-international.org/wp-content/uploads/ECMA-119_4th_edition_june_2019.pdf
or in HTML at
  https://wiki.osdev.org/ISO_9660#The_Primary_Volume_Descriptor

E.g. the info missing in xorriso's output of -pvd_info is at offsets:

   80 -  87 iso_volume_space_size
            (unsigned 32 bit, little endian first, then again as big endian)

  124 - 127 iso_volume_sequence_number
            (unsigned 16 bit, little endian first, then again as big endian)

  128 - 131 iso_logical_block_size
            (unsigned 16 bit, little endian first, then again as big endian)
            (If you don't read 2048 here, then you are in unchartered
             territory.)

The date format for e.g. iso_volume_creation_t at offset 813 - 829
is a decimal digit string YYYMMDDhhmmsshh with a trailing binary byte
value for the time zone with 15 minutes granularity. ("hh" means
hundredths of seconds.)
See also
  https://wiki.osdev.org/ISO_9660#Numerical_formats
  https://wiki.osdev.org/ISO_9660#Date.2Ftime_format


Have a nice day :)

Thomas

On Tue, Mar 30, 2021 at 10:57:07PM +0200, Thomas Schmitt
wrote:
> Hi,
> 
> thank you for flying xorriso. :))
> 
> But as its author and looking at
> 
>   https://github.com/libguestfs/libguestfs/blob/master/daemon/isoinfo.ml
> 
> i feel obliged to point out that using it as replacement for isoinfo
> will increase the consumption of RAM and CPU cycles substantially.
> 
> xorriso command -indev loads the whole directory tree of the ISO filesystem
> with names and metadata. Depending on that tree this can be dozens of
> megabytes and lots of insertion operations in a not much optimized tree
> model.
"megabytes" may not be a problem - and in fact hasn't been in my
testing.  Could it grow unbounded?
> On the other hand it is quite uncomplicated to implement an own ISO 9660
> PVD reader and to become independent of any external program for that
> purpose.
> 
> The Primary Volume Descriptor begins at LBA 16 (= byte offset 32768).
> Its layout is described in ECMA-119 8.4 (note: BP = byte offset + 1)
>  
https://www.ecma-international.org/wp-content/uploads/ECMA-119_4th_edition_june_2019.pdf
> or in HTML at
>   https://wiki.osdev.org/ISO_9660#The_Primary_Volume_Descriptor
> 
> E.g. the info missing in xorriso's output of -pvd_info is at offsets:
> 
>    80 -  87 iso_volume_space_size
>             (unsigned 32 bit, little endian first, then again as big
endian)
> 
>   124 - 127 iso_volume_sequence_number
>             (unsigned 16 bit, little endian first, then again as big
endian)
> 
>   128 - 131 iso_logical_block_size
>             (unsigned 16 bit, little endian first, then again as big
endian)
>             (If you don't read 2048 here, then you are in unchartered
>              territory.)
> 
> The date format for e.g. iso_volume_creation_t at offset 813 - 829
> is a decimal digit string YYYMMDDhhmmsshh with a trailing binary byte
> value for the time zone with 15 minutes granularity. ("hh" means
> hundredths of seconds.)
> See also
>   https://wiki.osdev.org/ISO_9660#Numerical_formats
>   https://wiki.osdev.org/ISO_9660#Date.2Ftime_format
OK, thanks.

Rich.

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