Android – How to give /dev/bus/usb permissions for libusb android applications

androidandroid-ndklibusbpermissionsusb

i am developing an Application that uses libusb via jni.
this application is currently targeted only to rooted, usb-host
Android 3+ machines.

the scenario is as follows:

<java Activity>
       loads <jni_wrapper.so>
               which wraps <my_main_lib.so>
                       that uses <libusb.so>
                               that needs rw access to: /dev/bus/usb/<device>

all the .so native libraries are part of an infrastructure that i install (as root) on /system/lib,
which then can be used by java Activities, run by the simple users.

that means that the whole usb communication must be done from the native side.

i got everything to work fine, except for one issue – the usb permissions:
the default permissions of /dev/bus/usb/ entries are (0660, uid= root, gid= usb).

obviously, a standard java process running a native code does not meet any of the above requirements, forcing me to "chmod 666" the corresponding /dev entry, to get this working.

this solution is not so good, though, because it does not survive reboot, or unplugging/replugging of the device (permissions go back to 0660).

the solution i'm looking for needs to:

survive reboot/replugging
be installed ~once~ by the root, and not involve bothering the user every time to gain permissions
be generic and run on all (/most) android 3+ machines
[not mandatory] gives the minimal credentials necessary

directions i thought of:

changing permissions in /init.rc or /ueventd.rc –> both are overridden on every reboot
mounting usbfs /proc/bus/usb with devmode=0666 –> survives replugging, but not reboot
making my process join the "usb" group? –> i tried android.permission.ACCESS_USB, but it doesnt work/supported… :/

any ideas? 🙂

thanks!

Best Answer

I've same problem and I solve it doing this in the ndk, changing /proc/bus/usb for where usb is mounted in your case. Hope it helps.

r = system("su -c \"chmod -R 777 /proc/bus/usb/\"");
if(r!=0) {
    LOGD("Could not grant permissions to USB\n");
    eturn CANT_GRAN_PERMISSION_USB;
}

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Linux – How to change permissions for a folder and its subfolders/files in one step

The other answers are correct, in that chmod -R 755 will set these permissions to all files and subfolders in the tree. But why on earth would you want to? It might make sense for the directories, but why set the execute bit on all the files?

I suspect what you really want to do is set the directories to 755 and either leave the files alone or set them to 644. For this, you can use the find command. For example:

To change all the directories to 755 (drwxr-xr-x):

find /opt/lampp/htdocs -type d -exec chmod 755 {} \;

To change all the files to 644 (-rw-r--r--):

find /opt/lampp/htdocs -type f -exec chmod 644 {} \;

Some splainin': ^{(thanks @tobbez)}

chmod 755 {} specifies the command that will be executed by find for each directory
chmod 644 {} specifies the command that will be executed by find for each file
{} is replaced by the path
; the semicolon tells find that this is the end of the command it's supposed to execute
\; the semicolon is escaped, otherwise it would be interpreted by the shell instead of find

Using libusb in Android application: how to allow application to access USB

In android, you cannot directly open usb device using libusb (this is the conclusion that you came to :).

why you were not able to open?

You need to get permission from Android system (!= Kernel) to open a device.

Do it in Java:

when you request, a pop is shown to user to accept or reject. so you need to open the device in java, and extract the fd using java and pass on to libusb so that it can communicate with device.

now how will libusb build a handle from fd?

Update: I maintain a libusb version specific modified for Android (with more fixes - tested/working on Android 5.1 and lower). see https://gitlab.com/madresistor/libusb/blob/android/README

OLD LINK (DEAD LINK) here the solution: https://github.com/martinmarinov/rtl_tcp_andro-/blob/master/jni/libusb-andro/libusb/core.c#L993

Best Answer

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