Re: [PATCH] iscsi-target: Reject immediate data underflow larger than SCSI transfer length

From: Nicholas A. Bellinger
Date: Thu Jul 13 2017 - 15:27:29 EST

On Tue, 2017-07-11 at 16:17 +0000, Bart Van Assche wrote:
> On Tue, 2017-07-11 at 00:22 -0700, Nicholas A. Bellinger wrote:
> > So rejecting this case as already done in commit abb85a9b51 is the
> > correct approach for >= v4.3.y.
> Hello Nic,
> I hope that you agree that the current target_cmd_size_check() implementation
> is complicated and ugly. Patch 30/33 of the patch series I referred to in my
> e-mail removes a significant number of lines of code from that function. So
> my patch series not only makes target_cmd_size_check() easier to maintain and
> to verify but it makes that function also faster. Hence please reconsider the
> approach from my patch series. For patch 30/33, see also

For reference, here is the patch your advocating:

diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index c28e3b58150b..6cd49fe578a7 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -1164,23 +1164,6 @@ target_cmd_size_check(struct se_cmd *cmd, unsigned int size)
" %u does not match SCSI CDB Length: %u for SAM Opcode:"
" 0x%02x\n", cmd->se_tfo->get_fabric_name(),
cmd->data_length, size, cmd->t_task_cdb[0]);
- if (cmd->data_direction == DMA_TO_DEVICE &&
- cmd->se_cmd_flags & SCF_SCSI_DATA_CDB) {
- pr_err("Rejecting underflow/overflow WRITE data\n");
- }
- /*
- * Reject READ_* or WRITE_* with overflow/underflow for
- */
- if (dev->dev_attrib.block_size != 512) {
- pr_err("Failing OVERFLOW/UNDERFLOW for LBA op"
- " CDB on non 512-byte sector setup subsystem"
- " plugin: %s\n", dev->transport->name);
- }
* For the overflow case keep the existing fabric provided
* ->data_length. Otherwise for the underflow case, reset

The original code is not 'complicated' or 'ugly', and the proposed
change doesn't make anything 'faster' nor removes a 'significant' number
of LoC.

So no, I don't buy any of that line of reasoning. ;-)

It comes down to two items. First, if SCF_SCSI_DATA_CDBs with
underflow/overflow will be allowed, and second the block_size != 512

For the former, I've still never seen a host environment in the wild
over the last 15 years that generates underflow/overflow for DATA CDBs
with an LBA. So I'm reluctant to randomly allow this for all cases and
fabrics, considering no host environment actually requires it.

That said, I do understand libiscsi generates this for WRITE_VERIFY, but
I'm still undecided if that's a good enough a reason to enable it for
all cases in upstream.

For the latter item, it's fine to drop the legacy block_size != 512
check, and I'll take a patch for that separate from the other bit.