[...] creating vfat filesystem on /dev/sda ... libguestfs: trace: mkfs "vfat" "/dev/sda" "label:TEST" [...] commandrvf: stdout=n stderr=y flags=0x0 commandrvf: mkfs.fat Usage: mkfs.fat [OPTIONS] TARGET [BLOCKS] Create FAT filesystem in TARGET, which can be a block device or file. Use only up to BLOCKS 1024 byte blocks if specified. With the -C option, file TARGET will be created with a size of 1024 bytes times BLOCKS, which must be specified. Options: -a Disable alignment of data structures -A Toggle Atari variant of the filesystem -b SECTOR Select SECTOR as location of the FAT32 backup boot sector -c Check device for bad blocks before creating the filesystem -C Create file TARGET then create filesystem in it -D NUMBER Write BIOS drive number NUMBER to boot sector -f COUNT Create COUNT file allocation tables -F SIZE Select FAT size SIZE (12, 16 or 32) -g GEOM Select disk geometry: heads/sectors_per_track -h NUMBER Write hidden sectors NUMBER to boot sector -i VOLID Set volume ID to VOLID (a 32 bit hexadecimal number) -I Ignore and disable safety checks -l FILENAME Read bad blocks list from FILENAME -m FILENAME Replace default error message in boot block with contents of FILENAME -M TYPE Set media type in boot sector to TYPE --mbr[=y|n|a] Fill (fake) MBR table with one partition which spans whole disk -n LABEL Set volume name to LABEL (up to 11 characters long) --codepage=N use DOS codepage N to encode label (default: 850) -r COUNT Make room for at least COUNT entries in the root directory -R COUNT Set minimal number of reserved sectors to COUNT -s COUNT Set number of sectors per cluster to COUNT -S SIZE Select a sector size of SIZE (a power of two, at least 512) -v Verbose execution --variant=TYPE Select variant TYPE of filesystem (standard or Atari) --invariant Use constants for randomly generated or time based values --offset=SECTOR Write the filesystem at a specific sector into the device file. --help Show this help message and exit [...] commandrvf: stdout=n stderr=y flags=0x0 commandrvf: mkfs -t vfat -I --mbr=n -n TEST /dev/sda

><fs> run ><fs> list-filesystems /dev/sda: vfat

"parted" incorrectly reports "loop" rather than "msdos" for the partition table type, when the (fake) partition table comes from the "--mbr" option of "mkfs.fat" (in dosfstools-4.2+), and the FAT variant in question is FAT16 or FAT32. (See RHBZ#2026224.) Work this around by - parsing the partition table ourselves, and - overriding "loop" with "msdos" when appropriate. Note that when the FAT variant is FAT12, "parted" fails to parse the fake MBR partition table completely (see RHBZ#2026220), which we cannot work around. However, FAT12 should be a rare corner case in libguestfs usage -- "mkfs.fat" auto-chooses FAT12 only below 9MB disk size, and even "-F 12" can only be forced up to and including 255MB disk size. Add the helper function "has_bogus_mbr" to the Utils module; we'll use it elsewhere too. Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1931821 Signed-off-by: Laszlo Ersek <lersek(a)redhat.com&gt; --- Notes: v2: - new patch (the hard one) daemon/parted.ml | 7 ++++++- daemon/utils.ml | 44 ++++++++++++++++++++++++++++++++++++++++++++ daemon/utils.mli | 10 ++++++++++ 3 files changed, 60 insertions(+), 1 deletion(-) diff --git a/daemon/parted.ml b/daemon/parted.ml index 29b98c4806c1..53077d91afdd 100644 --- a/daemon/parted.ml +++ b/daemon/parted.ml @@ -118,7 +118,12 @@ let part_get_parttype device = let fields = String.nsplit ":" device_line in match fields with | _::_::_::_::_::"loop"::_ -> (* If "loop" return an error (RHBZ#634246). *) - failwithf "%s: not a partitioned device" device + (* ... Unless parted failed to recognize the fake MBR that mkfs.fat from + * dosfstools-4.2+ created. In that case, return "msdos" for MBR + * (RHBZ#1931821). + *) + if Utils.has_bogus_mbr device then "msdos" + else failwithf "%s: not a partitioned device" device | _::_::_::_::_::ret::_ -> ret | _ -> failwithf "%s: cannot parse the output of parted" device diff --git a/daemon/utils.ml b/daemon/utils.ml index b56306055634..7ef3c206b71a 100644 --- a/daemon/utils.ml +++ b/daemon/utils.ml @@ -187,6 +187,50 @@ and compare_device_names a b = ) ) +let has_bogus_mbr device = + try + with_openfile device [O_RDONLY; O_CLOEXEC] 0 (fun fd -> + let sec0size = 0x200 + and sigofs = 0x1FE + and sysidofs = 0x003 and sysidsize = 0x008 + and pte1ofs = 0x1BE + and parttypes = [0x01; (* FAT12 *) + 0x04; (* FAT16 *) + 0x06; (* FAT12, FAT16, FAT16B *) + 0x0C; (* FAT32 LBA *) + 0x0E (* FAT16B LBA *)] in + let sec0 = Bytes.create sec0size in + let sec0read = Unix.read fd sec0 0 sec0size in + + (* sector read completely *) + sec0read = sec0size && + + (* boot signature present *) + Bytes.get_uint8 sec0 (sigofs ) = 0x55 && + Bytes.get_uint8 sec0 (sigofs + 0x1) = 0xAA && + + (* mkfs.fat signature present *) + Bytes.sub_string sec0 sysidofs sysidsize = "mkfs.fat" && + + (* partition bootable *) + Bytes.get_uint8 sec0 (pte1ofs ) = 0x80 && + + (* partition starts at C/H/S 0/0/1 *) + Bytes.get_uint8 sec0 (pte1ofs + 0x1) = 0x00 && + Bytes.get_uint8 sec0 (pte1ofs + 0x2) = 0x01 && + Bytes.get_uint8 sec0 (pte1ofs + 0x3) = 0x00 && + + (* partition type is a FAT variant that mkfs.fat is known to create *) + List.mem (Bytes.get_uint8 sec0 (pte1ofs + 0x4)) parttypes && + + (* partition starts at LBA 0 *) + Bytes.get_uint8 sec0 (pte1ofs + 0x8) = 0x00 && + Bytes.get_uint8 sec0 (pte1ofs + 0x9) = 0x00 && + Bytes.get_uint8 sec0 (pte1ofs + 0xA) = 0x00 && + Bytes.get_uint8 sec0 (pte1ofs + 0xB) = 0x00 + ) + with _ -> false

