Re: [Re: Linux 2.6.26-rc2] Write protect on on

Boaz Harrosh
2008-05-19T16:09:54Z

Alan Stern wrote:
> On Sun, 18 May 2008, Boaz Harrosh wrote:
>
>> Do you mean this diff below:
>>
>> @@ -796,133 +789,133 @@ kernel: usb-storage: *** thread awakened
>> kernel: usb-storage: Command MODE_SENSE (6 bytes)
>> kernel: usb-storage: 1a 00 3f 00 c0 00
>> kernel: usb-storage: Bulk Command S 0x43425355 T 0x4 L 192 F 128 Trg 0 LUN 0 CL 6
>> kernel: usb-storage: usb_stor_bulk_transfer_buf: xfer 31 bytes
>> kernel: usb-storage: Status code 0; transferred 31/31
>> kernel: usb-storage: -- transfer complete
>> kernel: usb-storage: Bulk command transfer result=0
>> kernel: usb-storage: usb_stor_bulk_transfer_sglist: xfer 192 bytes, 1 entries
>> kernel: usb-storage: Status code -32; transferred 0/192
>> kernel: usb-storage: clearing endpoint halt for pipe 0xc0008480
>> kernel: usb-storage: usb_stor_control_msg: rq=01 rqtype=02 value=0000 index=81 len=0
>> kernel: usb-storage: usb_stor_clear_halt: result = 0
>> kernel: usb-storage: Bulk data transfer result 0x2
>> kernel: usb-storage: Attempting to get CSW...
>> kernel: usb-storage: usb_stor_bulk_transfer_buf: xfer 13 bytes
>> kernel: usb-storage: Status code 0; transferred 13/13
>> kernel: usb-storage: -- transfer complete
>> kernel: usb-storage: Bulk status result = 0
>> kernel: usb-storage: Bulk Status S 0x53425355 T 0x4 R 192 Stat 0x0
>> kernel: usb-storage: scsi cmd done, result=0x0
>> kernel: usb-storage: *** thread sleeping.
>> -kernel: sd 2:0:0:0: [sda] Write Protect is off
>> -kernel: sd 2:0:0:0: [sda] Mode Sense: 00 00 00 00
>> +kernel: sd 2:0:0:0: [sda] Write Protect is on
>> +kernel: sd 2:0:0:0: [sda] Mode Sense: 09 50 f8 af
>> kernel: sd 2:0:0:0: [sda] Assuming drive cache: write through
>> kernel: usb-storage: queuecommand called
>>
>> ("+" is the new kernel and "-" the older one)
>
> That's right.
>
>> It looks like it used to be the same exact return and everything only that at
>> old kernel the 4 bytes used to be zero and now they are not.
>>
>> So It looks to me that it never used to work (Data was never actually read
>> from device) but by luck, the garbage data used to be a better default
>> "Write Protect is off"
>
> Yes, it never worked properly. But now it fails in a bad way whereas
> before it failed in a benign way.
>

You do realize that, that was pure lock to have a zero'ed buffer.

>> I do not think it is legal in scsi to return "Nothing was read" with no
>> error condition.
>
> I'm not aware of any part of the spec where it says that. In any case
> it doesn't matter what the spec says; we ought to be able to drive this
> device even if it isn't compliant with the spec.
>

Yes it is so. In read_x command a short read is an error status (or short
writes for write_x). The only commands that are allowed short reads
(with no error) are those commands that have optional sizes like inquiry
and stuff, so if the device does not have the extra info it will return
a short read, of exact number of bytes as permitted by the shorter version
of the command. The initiator will understand that the command return is of
the shorter version. And also REQUEST_SENSE can be short. But all other
commands must return the number of bytes requested or set a status. Here
the device does not return a shorter MODE_SENSE, it does not return it
at all. A status must be set/emulated.

>> You are probably right that we do not at all check resid
>> if status is 0, even though short reads are allowed with out error status
>> in some cases, as per command. But this is not the case here here nothing
>> was read at all, status must be returned. Or even worse if this command
>> is mandatory by scsi but not supported by some USB devices then it will
>> have to be emulated by usb_storage.
>
> The real question is how should we fix the problem. For the sake of
> argument, let's say that we fix it by changing scsi_mode_sense() --
> make the routine return 0 if the residue is so large that there isn't
> even a valid header.
>
> But how can this be done? Should we modify struct scsi_sense_hdr, by
> adding a "residue" field?
>

I would not do that, because:
- This is a per command filter and will not be easy to do right.
- All other drivers will need to be inspected, because mid-layer
behavior change. Where before the gate was status, then if set
look at resid. What you propose enables another gate for errors.
I'm much more lazy then you, and for my peace of mine will not do
such a subtle change, that I can never fully test for.

If I where you I would fix it at the usb level, where if *no* data
was read/written I would atleast set the DID_NOT_CONNECT status.
(OK I'm not sure what to do here it should be carefully considered)

Also be ware that I think the MODE_SENSE command is mandatory and
you must emulate it if the device does not supports it. (That is
filter for MODE_SENSE command and if returns error, set a sensible
returned buffer with info gathered by other means. Like good'ol
isd200 does it)

> Alan Stern
>

Boaz