daemon/parted: work around part table type misreporting by "parted"

"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@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20211125094954.9713-5-lersek@redhat.com>
[lersek@redhat.com: drop "fun" keyword, and use partial application, in
 the definition of "sec0at" [Rich]]

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

daemon/utils.ml

@@ -187,6 +187,51 @@ 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 = read fd sec0 0 sec0size in
+      let sec0at = Bytes.get_uint8 sec0 in
+
+      (* sector read completely *)
+      sec0read = sec0size &&
+
+      (* boot signature present *)
+      sec0at (sigofs       ) = 0x55 &&
+      sec0at (sigofs  + 0x1) = 0xAA &&
+
+      (* mkfs.fat signature present *)
+      Bytes.sub_string sec0 sysidofs sysidsize = "mkfs.fat" &&
+
+      (* partition bootable *)
+      sec0at (pte1ofs      ) = 0x80 &&
+
+      (* partition starts at C/H/S 0/0/1 *)
+      sec0at (pte1ofs + 0x1) = 0x00 &&
+      sec0at (pte1ofs + 0x2) = 0x01 &&
+      sec0at (pte1ofs + 0x3) = 0x00 &&
+
+      (* partition type is a FAT variant that mkfs.fat is known to create *)
+      List.mem (sec0at (pte1ofs + 0x4)) parttypes &&
+
+      (* partition starts at LBA 0 *)
+      sec0at (pte1ofs + 0x8) = 0x00 &&
+      sec0at (pte1ofs + 0x9) = 0x00 &&
+      sec0at (pte1ofs + 0xA) = 0x00 &&
+      sec0at (pte1ofs + 0xB) = 0x00
+    )
+  with _ -> false
+
 let proc_unmangle_path path =
   let n = String.length path in
   let b = Buffer.create n in

daemon/utils.mli

@@ -56,6 +56,16 @@ val sort_device_names : string list -> string list
     This also deals with partition numbers, and works whether or not
     [/dev/] is present. *)
 
+val has_bogus_mbr : string -> bool
+(** Check whether the first sector of the device contains a bogus MBR partition
+    table; namely one where the first partition table entry describes a
+    partition that starts at absolute sector 0, thereby overlapping the
+    partition table itself.
+
+    dosfstools-4.2+ creates bogus partition tables like this by default when
+    formatting non-removable, non-partitioned block devices. Refer to
+    RHBZ#1931821. *)
+
 val proc_unmangle_path : string -> string
 (** Reverse kernel path escaping done in fs/seq_file.c:mangle_path.
     This is inconsistently used for /proc fields. *)