2:40 a.m.
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>
> ---
>
> 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.
It's a matter of taste though, not a blocker.
Well I do want to learn good taste in OCaml. :)
Looks good, ACK.
Thanks!
Laszlo