Re: [PATCH 2/3] block layer varlen-cdb

[Boaz Harrosh <bharrosh@panasas.com>]

On Tue, Feb 12 2008 at 19:54 +0200, Boaz Harrosh <bharrosh@panasas.com> wrote:
> On Tue, Feb 12 2008 at 19:48 +0200, Christoph Hellwig <hch@infradead.org> wrote:
>> On Sun, Feb 10, 2008 at 09:09:41PM +0200, Boaz Harrosh wrote:
>>> - add varlen_cdb and varlen_cdb_len to hold a large user cdb
>>>   if needed. They start as empty. Allocation of buffer must
>>>   be done by user and held until request execution is done.
>>> - Since there can be either a fix_length command up to 16 bytes
>>>   or a variable_length, larger then 16 bytes, commands but never
>>>   both, we hold the two types in a union to save space. The
>>>   presence of varlen_cdb_len and cmd_len==0 signals a varlen_cdb
>>>   mode.
>> this one I'm a bit confused by, why can't we just set the length
>> of the variable length command in cmd_len aswell, and if cmd_len >
>> the length of the cmd array it's a variable length command?
>>
>> Note that this is both to keep the logic simpler and not to grow
>> struct request further.  Especially for the rather rare case
>> of a bidi command.
> 
> Because this will be dangerous for the Legacy block devices.
> Unlike scsi drivers block drivers do not have a .max_cmnd_len
> and upper layer will not check to make sure that the device supports
> the longer command. If such a command goes through, lets say bsg
> the drivers do blindly memcpy(,,rq->cmd_len) and will crash.
> Better safe then sorry, at no cost.
> 
> Boaz
> 
> -
PS: the struct request does not grow. Because before cmd_len was
unsigned but now both cmd_len and varlen_cdb_len are short so
it is the same as before.

Boaz