linux-kernel - Re: [PATCH v6 1/4] libata: Safely overwrite attached page in WRITE SAME xlat

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Date:   Mon, 22 Aug 2016 20:01:53 -0500
From:   Shaun Tancheff <shaun.tancheff@...gate.com>
To:     Tom Yan <tom.ty89@...il.com>
Cc:     Shaun Tancheff <shaun@...cheff.com>, linux-ide@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Hellwig <hch@....de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Damien Le Moal <damien.lemoal@...t.com>,
        Hannes Reinecke <hare@...e.de>,
        Josh Bingaman <josh.bingaman@...gate.com>,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [PATCH v6 1/4] libata: Safely overwrite attached page in WRITE
 SAME xlat

On Mon, Aug 22, 2016 at 6:49 PM, Tom Yan <tom.ty89@...il.com> wrote:
> The only 512 I can see in the old code is the one in:
>
>> -       used_bytes = ALIGN(i * 8, 512);
>
> where the alignment is necessary because 'used_bytes' will be returned
> as 'size', which will be used for setting the number of 512-byte block
> payload when we construct the ATA taskfile / command. It may NOT be a
> good idea to replace it with ATA_SECT_SIZE. Some comment could be
> nice.

Not sure I agree with that analysis. Could just as well assign it directly,
as ALIGN() is just superfluous here.
Later the count is always 512/512 -> 1. Always.

"i" is used here to limit the number of bytes that need to be memset()
after filling to payload. Personally I think memset is fast enough that
it is better to do before rather than later but I figure if the code works
let it be.

> So I don't think it makes any sense to check ATA_SCSI_RBUF_SIZE
> against 512 here.

Again, not sure I agree, but I don't really care on that point. Just many
years of defensive coding.

> On 22 August 2016 at 22:00, Shaun Tancheff <shaun.tancheff@...gate.com> wrote:
>> Because this is re-using the response buffer in a new way.
>> Such reuse could be a surprise to someone refactoring that
>> code, although it's pretty unlikely. The build bug on
>> gives some level of confidence that it won't go unnoticed.
>> It also codifies the assumption that we can write 512 bytes
>> to the global ata_scsi_rbuf buffer.
>>
>> As to using a literal 512, I was just following what was here
>> before.
>>
>> It looks like there is a ATA_SECT_SIZE that can replace most
>> of the literal 512's in here though. That might be a nice cleanup
>> overall. Not sure it belongs here though.
>>
>
>>>> +       used_bytes = ALIGN(i * 8, 512);
>>>> +       memset(buffer + i, 0, used_bytes - i * 8);
>>>> +       sg_copy_from_buffer(scsi_sglist(cmd), scsi_sg_count(cmd), buffer, 512);
>
> And I don't think you have a reason to use 512 here either. It appears
> to me that you should use ATA_SCSI_RBUF_SIZE instead (see
> ata_scsi_rbuf_put in libata-scsi). If not, it should probably be a
> _derived_ value (e.g. from `i`) that tells the _actual_ size of
> `buffer`.

Nope. We *must* copy the whole 512 byte payload into the sglist.
Otherwise what was the point of the memset to clear out an cruft?
Failing to move the whole payload into place could leave whatever
garbage is in the buffer to be interpreted as an actual trim and do
real damage. I certainly can't use ATA_SCSI_RBUF_SIZE because
the payload attached to the cmd need only be 512 bytes. Trying
to copy in 4k is going to cause bad things when you check
the return from sg_copy_from_buffer() and notice you failed to
copy in you payload.

> Again, note that even when we prefer to stick with _one_ 512-byte
> block TRIM payload, we should probably NEVER _hard code_ such limit
> (for it's really ugly and unnecessary) in functions like this. All we

The interface requires well formed 512 byte chunks so we have to
at least have to do enough to ensure that we work in multiples of
512. Since 512 is all over the spec for this type of thing I think
it would be reasonable to have a define or enum if you don't think
reusing ATA_SECT_SIZE is good maybe something
like ATA_CMD_XFER_SIZE ?

> need to do is to advertise the limit (or lower) as Maximum Write
> Length, and make sure our SATL works as per SBC that it will reject
> SCSI Write Same commands that is "larger" than that.
>>>> +       spin_unlock_irqrestore(&ata_scsi_rbuf_lock, flags);
>>>> +
>>>> +       return used_bytes;
>>>> +}

-- 
Shaun Tancheff