Few days ago I replaced front glass on samsung i9300 and flashed LineageOS 14.1. After that I’ve found that proximity sensor stays in triggered state, it may happened because of lack of experience (I’ve used too much UV-glue, so it was everywhere) or because of additional screen protector which been installed. Anyway, always-triggered-proximity-sensor made phone partially usable (you can’t cancel any call without pushing power button few times). I’ve found a lot of articles how to calibrate proximity sensor like this one. More over I’ve found that I shouldn’t do any calculation to update /efs/prox_cal, after auto-calibration /efs/prox_cal updated automatically (at least with kernel that shipped by default), but anyway it didn’t help me. Every reboot calibration  was reseted to zero.

For a first time, I’ve used proximity threshold value to fix proximity sensor, but later I saw that kernel driver read calibration directly from file and SELinux could be a reason why /efs/prox_cal haven’t effect.

Part that read calibration value looks like that:

#define CANCELATION_FILE_PATH "/efs/prox_cal"
...
int proximity_open_calibration(struct ssp_data *data)
{
 int iRet = 0;
 mm_segment_t old_fs;
 struct file *cancel_filp = NULL;
 
old_fs = get_fs();
 set_fs(KERNEL_DS);
 
cancel_filp = filp_open(CANCELATION_FILE_PATH, O_RDONLY, 0666);
 if (IS_ERR(cancel_filp)) {
 iRet = PTR_ERR(cancel_filp);
 if (iRet != -ENOENT)
 pr_err("[SSP]: %s - Can't open cancelation file\n",
 __func__);
 set_fs(old_fs);
 goto exit;
}

I’ve checked logcat and here is it:

05-06 21:29:12.916 3219 3219 W Binder:2377_A: type=1400 audit(0.0:39): avc: denied { read } for name="prox_cal" dev=mmcblk0p3 ino=46 scontext=u:r:system_server:s0 tcontext=u:object_r:efs_device_file:s0 tclass=file permissive=0

Definitely SELinux forbid reading of calibration file, I was surprised that SElinux capable to forbid kernel read call and now I feel a shame because usually I just disable it.

First I wanted to create new policy to allow reading of that file for kernel, but later I’ve found that /efs partition contains other calibration files, for example /efs/gyro_cal_data, I’ve checked security context of that files and found that it differs from /efs/prox_cal, it was u:object_r:sensors_data_file:s0 but prox_cal was created with default for /efs partition context u:object_r:efs_file:s0, so I’ve changed context:

# chcon u:object_r:sensors_data_file:s0 /efs/prox_cal

After that kernel started to load calibration value every boot. Looks like instructions like one mentioned above works for everyone who modified factory shipped prox_cal file with right security context, but I haven’t /efs/prox_cal before and it was created with wrong context.
I hope that story may help someone.