Am 25.10.20 um 02:25 schrieb Bart Van Assche: > On 10/24/20 5:13 AM, Roman Bolshakov wrote: >> iSCSI doesn't specify a specific code but mentions a possibility of CHECK >> CONDITION for residuals (11.4.5.1. Field Semantics): >> >> Targets may set the residual count, and initiators may use it when the >> response code is Command Completed at Target (even if the status >> returned is not GOOD). > > My interpretation of the above text is that it neither allows nor > requires to change the status GOOD into something else if there is a > residue. > >>> Additionally, what benefits does it provide to report a CHECK CONDITION >>> upon residual overflow? >> >> Typical use case for CHECK CONDITION in case of Underflow/Overflow is >> extra robustness against buggy initiators [1][2]. Failing both READ and >> WRITE is the most solid approach in that sense [3][4][5] as it prevents >> data corruption at all costs. >> >> Suppose an initiator wants to WRITE 8 LBA. For 512-byte formatted LUN, >> 8 LBAs need a buffer of 4K bytes. For 4096-byte formatted LUN the >> command would need 32K data buffer. >> >> An Overflow happens if initiator treats 4Kn device like 512n one but >> provides a buffer of 4K. i.e. to complete the WRITE target needs to >> consume 28K more data, otherwise only 1 LBA would be written and the >> rest 7 LBAs would have indeterminate content. >> >> An Underflow happens if initiator confuses 512n device with 4Kn one and >> provides a buffer of 32K, i.e. target doesn't utilize all buffer for the >> command. > > Thanks for the additional background information, this really helps. How > about only rejecting SCSI commands for which the data buffer size is not > a multiple of the block size? I'm concerned that flagging all SCSI > commands that have a residue as invalid will break SCSI tape software. AFAICS, there is no risk to break tape handling. target_cmd_size_check() is mainly used by sbc_parse_cdb(), while passthrough_parse_cdb() optionally calls it for PERSISTENT_RESERVFE_IN/_OUT and RESERVE(_10)/RELEASE(_10) only. sbc_parse_cdb is not usable for tape devices anyway, since CDB 'length' field in READ/WRITE for SSC devices needs special processing. Depending on current state of the device, length 1 can have the meaning 1 byte or 1 times the optionally set fixed block size. So the only way to set up a tape target LUN is to use tcmu or pscsi. > > Thanks, > > Bart. >
