Some interface files in debugfs support the read method
dfs_file_read(), but their rwx permissions is shown as
unreadable.

In the user mode, the following problem can be clearly seen:

 # ls -l /sys/kernel/debug/ubi/ubi0/
 total 0
 --w------- 1 root root 0 Oct 22 16:26 chk_fastmap
 --w------- 1 root root 0 Oct 22 16:26 chk_gen
 --w------- 1 root root 0 Oct 22 16:26 chk_io
 -r-------- 1 root root 0 Oct 22 16:26 detailed_erase_block_info
 --w------- 1 root root 0 Oct 22 16:26 tst_disable_bgt
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_bitflips
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_io_failures
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_power_cut
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_power_cut_max
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_power_cut_min

It shows that these files do not have read permission 'r',
but we can actually read their contents.

 # echo 1 > /sys/kernel/debug/ubi/ubi0/chk_io
 # cat /sys/kernel/debug/ubi/ubi0/chk_io
 1

User's permission access is determined by capabilities.
Of course, the root user is not restricted from reading
these files.

When reading a debugfs file, the process is as follows:

 ksys_read()
   vfs_read()
     if (file->f_op->read)
       file->f_op->read()
         full_proxy_open()
           real_fops->read()
             dfs_file_read() -- Read method of debugfs file.
     else if (file->f_op->read_iter)
       new_sync_read()
     else
       ret = -EINVAL -- Return -EINVAL if no read method.

This indicates that the debugfs file can be read as long as the read
method of the debugfs file is registered. This patch adds the read
permission display for file that support the read method.
Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>