linux-kernel - Re: [PATCH 2/2] block: support > 16 byte CDBs for SG

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Date:	Sun, 20 Jul 2014 15:27:25 +0200
From:	Christoph Hellwig <hch@....de>
To:	Boaz Harrosh <openosd@...il.com>
Cc:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] block: support > 16 byte CDBs for SG_IO

On Sun, Jul 20, 2014 at 02:47:49PM +0300, Boaz Harrosh wrote:
> 
> So two things here:
> - hdr->cmd_len is char so can be MAX of 255. I understand that 4 bytes alignment is a SCSI
>   thing so you found no point of checking any max size?

I don't see any point to force the aligmnet - the devices need to reject
garbage commands, and if for some reason we'd see future commands
that are > 252 and < 255 we are prepared to handle them.

> - Why the zero alloc, if you are going to paste over it the exact same length. Now if like in scsi
>   you need 4 bytes alignment and zero padding yes, but here you do not do this (and probably shouldn't).
>   Hence why zero-alloc?

No good reason except that's what sg and bsg do.

> - Looking at sg.h where hdr->cmd_len is defined it stills has a comment of <= 16 you might want to
>   remove the comment by now.

The sg changes to support > 16 byte CDs remove the comment, take a look at my
core-for-3.17 tree which has them applied.

> Inside here: blk_fill_sghdr_rq() calls blk_verify_command() which does:
> (Below cmd[] is the buffer copied from user)
> 
> 	/* Anybody who can open the device can do a read-safe command */
> 	if (test_bit(cmd[0], filter->read_ok))
> 		return 0;
> 
> 	/* Write-safe commands require a writable open */
> 	if (test_bit(cmd[0], filter->write_ok) && has_write_perm)
> 		return 0;
> 
> Now I am not sure what type "Commands" you guys intend for these large commands
> but if they are say SCSI-VARLEN this test will not work. Also a user might fool
> the system with pretending to be "read" a device modifying command.
> 
> I would pass len to this test function and only permit these above if command is
> <= 16. Any "special" large command is root only.

Honestly that whole filter crap should never have been merged to start with,
you'll just need proper CAP_SYS_RAWIO for variable length commands.

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