On Wed, 2008-07-02 at 15:28 -0500, James Bottomley wrote: > On Wed, 2008-07-02 at 20:40 +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. > > >> > > > > > >Thanks for the quick reply, James. > > > > > >We're talking about this snippet of code from st.c, I guess? > > > > > >--- > > >switch (cmd_in) { > > > case SCSI_IOCTL_GET_IDLUN: > > > case SCSI_IOCTL_GET_BUS_NUMBER: > > > break; > > > default: > > > if ((cmd_in == SG_IO || > > > cmd_in == SCSI_IOCTL_SEND_COMMAND || > > > cmd_in == CDROM_SEND_PACKET) && > > > !capable(CAP_SYS_RAWIO)) > > > i = -EPERM; > > > else > > > i = scsi_cmd_ioctl(file, STp->disk->queue, > > > STp->disk, cmd_in, p); > > > if (i != -ENOTTY) > > > return i; > > > break; > > >} > > >--- > > > > > >Obviously. (I just found the discussion about this dating from > > >April '05). > > > > > >What's the way to go then in order to access a tape as a user, when > > >the user would like to get the sense bytes in case of problems? > > > > > >Should the user process get CAP_SYS_RAWIO? > > > > The user process in my case is forked by another process which runs > > as root. But since this process does not have CAP_SETPCAP it cannot > > set the child's capabilities (which is how I naively thought one could > > implement this). > > > > What options are left? Running a patched kernel where the "SG_IO in st > > requires CAP_SYS_RAWIO" is taken out? > > Erm, well capabilities are designed to be malleable, especially with > things like sucap and execap, which root should be able to use. But you need to change and recompile your kernel to use that, as init needs CAP_SETPCAP to be set, no? Cheers, Arne -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
