[PATCH] SG_IO block filter whitelist missing MMC SET READ AHEAD command

[PATCH] SG_IO block filter whitelist missing MMC SET READ AHEAD command

[xiphmont]

Hi Jens,

I have another request for the block filter SG_IO command whitelist,
specifically the MMC streaming command set SET READ AHEAD command.
The command applies only to MMC CDROM/DVDROM drives with the streaming
optional feature set.  The command is useful to cdparanoia in that it
allows explicit cache control side effects that are, on many drives,
cdparanoia's most efficient way to flush/disable the media cache on
cdrom drives. I am aware of no reason why it should not be accessible
from usespace.

Also note that the command is already fully accessible through the
SCSI-native version of the SG_IO ioctl as well as the traditional SG
interface.  The command is only being refused on block devices.  That
means that on a typical stock distro, the command is available through
/dev/sg* but not /dev/scd* although both are typically available and
accessible.  Filtering the command is not providing any protection,
only a confusing inconsistency.

For convenience's sake, trivial patch below.

diff -ru linux-2.6.26.3/block/scsi_ioctl.c
linux-2.6.26.3-monty/block/scsi_ioctl.c
--- linux-2.6.26.3/block/scsi_ioctl.c   2008-08-20 14:11:37.000000000 -0400
+++ linux-2.6.26.3-monty/block/scsi_ioctl.c     2008-08-22
04:17:56.194248000 -0400
@@ -188,6 +188,7 @@
                safe_for_write(GPCMD_PREVENT_ALLOW_MEDIUM_REMOVAL),
                safe_for_write(GPCMD_LOAD_UNLOAD),
                safe_for_write(GPCMD_SET_STREAMING),
+               safe_for_write(GPCMD_SET_READ_AHEAD),
        };
        unsigned char type = cmd_type[cmd[0]];


Thanks,
Monty

Re: [PATCH] SG_IO block filter whitelist missing MMC SET READ AHEAD command

[Jens Axboe]

(apologies to Monty who will receive this email twice)

> Hi Jens,
> 
> I have another request for the block filter SG_IO command whitelist,
> specifically the MMC streaming command set SET READ AHEAD command.
> The command applies only to MMC CDROM/DVDROM drives with the streaming
> optional feature set.  The command is useful to cdparanoia in that it
> allows explicit cache control side effects that are, on many drives,
> cdparanoia's most efficient way to flush/disable the media cache on
> cdrom drives. I am aware of no reason why it should not be accessible
> from usespace.
> 
> Also note that the command is already fully accessible through the
> SCSI-native version of the SG_IO ioctl as well as the traditional SG
> interface.  The command is only being refused on block devices.  That
> means that on a typical stock distro, the command is available through
> /dev/sg* but not /dev/scd* although both are typically available and
> accessible.  Filtering the command is not providing any protection,
> only a confusing inconsistency.
> 
> For convenience's sake, trivial patch below.
> 
> diff -ru linux-2.6.26.3/block/scsi_ioctl.c
> linux-2.6.26.3-monty/block/scsi_ioctl.c
> --- linux-2.6.26.3/block/scsi_ioctl.c   2008-08-20 14:11:37.000000000 -0400
> +++ linux-2.6.26.3-monty/block/scsi_ioctl.c     2008-08-22
> 04:17:56.194248000 -0400
> @@ -188,6 +188,7 @@
>                safe_for_write(GPCMD_PREVENT_ALLOW_MEDIUM_REMOVAL),
>                safe_for_write(GPCMD_LOAD_UNLOAD),
>                safe_for_write(GPCMD_SET_STREAMING),
> +               safe_for_write(GPCMD_SET_READ_AHEAD),
>        };
>        unsigned char type = cmd_type[cmd[0]];

Looks harmless enough to me, I've adapted your patch for current -git.
Note that with 2.6.27, you can actually modify the command table at
runtime as well. But we should still have sane defaults.

-- 
Jens Axboe