Why does Ruby on Linux return true to File.writable?('/tmp/file') but raises Errno::EACCES while trying to write the file?

Question

I am using a GNU/Linux system.

Firstly, I have:

• Moved to the /tmp/ directory.

• Created a file called ruby.rb as a non-root user.

• Opened irb as su user.

Now in IRB:

┌┄┄[root::archlinux]┈[/tmp]
└──╼⮚ irb
irb(main):001:0> File.writable?('ruby.rb')
=> true
irb(main):002:0> File.stat('ruby.rb')
=> #<File::Stat dev=0x2d, ino=819138, mode=0100644, nlink=1, uid=1000, gid=1000, rdev=0x0, size=0, blksize=4096, blocks=0, atime=2019-07-14 04:44:13 +0530, mtime=2019-07-14 04:44:13 +0530, ctime=2019-07-14 04:44:13 +0530>
irb(main):003:0> File.write('ruby.rb', '#!/usr/bin/ruby -w')
Traceback (most recent call last):
        3: from /root/.irb:351:in `<main>'
        2: from (irb):3
        1: from (irb):3:in `write'
Errno::EACCES (Permission denied @ rb_sysopen - ruby.rb)
irb(main):004:0> 

Apart from this, actually I am trying to do is writing a log file. I am checking if the file is writable. If not, it sends notification to the user.

I have faced the problem previously where File#writable?(str) returned true in such case (and it works on the /tmp/ directory). And just used a begin <...> rescue Errno::EACCES block to fix the problem. But in the current project, I don't want to use a rescue block.

Why does the File#writable?(str) return true in the first place?

EDIT: First of all this should be moved to unix.stackexchange.

Secondly I understood

• You can't modify a file on a tmp filesystem: I have mounted a mere 4 MiB partition as tmpfs on /mnt. Created a file as local user. Then changed the permission to omnipotent 777! Then I changed my account to root. I tried to edit the contents with nano. It's not possible.

• Ruby's File#writable? probably doesn't detect the mounted filesystem type. Whether it's XFS or EXT4 or procfs or tmpfs. If all it does is checking the mode, in such scenario, the File#writable? returns true while in reality, it's not actually writable!

Edit 2: I have uploaded 2 screenshots at imgur:

Answer 1

Okay this is not a Ruby specific problem. That said, any programming language will have such problem because it's all about the file system protection implemented in systemd.

The option fs.protected_regular is implemented in Linux Kernel 4.19+ to make data spoofing attacks harder.

So, firstly you need to check:

sysctl fs.protected_regular

In your case, it should print 1

So, change the value to 0:

sysctl fs.protected_regular=0

Hopefully the problem will get solved. Or try this:

sysctl fs.protected_regular=0
sysctl fs.protected_fifos=0

Note that the option is enabled by default. You may not prefer telling every user of your program to set the above to 0! They may not even prefer that!

So, in Ruby:

1. You can delete the file:

File.delete('file') if File.exist?('file')
# Better use the '/tmp/file' or File.join(%w(/ tmp file)) otherwise the file will get delete from the Dir.pwd

Which will delete the file every time the script runs.

2. You can also use begin ... rescue block and use Warning.warn or Kernel.warn or STDERR.puts to print a warning message.

Answer 2

Could be one of 2 things, file is not being opened for writing, or permissions issue with user.

Try to open the file for writing with either 'w' or 'w+'

File.open('ruby.rb', 'w') { |file| file.write("#!/usr/bin/ruby -w") }

If that doesn't work, try using chmod to update file permissions for ruby.rb with

chmod -R 777 <path_to_file>