>>> 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? 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
