[Libguestfs] Consider to replace isoinfo by own code rather than by xorriso

[Richard W.M. Jones]

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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