Mfrhtyj

I am already a member of the group "aid_sdcard_rw":

android@localhost:~$ whoami

android

android@localhost:~$ groups

android aid_radio ...... aid_sdcard_rw ......

you can see that i am already in group aid_sdcard_rw. Then, when i
access directory /storage/sdcard0 :

android@localhost:~$ ls /storage/sdcard0/

ls: cannot open directory '/storage/sdcard0/': Permission denied

but:

android@localhost:~$ ls -l /storage/

total 8

d---rwxr-x 17 android aid_sdcard_rw 8192 1月   1  1970 sdcard0

doesn't group "aid_sdcard_rw" members have read permission?
why is this Permission denied happened?

The system i run is an ubuntu 16.04 and running in my android phone using
an app called Linux Deploy, and my phone have external sd card inserted :

Welcome to Ubuntu 16.04 LTS (GNU/Linux 3.4.5 armv7l)



 * Documentation:  https://help.ubuntu.com/

Ubuntu 16.04 LTS [running via Linux Deploy]

The directory /storage/sdcard0 is my external sd card mount directory(see the last line below):

android@localhost:~$ cat /etc/mtab

/dev/loop1 / ext4 rw,relatime,data=ordered 0 0

proc /proc proc rw,relatime 0 0

sys /sys sysfs rw,relatime 0 0

tmpfs /dev tmpfs rw,nosuid,relatime,mode=755 0 0

devpts /dev/pts devpts rw,relatime,gid=5,mode=620 0 0

tmpfs /dev/shm tmpfs rw,relatime 0 0

/dev/block/vold/179:97 /storage/sdcard0 vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0

I have tried umount /storage/sdcard0 and remount it with option -o umask=0000 , but command ls -l /storage get same result as above.

If the user that owns the file or directory is explicitly denied access to the file or directory through the permissions, then that user can not access the file or directory. It does not matter if the owner is a member of the group that has access.

Or in other words, to access the file or directory as the owner, it has to allow access to the owner.

In your case, you have a directory without x permissions for the owner. The owner can therefore not access the directory.

To fix this, use

chmod u+x /storage/sdcard0

If the owner additionally needs to be able to list the contents of the directory, also give the owner r permissions. To create or delete files or subdirectories, you will need w permissions.

You should be able to set the permissions on the directory with the card unmounted. When mounting the SD card, the permissions on the mount point should be inherited from the directory permissions.

The android user is also the owner of the directory, owner that has no rights (even if android user is member of a valid group). The solution would be to change the owner of the directory with chown or try it with another user (that belongs to a valid group).

File permissions have a precedence order:

• User permissions affect the owner of the file.


• Group permissions affect everyone in the file's group.


• Other permissions affect everyone else.

The permissions aren't merged -- they're tested in order. This means the group permissions don't affect the owner, and other permissions don't affect the owner and group. This then permits the situation where wider sets of users can have more permissions than more specific ones -- this is occasionally useful when you have a file or directory that should generally be accessible, but you want to exclude a specific user or group.

However, this is mostly ineffectual when it comes to excluding a user. The owner of a file also has permission to change its permissions, so the owner can do

chmod u+rwx /storage/sdcard0

However, if the user is in a restricted environment where they can't use chmod, this can be effective.

Try to execute like this:

newgrp aid_sdcard_rw 

ls /storage/sdcard0/