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[PATCH 0/2] fs/iso9660: Delay CE hop until end of current SUSP area

From: Thomas Schmitt
Subject: [PATCH 0/2] fs/iso9660: Delay CE hop until end of current SUSP area
Date: Tue, 7 Mar 2023 17:56:49 +0100


SUSP 1.12 says:

  The "CE" System Use Entry indicates a Continuation Area that shall be
  processed after the current System Use field or Continuation Area is

But GRUB rather takes an encountered CE entry as reason to immediately
switch reading to the location that is given by the CE entry.
This can skip over important information.

The usual ISO 9660 producers on GNU/Linux write the CE entry as last
entry of System Use field or Continuation Area. So the problem does not
show up with their output. Nevertheless, Linux and libisofs obey the
specs whereas GRUB does not.

As demonstration i crafted a small ISO, where the CE entry comes before
the NM entry which tells the Rock Ridge file name "RockRidgeName:x".
Linux shows the NM name, nevertheless:
  $ sudo mount iso9660_early_ce.iso /mnt/iso
  mount: /mnt/iso: WARNING: source write-protected, mounted read-only.
  $ ls /mnt/iso

GRUB does not see the NM entry and thus shows the dull ISO 9660 name
(which is actually "ROCKRIDG.;1"):
  $ ./grub-fstest iso9660_early_ce.iso ls /

After the code change of my patch, i get:
  $ ./grub-fstest iso9660_early_ce.iso ls /

A new code block in tests/ verifies that the patched code
is in effect:
  make check TESTS=iso9660_test
detects the old code state and shows that the new code still has the
capability to cope with endless CE loops.

How to create an ISO 9660 filesystem where CE is not the last SUSP entry
of a file's directory record:

   # Deliberately chosen names
   # rr_path is longer than 8, mixed-case, with non-ISO-9660 character

   # A dummy file as payload
   echo x >x

   # 250 fattr characters to surely exceed the size of a directory record

   # Create ISO with the payload file and attached fattr named user.dummy .
   # Make it small with the most restrictive ISO 9660 file name rules.
   test -e "$iso" && rm "$iso"
   xorriso -compliance no_emul_toc:iso_9660_level=1 \
           -padding 0 \
           -outdev "$iso" \
           -xattr on \
           -map x "$rr_path" \
           -setfattr user.dummy "$long_string" "$rr_path" --

   # Cut out the NM field and the CE field from the directory record
   # of $rr_path. The numbers were determined by looking at a hex dump.
   dd if="$iso" bs=1 skip=37198 count=20 of=nm_field
   dd if="$iso" bs=1 skip=37218 count=28 of=ce_field

   # Put them back in reverse sequence
   dd conv=notrunc if=ce_field bs=1 seek=37198 of="$iso"
   dd conv=notrunc if=nm_field bs=1 seek=37226 of="$iso"


Have a nice day :)


Thomas Schmitt (2):
  fs/iso9660: Delay CE hop until end of current SUSP area
  tests: Add test for iso9660 delayed CE hop

 grub-core/fs/iso9660.c        |  84 ++++++++++++++++++----------------
 tests/iso9660_early_ce.iso.gz | Bin 0 -> 709 bytes
 tests/         |  24 ++++++++++
 3 files changed, 68 insertions(+), 40 deletions(-)
 create mode 100644 tests/iso9660_early_ce.iso.gz


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