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Re: grub-mkrescue fails with HFS+ error possibly due to directory size

From: Thomas Schmitt
Subject: Re: grub-mkrescue fails with HFS+ error possibly due to directory size
Date: Fri, 12 Jun 2020 11:30:56 +0200


(cc-ing and the reporter of the problem.)

I now have the Guix ISO which fails when created by grub-mkrescue
with HFS+ tree.

To my newest theory it is not about the number of files in a directory
but about the total number of files in the tree and their name lengths.

I wonder how to describe this limit to the users of grub-mkrescue.

  grub-mkrescue for platforms I386_EFI, X86_64_EFI, and POWERPC_IEEE1275
  has a limit on the number of files multiplied by their average name
  length. Beginning with about 300,000 files of usual name length
  expect a xorriso error
    libisofs: FAILURE : HFS+ map nodes aren't implemented
  plus a rather misleading error message
    libisofs: FAILURE : Too much files to mangle, cannot guarantee
                        unique file names

  The limit can only be avoided by suppressing xorrisofs option -hfsplus.
  This can be done by using xorriso script
  with MKRESCUE_SED_MODE="mbr_only" or MKRESCUE_SED_MODE="gpt_appended".
  Another method is to add as last arguments of grub-mkrescue these two
    -- -hfsplus off
  in order to leave xorriso's mkisofs emulation mode and to disable HFS+
  production by a generic xorriso command.

I still hope for a clarifying comment by Vladimir Serbinenko.


Riddling what is overflowing in hfsplus.c i found some description of HFS+

Since hfsplus.c reports to need "map nodes" i assume that it is the
"header node" which contains a "map record". Map nodes would then
be data structures which contain more map records.
So for now i believe the overflow is in the "B-tree Map Record" of
the header node.
  "It is a bitmap that indicates which nodes in the B-tree are used
   and which are free. The bits are interpreted in the same way as the
   bits in the allocation file."

The number in  target->hfsp_nnodes  which causes the overflow is 35487.
This is not the number of files 434,920. The number in
is 869842 which is (434920+1)*2.
The loop which accumulates the number of target->hfsp_nnodes iterates
over this number of 869842.

I guess that the 869842 leafs are planned to get stored in allocation
blocks of target->hfsp_cat_node_size which i now know is 4096.
Each leaf occupies 50 to 200 bytes in the allocation block.
This roughly matches the ratio of 35487 * 4096 / 869842  = 167.

So it seems to be about the number of files and the sum of their name
lengths. The size limit gets exceeded by about 19,525,632 bytes or
(estimated by above ratio) 116,920 files.

Have a nice day :)


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