RFE: add O_RDRW mode for sat/scsi/ata devices

[George Shuklin]

Right now smartmontool could not gather metrics from devices without root access, because to open device with O_RDONLY mode. It is explained in source code as a way to avoid spindle spin-up when device is closed. Unfortunately this made impossible to use all SG_IO commands, as they require RDWR access to the device.

My proposal is to add device types 'scsi-rw', 'sat-rw', 'ata-rw', etc to allow to run smartmontools with limited capabilities and permissions (e.g. not from the root).

[Christian Franke]

... Unfortunately this made impossible to use all SG_IO commands, as they require RDWR access to the device.

Which smartmontools functionality using SG_IO does not work due to missing O_RDWR access?

My proposal is to add device types 'scsi-rw', 'sat-rw', 'ata-rw', etc to allow to run smartmontools with limited capabilities and permissions (e.g. not from the root).

Why should O_RDWR access work without root permissions if O_RDONLY access already requires root permissions?

[George Shuklin]

I apologize for premature conclusion. I've tested smartmontools with O_RDWR access and it does not help.

My issue was that I wasn't able to make smartmonctl to work under non-root user. I've tried to give that user rw access to the block device file, and to set cap_sys_rawio+ie on smartctl binary. It didn't helped.

I saw in strace that smartctl uses O_RDONLY mode and there is an article which says that sg_io can be used only in RW mode: ​https://www.tldp.org/HOWTO/SCSI-Generic-HOWTO/sg_io.html.

I made a mistake thinking that O_RDONLY is a source of the problem.

[Christian Franke]

According to function blk_verify_command() from current kernel sources (see ​block/scsi_ioctl.c), O_RDONLY or O_RDWR make no difference if device was opened as root (or with CAP_SYS_RAWIO).

The SCSI commands listed in function blk_set_cmd_filter_defaults() show that some of the smartctl -d scsi functionality might work with O_RDONLY for non-root users. Some more might work with O_RDWR.

But smartctl -d sat (to access SATA devices) won't work at all because the SCSI commands ATA_12 and ATA_16 (see ​scsi_proto.h) are always blocked for non-root users.

[Christian Franke]

There is no benefit if device is opened with O_RDWR mode for SG_IO access.