Re: SG_IO permissions

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Wed, 2008-07-02 at 15:20 +0200, Arne Wiebalck wrote:
> Hi all,
> 
> I am trying to replace some read/write calls in our application
> by SG_IO commands in order to have access to the sense bytes in 
> case of an error. The underlying devices are tape drives. 
> 
> Part of our application, such as positioning or reading labels
> from the tape, are run as root. This seems to work fine, I get 
> the data I expect and the sense bytes in case of an error. 
> 
> However, the actual data transfer from and to the device is run 
> under a user's ID. This part does not work anymore when switching
> from read/write to SG_IO: 'Operation not permitted'. 
> 
> Does a user need some special rights to issue SG_IO (read) commands
> (on a file descriptor that he opened for reading and that he 
> can use without problems for read() calls)? 
> 
> The device node that the processes are accessing is a char special
> file owned by the user and with all user bits set. This special file
> is created on a per tape request basis. I also tried to use /dev/nst0
> instead, but that made no difference. 
> 
> I am running a relatively old kernel (2.6.9 based), could that cause
> any problem?
> 
> BTW, why does it say "except st" on the permission requirements table on
> http://sg.torque.net/sg/sg_io.html ? :)
> 
> 
> Any hints appreciated.

SG_IO access requires CAP_SYS_RAWIO to defeat the command verifier.

James