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*** Description of problem:
In dosfstools-4.2 a new option appeared, called "--mbr". Under circumstances that matter much to libguestfs, namely when the disk being formatted is (a) non-removable and (b) unpartitioned (that is, a whole disk, not a partition), the "--mbr" option defaults to "yes". (For the sake of the reproducer below, we'll use an explicit "--mbr=yes".) When this option is in effect, "mkfs.fat" overlays the first sector of the medium (the FAT boot sector) with a bogus MBR partition table. The table has one entry, and it describes a partition that spans the entire disk, covering the partition table itself, resident in the first sector of the medium. This is supposed to placate Windows (otherwise Windows does not recognize the FAT filesystem on a non-removable device), and to work with Linux utilities as well (the kernel itself, and blkid, and so on).
In case the chosen FAT variant is FAT16 or FAT32, parted manages to parse the partition table, and prints the one entry in the table correctly. However, parted incorrectly reports the partition table type as "loop", rather than "msdos".
*** Version-Release number of selected component (if applicable):
- parted-3.4-6.el9.x86_64
(it should reproduce with upstream parted too, as of commit b20227adf575)
- dosfstools-4.2-3.el9.x86_64
*** How reproducible:
Always.
*** Steps to Reproduce:
(1) rm -f image
fallocate -l 255M image
mkfs.fat -F 16 --mbr=yes image
parted -m -s -- image unit b print
(2) rm -f image
fallocate -l 255M image
mkfs.fat -F 32 --mbr=yes image
parted -m -s -- image unit b print
*** Actual results:
parted prints the following:
(1)
BYT;
/root/image:267386880B:file:512:512:loop::;
1:0B:267386879B:267386880B:fat16::;
(2)
BYT;
/root/image:267386880B:file:512:512:loop::;
1:0B:267386879B:267386880B:fat32::;
Note "loop" in both outputs.
*** Expected results:
Create a "normal" partition table:
rm -f image
fallocate -l 255M image
fdisk image
Command (m for help): n
Select (default p): p
Partition number (1-4, default 1): 1
First sector ...: [accept default]
Last sector ...: [accept default]
Command (m for help): w
List it with "parted":
parted -m -s -- image unit b print
Output:
BYT;
/root/image:267386880B:file:512:512:msdos::;
1:1048576B:267386879B:266338304B:::;
Note that the device line contains "msdos", not "loop".
loop is correct, parted it detecting the filesystem at the start of the disk. It isn't a valid msdos disk, you can't partition it any further without damage, and it isn't even a valid MBR -- it just happens to fit into some extra space in the first 512 bytes of the filesystem. It's safer to treat it as a loop mountable device/image than a real msdos disk.
Comment 2RHEL Program Management
2021-12-03 00:26:14 UTC
Development Management has reviewed and declined this request. You may appeal this decision by reopening this request.
*** Description of problem: In dosfstools-4.2 a new option appeared, called "--mbr". Under circumstances that matter much to libguestfs, namely when the disk being formatted is (a) non-removable and (b) unpartitioned (that is, a whole disk, not a partition), the "--mbr" option defaults to "yes". (For the sake of the reproducer below, we'll use an explicit "--mbr=yes".) When this option is in effect, "mkfs.fat" overlays the first sector of the medium (the FAT boot sector) with a bogus MBR partition table. The table has one entry, and it describes a partition that spans the entire disk, covering the partition table itself, resident in the first sector of the medium. This is supposed to placate Windows (otherwise Windows does not recognize the FAT filesystem on a non-removable device), and to work with Linux utilities as well (the kernel itself, and blkid, and so on). In case the chosen FAT variant is FAT16 or FAT32, parted manages to parse the partition table, and prints the one entry in the table correctly. However, parted incorrectly reports the partition table type as "loop", rather than "msdos". *** Version-Release number of selected component (if applicable): - parted-3.4-6.el9.x86_64 (it should reproduce with upstream parted too, as of commit b20227adf575) - dosfstools-4.2-3.el9.x86_64 *** How reproducible: Always. *** Steps to Reproduce: (1) rm -f image fallocate -l 255M image mkfs.fat -F 16 --mbr=yes image parted -m -s -- image unit b print (2) rm -f image fallocate -l 255M image mkfs.fat -F 32 --mbr=yes image parted -m -s -- image unit b print *** Actual results: parted prints the following: (1) BYT; /root/image:267386880B:file:512:512:loop::; 1:0B:267386879B:267386880B:fat16::; (2) BYT; /root/image:267386880B:file:512:512:loop::; 1:0B:267386879B:267386880B:fat32::; Note "loop" in both outputs. *** Expected results: Create a "normal" partition table: rm -f image fallocate -l 255M image fdisk image Command (m for help): n Select (default p): p Partition number (1-4, default 1): 1 First sector ...: [accept default] Last sector ...: [accept default] Command (m for help): w List it with "parted": parted -m -s -- image unit b print Output: BYT; /root/image:267386880B:file:512:512:msdos::; 1:1048576B:267386879B:266338304B:::; Note that the device line contains "msdos", not "loop".