On 11/24/21 17:45, Richard W.M. Jones wrote: > On Wed, Nov 24, 2021 at 11:37:46AM +0100, Laszlo Ersek wrote: >> "parted" incorrectly reports "loop" rather than "msdos" for the partition >> table type, when the (fake) partition table comes from the "--mbr" option >> of "mkfs.fat" (in dosfstools-4.2+), and the FAT variant in question is >> FAT16 or FAT32. (See RHBZ#2026224.) Work this around by >> - parsing the partition table ourselves, and >> - overriding "loop" with "msdos" when appropriate. >> >> Note that when the FAT variant is FAT12, "parted" fails to parse the fake >> MBR partition table completely (see RHBZ#2026220), which we cannot work >> around. However, FAT12 should be a rare corner case in libguestfs usage -- >> "mkfs.fat" auto-chooses FAT12 only below 9MB disk size, and even "-F 12" >> can only be forced up to and including 255MB disk size. >> >> Add the helper function "has_bogus_mbr" to the Utils module; we'll use it >> elsewhere too. >> >> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1931821 >> Signed-off-by: Laszlo Ersek <lersek(a)redhat.com&gt; >> --- >> >> Notes: >> v2: >> >> - new patch (the hard one) >> >> daemon/parted.ml | 7 ++++++- >> daemon/utils.ml | 44 ++++++++++++++++++++++++++++++++++++++++++++ >> daemon/utils.mli | 10 ++++++++++ >> 3 files changed, 60 insertions(+), 1 deletion(-) >> >> diff --git a/daemon/parted.ml b/daemon/parted.ml >> index 29b98c4806c1..53077d91afdd 100644 >> --- a/daemon/parted.ml >> +++ b/daemon/parted.ml >> @@ -118,7 +118,12 @@ let part_get_parttype device = >> let fields = String.nsplit ":" device_line in >> match fields with >> | _::_::_::_::_::"loop"::_ -> (* If "loop" return an error (RHBZ#634246). *) >> - failwithf "%s: not a partitioned device" device >> + (* ... Unless parted failed to recognize the fake MBR that mkfs.fat from >> + * dosfstools-4.2+ created. In that case, return "msdos" for MBR >> + * (RHBZ#1931821). >> + *) >> + if Utils.has_bogus_mbr device then "msdos" >> + else failwithf "%s: not a partitioned device" device >> | _::_::_::_::_::ret::_ -> ret >> | _ -> >> failwithf "%s: cannot parse the output of parted" device >> diff --git a/daemon/utils.ml b/daemon/utils.ml >> index b56306055634..7ef3c206b71a 100644 >> --- a/daemon/utils.ml >> +++ b/daemon/utils.ml >> @@ -187,6 +187,50 @@ and compare_device_names a b = >> ) >> ) >> >> +let has_bogus_mbr device = >> + try >> + with_openfile device [O_RDONLY; O_CLOEXEC] 0 (fun fd -> >> + let sec0size = 0x200 >> + and sigofs = 0x1FE >> + and sysidofs = 0x003 and sysidsize = 0x008 >> + and pte1ofs = 0x1BE >> + and parttypes = [0x01; (* FAT12 *) >> + 0x04; (* FAT16 *) >> + 0x06; (* FAT12, FAT16, FAT16B *) >> + 0x0C; (* FAT32 LBA *) >> + 0x0E (* FAT16B LBA *)] in >> + let sec0 = Bytes.create sec0size in >> + let sec0read = Unix.read fd sec0 0 sec0size in >> + >> + (* sector read completely *) >> + sec0read = sec0size && >> + >> + (* boot signature present *) >> + Bytes.get_uint8 sec0 (sigofs ) = 0x55 && >> + Bytes.get_uint8 sec0 (sigofs + 0x1) = 0xAA && >> + >> + (* mkfs.fat signature present *) >> + Bytes.sub_string sec0 sysidofs sysidsize = "mkfs.fat" && >> + >> + (* partition bootable *) >> + Bytes.get_uint8 sec0 (pte1ofs ) = 0x80 && >> + >> + (* partition starts at C/H/S 0/0/1 *) >> + Bytes.get_uint8 sec0 (pte1ofs + 0x1) = 0x00 && >> + Bytes.get_uint8 sec0 (pte1ofs + 0x2) = 0x01 && >> + Bytes.get_uint8 sec0 (pte1ofs + 0x3) = 0x00 && >> + >> + (* partition type is a FAT variant that mkfs.fat is known to create *) >> + List.mem (Bytes.get_uint8 sec0 (pte1ofs + 0x4)) parttypes && >> + >> + (* partition starts at LBA 0 *) >> + Bytes.get_uint8 sec0 (pte1ofs + 0x8) = 0x00 && >> + Bytes.get_uint8 sec0 (pte1ofs + 0x9) = 0x00 && >> + Bytes.get_uint8 sec0 (pte1ofs + 0xA) = 0x00 && >> + Bytes.get_uint8 sec0 (pte1ofs + 0xB) = 0x00 >> + ) >> + with _ -> false > > It's not wrong, but you might consider factoring out some of those > common functions. > > First of all, take a look at this code for inspiration since it's > doing something which is a bit similar: > https://github.com/libguestfs/libguestfs/blob/e7f72ab146b9c2aaee92a600a1f... - Upon reading the documentation of the Bytes module <https://ocaml.org/api/Bytes.html>;, I've found the to_string function. That just rubbed me the wrong way, completely. I don't want to deal with characters here (except for the "system id", where I do use sub_string), but with uint8_t values. IOW I wouldn't like to think about characters here, and make comparisons against character constants. Anyway that's just a generic comment.

