Android NDK open() 设备权限被拒绝

Question

In a native call, I'm trying to open("/dev/video4", O_RDWR) but I get errno EACCES 13 "permission denied".

If I run the same code* in an executable, on the same Android host, as the same UID of the installed app I'm running above, it works fine. (* minor differences like main() instead of Java_com_test_testOpen() )

I've tried chmod 666 /dev/video4 and still get EACCES, which is especially strange.

Why does the same code , on the same host , as the same user , give EACCESS when called via JNI, and success when called as standalone executable?

The test device is rooted and running Cyanogenmod 12.1 (API 22) and I'm targeting >= API 21 (5.0 Lollipop) on rooted devices. Thanks for your help.

Answer 1

Since you get an error when running your code from an Android Java application, I would guess that you are missing a permission. It's the camera that you are trying to access, if I am not mistaken, so if you add:

 <uses-permission android:name="android.permission.CAMERA" />

to your AndroidManifest.xml, your application should run fine.

Answer 2

Since I'm building Cyanogenmod 12.1 (API 22) with other minor hacks I was able to get permissions for /dev/video* in my app by using the following hacks:

1. For standard Linux permissions, android.permission.CAMERA no longer seems to allow access to /dev/video* even though they're owned by system:camera . Instead, I edited device/samsung/klte-common/rootdir/etc/ueventd.qcom.rc and changed the /dev/video* line to 0666 .
2. For SE Linux permissions, I added the line allow untrusted_app video_device:chr_file rw_file_perms; to external/sepolicy/untrusted_app.te .

After rebuilding and installing the image, my JNI lib is able to access /dev/video* and my client is happy!

Answer 3

The answers was not enough for me, so I left one more anwser. open API has more parameter 'mode'.

int open(const char *pathname, int flags, mode_t mode);

check out the link http://man7.org/linux/man-pages/man2/open.2.html

you might need to use 'open' with S_IRWXU option. like

open("/dev/video4", O_RDWR | O_CREAT, S_IRWXU )