Ubuntu – File permissions: why can’t I list a directory?

x bit for directory is also called as search bit. Actually, it enables you to access the inodes of the files listed inside the folder. So if you want to access /home/user/foo/bar.txt then you must have search access on every ancestor of bar.txt

Quoting from the page

Because directories are not used in the same way as regular files, the permissions work slightly (but only slightly) differently. An attempt to list the files in a directory requires read permission for the directory, but not on the files within. An attempt to add a file to a directory, delete a file from a directory, or to rename a file, all require write permission for the directory, but (perhaps surprisingly) not for the files within. Execute permission doesn't apply to directories (a directory can't also be a program). But that permission bit is reused for directories for other purposes.

Execute permission is needed on a directory to be able to cd into it (that is, to make some directory your current working directory).

Execute is needed on a directory to access the inode information of the files within. You need this to search a directory to read the inodes of the files within. For this reason the execute permission on a directory is often called search permission instead.

Search permission is required in many common situations. Consider the command cat /home/user/foo. This command clearly requires read permission for the file foo. But unless you have search permission on /, /home, and /home/user directories, cat can't locate the inode of foo and thus can't read it! You need search permission on every ancestor directory to access the inode of any file (or directory), and you can't read a file unless you can get to its inode.

Please read more at file permission directory section.

Update: Leo raised a very good question. If we know the inode then can we access a file from a directory having it's x bit unset? I believe, we should not be able to do so. I did not test it by c program but rather used some handy bash commands to confirm it.

user@user-desktop:~/test$ ls -lart
total 12
drwxr-xr-x 49 user user 4096 2011-11-30 22:37 ..
drwxr-xr-x  3 user user 4096 2011-11-30 22:37 .
drwxr-xr-x  2 user user 4096 2011-11-30 22:38 level1
user@user-desktop:~/test$ ls -lart level1/
total 12
drwxr-xr-x 3 user user 4096 2011-11-30 22:37 ..
drwxr-xr-x 2 user user 4096 2011-11-30 22:38 .
-rw-r--r-- 1 user user    8 2011-11-30 22:38 file1
user@user-desktop:~/test$ stat level1
  File: `level1'
  Size: 4096        Blocks: 8          IO Block: 4096   directory
Device: 808h/2056d  Inode: 95494       Links: 2
Access: (0755/drwxr-xr-x)  Uid: ( 1000/    user)   Gid: ( 1000/    user)
Access: 2011-11-30 22:46:16.576702105 +0530
Modify: 2011-11-30 22:38:12.386701913 +0530
Change: 2011-11-30 22:46:08.876702102 +0530
user@user-desktop:~/test$ stat level1/file1 
  File: `level1/file1'
  Size: 8           Blocks: 8          IO Block: 4096   regular file
Device: 808h/2056d  Inode: 60775       Links: 1
Access: (0644/-rw-r--r--)  Uid: ( 1000/    user)   Gid: ( 1000/    user)
Access: 2011-11-30 22:38:19.846701917 +0530
Modify: 2011-11-30 22:38:16.366701915 +0530
Change: 2011-11-30 22:38:16.366701915 +0530
user@user-desktop:~/test$ chmod -x level1
user@user-desktop:~/test$ stat level1/file1 
stat: cannot stat `level1/file1': Permission denied
user@user-desktop:~/test$ ls -lart level1/
ls: cannot access level1/..: Permission denied
ls: cannot access level1/.: Permission denied
ls: cannot access level1/file1: Permission denied
total 0
-????????? ? ? ? ?                ? file1
d????????? ? ? ? ?                ? ..
d????????? ? ? ? ?                ? .
user@user-desktop:~/test$ cat level1/file1
cat: level1/file1: Permission denied
user@user-desktop:~/test$ find . -inum 95494
./level1
user@user-desktop:~/test$ find . -inum 60775
user@user-desktop:~/test$ find ./level -inum 60775
find: `./level': No such file or directory
user@user-desktop:~/test$ find ./level1 -inum 60775

What it says there is that the owner of the disk is root. So you will need to change the permissions. Since it's ext4 the permissions are stored on the disk. It's not odd GParted made it read-only except for root because ext4 is not commonly used for USB drives. It seems GParted assumed it is most likely you would use ext4 for an operating system, for which it would be best to only have root access to the root of the partition.

It will always be possible to mount the drive and use the root user from a different computer to change the permissions.