- What does the "|>" operator do?

> Using factoring you could write it as: > > let is offs b = Bytes.get_uint8 sec0 offs = b in > is sigofs 0x55 && > is (sigofs + 0x1) 0xaa && ... I was slightly annoyed by having to write down "Bytes.get_uint8 sec0" so many times; on the other hand, I feel "attached" to the equal sign (perhaps because it resembles C more?). My preferred form would be something that resembles array indexing (but that was when I found out about "to_string", and I refused to go through that, even temporarily, for conversion to a byte array perhaps). Maybe I could do let sec0_at = fun offset -> Bytes.get_uint8 sec0 offset

and then write sec0_at (sigofs ) = 0x55 && sec0_at (sigofs + 0x1) = 0xAA && (... It's less concise than your "is", but with "maximal factoring", every such block reads like its own domain-specific language.) Does "sec0_at" feel like an improvement to you? If it does, I'll respin.

The "is_partition_can_hold_filesystem" function calls "Parted.part_get_gpt_type" on the partition if: - the partition table type is GPT, - or the partition table type is MBR, and the partition is primary or logical. The one entry in the fake MBR partition table described in the previous patch passes the second branch of this check, therefore "Parted.part_get_gpt_type" is reached, and it invokes "sgdisk -i 1" on the disk. Surprisingly (not), while "sgdisk -i" copes fine with valid MBR partition tables, it chokes on the fake one. The output does not contain the "Partition GUID code" line, and so "sgdisk_info_extract_field" throws an exception. Prevent calling "Parted.part_get_gpt_type" on a bogus MBR partition table, similarly to the "extended entry in MBR partition table" case; the difference is that the bogus primary entry, unlike a valid extended entry, *can* hold a filesystem. Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1931821 Signed-off-by: Laszlo Ersek <lersek(a)redhat.com&gt; --- Notes: v2: - new patch (sigh) daemon/listfs.ml | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/daemon/listfs.ml b/daemon/listfs.ml index d8add5005ade..63ced260a14d 100644 --- a/daemon/listfs.ml +++ b/daemon/listfs.ml @@ -109,6 +109,8 @@ and is_partition_can_hold_filesystem partition = if is_gpt_or_mbr then ( if is_mbr_extended parttype device partnum then false + else if is_mbr_bogus parttype device partnum then + true else ( (* MBR partition id will be converted into corresponding GPT type. *) let gpt_type = Parted.part_get_gpt_type device partnum in @@ -130,6 +132,9 @@ and is_mbr_extended parttype device partnum = parttype = "msdos" && Parted.part_get_mbr_part_type device partnum = "extended" +and is_mbr_bogus parttype device partnum = + parttype = "msdos" && partnum = 1 && Utils.has_bogus_mbr device + (* Use vfs-type to check for a filesystem of some sort of [device]. * Appends (device, vfs_type) to the ret parameter (there may be * multiple devices found in the case of btrfs).