[dm-devel] bugs in handling of errors for SG_IO and SCSI_IOCTL_SEND_COMMAND ioctls to block device

[Mike Christie]

goggin, edward wrote:
> Found several problems in both the upstream kernel (at least up to
> 2.6.12-rc2)
> and the SuSE SLES 9 SP2-RC(2/3/4) kernels regarding the handling of errors
> occurring during the servicing of both an SG_IO and a
> SCSI_IOCTL_SEND_COMMAND
> SCSI ioctl command sent to a block device.  Haven't verified this problem
> with a Red Hat
> SP2 kernel yet.
> 
> Looks like three bugs, starting from the bottom up.
> 
> (1)	For the SuSE SP2 kernels, scsi_io_completion in
> drivers/scsi/scsi_lib.c is ignoring
> 	a whole class of errors involving the higher order 24 bits of the
> 32-bit result when
> 	setting the errors field of a REQ_BLOCK_PC io request.  Since most
> FC cable
> 	failures are generating a DID_NO_CONNECT (as the result of a scsi
> command
> 	timeout) status in the third byte of this field without any sense
> data, the current
> 	code which only pays attention only to the availability of sense
> data or the low
> 	order 8 bits of the scsi command's result field, simply sets the
> errors field of the
> 	pass through io request to zero for most if not all cable failures.
> 
> 	This problem is corrected in at least the version 2.6.12-rc2
> upstream kernel.

I think I brought this one up at the meeting two weeks ago by accident. 
It is fixed in the current RHEL kernel.

> 
> (2)	sg_scsi_ioctl is only referencing the low order 8 bits of the errors
> field of the
> 	REQ_BLOCK_PC io request just serviced.  This is the case in both the
> SuSE
> 	SP2 kernels and the upstream 2.6.12-rc2 kernel.  While this is not a
> problem
> 	for multipath, and the SCSI_IOCTL_SEND_COMMAND interface is
> deprecated,
> 	this is still a problem.
> 

not for us :) yippeee. close our eyes.


> (3)	Why do both the bio_uncopy_user and bio_unmap_user functions of
> fs/bio.c
> 	always copy_to_user the entire bio's worth of data for a read?
> Seems like they
> 	should only do the copy_to_user up to a byte length which should be
> specified as a
> 	parameter to each function passed through by blk_rq_unmap_user.  For
> 
> 	REQ_BLOCK_PC io requests, this would be the byte size of the io
> transfer
> 	minus the residual after an error during the transfer.  In the event
> of a completely
> 	failed io due to a cable disconnect, no data should be transferred
> to user space.

I don't think some LLDs maintain the resid correctly so the problem may 
be a little larger.


> 	The bio handling for these REQ_BLOCK_PC requests shouldn't be
> treated any
> 	differently than the more typical REQ_CMD type block io request.

what is meant by this last comment specifically?


> 
> 
> All of this combines to cause scsi pass through commands sent to a scsi
> block device
> to appear to succeed when they actually have failed when sent along a failed
> path.  This
> is what is causing both tur and readsector0 path check functions to yield
> false positive
> path test results.
> 
> These bugs even combine to cause the emc_clariion path checker to
> occasionally yield false negative results by tripping onto another problem
> in that path
> checker which causes multipathd to think a path is down when it really is
> not, which
> prevents the path from being restored to a useful state unless multipath(8)
> is run or
> multipathd is restarted.
> 
> --
> dm-devel mailing list
> dm-devel at redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel