What capabilities required for ioctl() on emmc on systemd? - systemd

Question

I want to run my program with systemd with a regular user ( non-root). This program uses ioctl() syscall to access emmc registers.I want to learn which capabilities required to be added to my systemd unit file.
I tried with below unit file:
[Unit]
Description=EMMC-LIFETIME UTILITY
[Service]
User=tron
Group=disk
ExecStart=/HARICI/emmc-lifetime /dev/mmcblk0 -v
CapabilityBoundingSet=CAP_SYS_ADMIN
DeviceAllow=/dev/mmcblk0 rw
[Install]
WantedBy=multi-user.target
Here is the code of emmc-lifetime:
int main(int argc, char **argv)
{
if(argc < 2){
printf("Usage: %s <mmcfilename> (-v)\n", argv[0]);
printf("Example: %s /dev/mmcblk1 -v\n", argv[0]);
return 1;
}
char ext_csd[512], ext_csd_rev;
int fd, ret;
fd = open(argv[1], O_RDWR);
if (fd < 0) {
printf("Failed to open eMMC device, please check which path you have passed\n");
return 1;
}
struct mmc_ioc_cmd idata;
memset(&idata, 0, sizeof(idata));
memset(ext_csd, 0, sizeof(char) * 512);
idata.write_flag = 0;
idata.opcode = MMC_SEND_EXT_CSD;
idata.arg = 0;
idata.flags = MMC_RSP_SPI_R1 | MMC_RSP_R1 | MMC_CMD_ADTC;
idata.blksz = 512;
idata.blocks = 1;
mmc_ioc_cmd_set_data(idata, ext_csd);
ret = ioctl(fd, MMC_IOC_CMD, &idata);
if (ret){
printf("ioctl failed, are you sure it is an MMC device???\n");
return ret;
}
ext_csd_rev = ext_csd[EXT_CSD_REV];
if (ext_csd_rev >= 7) {
if(argc==3 && !strcmp(argv[2],"-v")){
printf("EXT_CSD_DEVICE_LIFE_TIME_EST_TYP_A: 0x%02x\n",
ext_csd[EXT_CSD_DEVICE_LIFE_TIME_EST_TYP_A]);
printf("EXT_CSD_DEVICE_LIFE_TIME_EST_TYP_B: 0x%02x\n",
ext_csd[EXT_CSD_DEVICE_LIFE_TIME_EST_TYP_B]);
printf("EXT_CSD_PRE_EOL_INFO: 0x%02x\n",
ext_csd[EXT_CSD_PRE_EOL_INFO]);
}else{
printf("%d\n",ext_csd[EXT_CSD_DEVICE_LIFE_TIME_EST_TYP_B]*10);
}
}
if(fd)
close(fd);
return ret;
If I comment out "User=tron" in my unit file, everything works expectedly:
Nov 03 01:17:03 tron systemd[1]: Started EMMC-LIFETIME UTILITY.
Nov 03 01:17:03 tron emmc-lifetime[28294]: EXT_CSD_DEVICE_LIFE_TIME_EST_TYP_A: 0x01
Nov 03 01:17:03 tron emmc-lifetime[28294]: EXT_CSD_DEVICE_LIFE_TIME_EST_TYP_B: 0x01
Nov 03 01:17:03 tron emmc-lifetime[28294]: EXT_CSD_PRE_EOL_INFO: 0x01
But if I uncomment "User=tron", here is the result:
Nov 03 00:57:17 tron systemd[1]: Started EMMC-LIFETIME UTILITY.
Nov 03 00:57:17 tron emmc-lifetime[27706]: ioctl failed, are you sure it is an MMC device???
Nov 03 00:57:17 tron systemd[1]: emmc-info.service: Main process exited, code=exited, status=255/n/a
Nov 03 00:57:18 tron systemd[1]: emmc-info.service: Unit entered failed state.
Nov 03 00:57:18 tron systemd[1]: emmc-info.service: Failed with result 'exit-code'.
What capabilities are required in my unit file to run my executable as "tron" user ?

Answer 1

Solved. For the capabilities side, it must have CAP_SYS_RAWIO.
We can use :
setcap cap_sys_rawio=+eip /HARICI/emmc-lifetime
And by this way, we can run "emmc-lifetime" binary with non-root user. Please note that, this user must be in "disk" group to be able to open "/dev/mmcblkX"
But unfortunately, this doesn't work in systemd unit files.
In systemd unit files, if you write:
User=some-non-root-user
CapabilityBoundingSet=SOME_CAPABILITY
Does not work. Thats why my above code allways fails.
I must find another way to run my binary with non-root privileges.