On Mon, Jul 22, 2013 at 06:15:38PM +0200, David Vasek wrote: > On Mon, 22 Jul 2013, Kenneth R Westerback wrote: > > >On Mon, Jul 22, 2013 at 01:56:16PM +0200, David Vasek wrote: > >>Hello. > >> > >>From the experiment below it seems that the kernel modifies its > >>knowledge of a sector size of a disk hardware according to what can > >>be found in the disk label sector. It does so even if the value > >>found does not make sense hardware-wise and in spite of a sector > >>size the hardware device driver works with. It can make the device > >>completly inaccessible. The disklabel(8) command doesn't allow to > >>set/change the sector size value even if it were accessible. > > > >The kernel obtains the sector size from the hardware device whenever > >the disklabel is created. It does trust the disklabel it reads from > >the disk. People can abuse that trust. The interesting gyrations to > >automatically convert all the fields in the disklabel from one > >sector size to another are, well, *interesting*. And have never been > >considered useful since there is no general mechanisms for the kernel > >to read incorrectly blocked data from the disk. > > Well, it trusts the disk label in what it says the sector size is, > but on the other hand it ignores what the disklabel says about the > total number of sectors (and the size of the 'c' partition). > Strange.
Not at all. It always ensures the magic 'c' partition reflects the current hardware provided information. The other partitions sizes are in the units of the original disklabel, and are left alone with the original sector size so the two are consistant. > > >>Such situation also occurs if the disk label is read from a sector > >>where a disk label is not expected - this was already a subject of > >>my previous report. > > > >Still working on puzzling out that report. > > Don't hesitate to ask if anything is not clear in that report. > > >>It effectively means that if there is wrong data on the drive, you > >>don't have a chance to analyze the situation or fix it. You can't > >>even view the fdisk (MBR) partition table. > > > >Correct. > > One would hope there is a chance to edit (fix) the sector size value > with disklabel(8). But obviously, once it has been changed, the disk > label cannot be written to disk again. > > >Bits directly plopped from a device with one sector size to another > >device with a different sector size all pass through a gate having > >a large sign over it containing the words: "Lasciate ogne speranza, > >voi ch'intrate". > > In some cases disks can be connected to computers in more than on > way and in each way its host will be presented with a different > sector size. I made an attemt to simulate it. I just didn't expect > it could silently influence hardware drivers. Doesn't influence hardware drivers, outside of the drivers doing what the hardware tells them. If the disk tells one story when plugged in via A and another story when plugged in via B I not sure how A or B are supposed to determine they should ignore what they are told. > > >>My guess is that this issue could be related to this one about a year ago: > >>http://marc.info/?l=openbsd-misc&m=134027998905971&w=2 > > > >Nope. That problem had to do with wd(4) not supporting 4K sectors. > >Only sd(4) will even make an attempt to handle sector sizes other > >than 512 bytes. > > I don't think so. If I see the number of sector the drive reported > both when inside an USB enclosure and connected directly to SATA > controller, I believe that it was a standard 512-byte sector SATA > hard drive and only the kernel got fooled by the disk label it found > on it. I think so since I have the subsquent email trail with the submitter and others, and that is what we concluded to close that issue. His bios had no AHCI option for the disk controller, so the disk always appeared as a wd(4) when plugged in directly. And wd(4) does not support 4K size sectors. When plugged in via usb(4) it appears as an sd(4) disk and worked fine. > > In fact, that was the real life situation I wanted to model in my > experiments - what will happen if the disk is taken out from a USB > enclosure which pretends it to be a 4k-sector drive and is connected > to a SATA controller as a standard 512-byte/sector drive. It won't work, at least in the sense that you could reliably access existing information. Even worse if you go the other way. And it's not intended to work. .... Ken
