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UDF filesystem fixes for grub

From: Giuseppe Caizzone
Subject: UDF filesystem fixes for grub
Date: Tue, 2 Nov 2010 14:10:08 +0100


I've made 4 small patches that improve the UDF filesystem support in
grub. With these changes, I've been able to read any regular file in
UDF filesystems created by both Linux and Windows on both optical
storage and USB sticks. Perhaps they might be useful for other grub
I don't know if this list is the most appropriate place to post them,
I apologise for the noise if it isn't.

I'm attaching the patches, which are against last week's trunk, and a
simple bash test case for each.

- - - -

1/4) Support for large (> 2GiB) files

I know that grub isn't a typical consumer of huge files, but since the
fix is apparently trivial...

How to test that the patch is needed (tried with grub trunk on linux 2.6.36):

# dd if=/dev/zero of=/test.bin bs=1M count=3072
# mkfs.udf /test.bin
# mount -o loop /test.bin /mnt/m1
# dd if=/dev/urandom of=/mnt/m1/bigfile.bin bs=1M count=2560
# umount /mnt/m1
# grub-fstest /test.bin crc "(loop0)/bigfile.bin"

Result: grub will segfault instead of reading the whole file (tested
on a 32-bit machine).
The patch just changes an int and a grub_uint32_t to be grub_off_t.

2/4) Support for deleted files

On real-world, non-readonly media, deleted files are likely to appear
inside directories. These files' directory entries have a "deleted"
bit, which grub currently ignores.

Test case:

# dd if=/dev/zero of=/test.bin bs=1M count=32
# mkfs.udf /test.bin
# mount -o loop /test.bin /mnt/m1
# touch /mnt/m1/file{1,2,3,4,5}
# rm /mnt/m1/file3
# umount /mnt/m1
# grub-fstest /test.bin ls "(loop0)/"

Result: only the first two files will be listed, because grub will
"crash" when, encountering the third file's deleted directory entry,
it tries to "dereference" a zeroed field.
The patch just skips directory entries with the "deleted" bit set, in

3/4) Support for a generic block size

Currently grub assumes that all UDF filesystems have a logical block
size of 2048 bytes. But the UDF standard states that the block size
should match the physical sector size of the hosting device, so for
instance Windows only creates filesystems with a block size of 512
bytes on USB sticks. Under Linux, mkudffs allows for other block sizes
as well, up to 4096 bytes. None of these are currently readable by

Test case:

# dd if=/dev/zero of=/test.bin bs=1M count=32
# mkfs.udf --blocksize=512 /test.bin
# grub-fstest /test.bin ls "(loop0)/"

Result: the file system won't be recognised as UDF by grub.
The patch repeats the superblock search in grub_udf_mount() for 512,
1024, 2048 and 4096 block sizes, stopping if it finds a valid one AND
the sector offset recorded in that superblock matches the detected
superblock offset. So for instance it won't misdetect a superblock
located at sector 256 with blocksize 2048 for a superblock located at
sector 512 with blocksize 1024.
The log2(blocksize/512) is then stored in a new field called "lbshift"
in struct grub_udf_data, which gets used instead of the previous
constant GRUB_UDF_LOG2_BLKSZ, while the constant GRUB_UDF_BLKSZ gets
replaced with the lvd.bsize field already present in struct

4/4) Support for allocation descriptor extensions

When files on an UDF file system grow and fragmentation appears, an
"allocation descriptor extension" entry can be recorded, which points
to a new block containing more allocation descriptors for the file.
Currently grub ignores these entries, and is thus unable to read such
fragmented files properly.


# dd if=/dev/zero of=/test.bin bs=1M count=32
# mkfs.udf --blocksize=512 /test.bin
# mount -o loop /test.bin /mnt/m1
# for ((i=0;i<8192;i++)) ; do touch "/mnt/m1/file$i" ; done
# umount /mnt/m1
# grub-fstest /test.bin ls "(loop0)/"

Result: only the first 4820 files appear (because the directory itself
gets fragmented, and when grub reaches the jump between the two
fragments, it erroneously tries to read the allocation extension
descriptor as the directory's body).
The patch checks, in grub_udf_read_block(), whether the "allocation
descriptor type", previously ignored, is 3, and in this case, it loads
the referenced block, checks that it's a valid allocation extension
descriptor, and sets the "ad" pointer (and the "len" limit) to
continue the iteration from the allocation descriptors contained in
that block.

This last one is the only patch which actually contains a proper new
block of code, and it allocates a big structure on the stack (it's a
sector, so it's up to 4K big): I don't know if this is OK, or if
perhaps I should use grub_malloc() instead. Also, the patch depends on
the previous "generic block size" patch.

-- Giuseppe Caizzone <address@hidden>

Attachment: 1-udf-large-files.patch
Description: Binary data

Attachment: 2-udf-deleted-files.patch
Description: Binary data

Attachment: 3-udf-generic-block-size.patch
Description: Binary data

Attachment: 4-udf-ad-extents.patch
Description: Binary data

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