Recently while testing with the scsi_debug driver I was able to trick the block layer into reading random data which the block layer thought was valid ***. Best to start with an example, say LBA ** 4660 has an unrecoverable error (aka medium error) and the block layer fires off a SCSI READ for 8 blocks (512 byte variety) at LBA 4656. The response will be a medium error with the sense buffer info field indicating LBA 4660. Now are the 4 blocks that precede it (i.e. LBA 4656 to 4659) possibly sitting in the data-in buffer and valid?? The block layer thinks they are. This is what my term "short read" in the title alludes to. So I put this question to the T10 reflector: http://www.t10.org/t10r.htm titled "sbc: reading blocks prior to a medium error". And the answers were pretty clear. And the one from George Penokie of LSI is interesting because Linux's block layer assumption breaks some of LSI's equipment. On the other hand, big array vendors and database vendors want exactly what the block layer is doing at the moment. So those guys don't want a change. [Please correct me if that is too sweeping.] Also I'm informed some other OSes do this as well. I would like to propose a solution, at least in the SCSI subsystem context. The 'resid' field was added 11 years ago and is used by a HBA driver to indicate how many bytes less than requested were placed in the scatter gather list (i.e. the data-in buffer). It defaults to zero (meaning all requested bytes have been read). Usually for a medium error one would not bother setting resid (so resid would remain 0). Somewhat surprisingly the block layer has always ignored resid. I propose in the case of a short read caused by a MEDIUM ERROR the block layer checks resid. And if resid equals the requested number of bytes then that means no data in the scatter gather list is valid. So the block layer should act on this information. To this end I propose to change the scsi_debug driver to set resid equal to bufflen when it simulates a medium error. Changes in the block layer and drivers from vendors who want the strict "T10" handling of medium errors would also be required. Maybe the USB mass storage (and UAS) folks might also check if this impacts them. Doug Gilbert ** LBA is Logical Block Address (origin 0) *** Using 'modprobe scsi_debug opts=2' will set up a pseudo device which the example in the second paragraph is based on. Write a known pattern into the pseudo device (only 8 MB long) and use dd to read that device. Due to the 4 KB blocks used by the block layer, the read ends at LBA 4655. In my tests LBAs 4576 through to 4655 are corrupted (i.e. not what is actually on the pseudo device). -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
