On 1/18/19 7:47 AM, Vladimir Sementsov-Ogievskiy wrote:
>
> > Only expose MBR partition @var{num}. Understands physical partitions
> > 1-4 and logical partitions 5-8.
>
> I'm afraid, I'm too lazy to sort out these things I don't know, so just
> believe. It at least
> corresponds to limits in code that it should be 1 <= partition <= 8 ;)It matches the code, but I just learned the code is buggy for anything larger than 5. According to http://tldp.org/HOWTO/Large-Disk-HOWTO-13.html, MBR Extended/Logical partitions form a linked-list, something like: MBR: EBR1 EBR2 +----------+ +----------+ +----------+ + Part 1 + |->+ Part 5 + |->+ Part 6 + + Part 2 + | + Next --------| + 0 + + Part 3 + | + 0 + + 0 + + Part 4 ------- + 0 + + 0 + +----------+ +----------+ +----------+ In reality, ANY of part 1-4 can point to the first EBR, as long as there is at most one primary partition pointing to the extended partition - even crazier is that there are different magic numbers in historical use, such as type 5 for Part 3 and type 85 for Part 4, where MS-DOS would treat Partition 3 as the extended partition and ignore Part 4, but Linux would follow both chains (where the second extended partition thus allowed Linux access to logical partitions residing in space beyond what DOS could access). But once you start the extended partition chain, all logical partitions within the chain each have their own EBR table with one entry for the partition and the next entry pointing to the next EBR, meaning you can have more than 8 logical partitions (provided your disk is big enough). But our find_partition() code stupidly assumes that if there is any extended partition type in the MBR (recognizing only type 5 or type F, but not Linux' type 85), then all four entries in that EBR are logical partitions 5-8 (and no more, rather than a linked list chain, as in: MBR: EBR +----------+ +----------+ + Part 1 + |->+ Part 5 + + Part 2 + | + Part 6 + + Part 3 + | + Part 7 + + Part 4 ------- + Part 8 + +----------+ +----------+ It's highly unlikely that there are any BIOS implementations that would actually recognize such a partition beyond 5 as being bootable (there might be OSs which are a bit more tolerant, since MBR doesn't seem to have any one hard canonical specification). I'd have to compare what the Linux kernel MBR code does, to see if we even stand a chance of being interoperable in any manner. It doesn't help that nbdkit does not yet support Logical MBR partitions. Oh well, another project for another day; this documentation change is going in as-is because it at least matches the code, even if the code is buggy. (I'm tempted to fix nbdkit to fully support MBR logical partitions, then rip out the partitioning code in qemu as redundant as you could get it via nbdkit if you really need it - qemu-nbd's -P 8 has unchanged, and thus buggy, since at least commit 7a5ca8648 in May 2008, with no user complaining of a bug for 11 years) -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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