trick question about sudo

Phil Meyer pmeyer at
Mon Feb 18 20:03:58 UTC 2008

Valent Turkovic wrote:
> Hi,
> I have one question about sudo.
> While looking for uid/gid override for samba mounts I found this solution:
> sudo echo 0 > /proc/fs/cifs/LinuxExtensionsEnabled
> But this obviously doesn't work on Fedora but this works:
> sudo sh -c 'echo 0 > /proc/fs/cifs/LinuxExtensionsEnabled'
> But the trick question is in which setups and which circumstances can
> the first sudo work?
> Cheers,
> Valent.

The answer: Never

Here is why: plumbing

The command sudo as I type it, belongs to me, and the connections to it, 
ie: STDIN, STDOUT, and STDERR (file handles 0, 1, and 2) are owned by 
me, and controlled by the shell that I am in.

These are attached by the shell/OS when sudo is run as a part of loading 
it, and well before the actual sudo code begins to execute.

Think of it as plumbing and everything in the pipeline before and after 
the command is mine.

mine >command> mine

ls > /tmp/file

/tmp/file is created with my permissions

any_command > /tmp/file


sudo any_command > /tmp/file


command < /tmp/file1 > /tmp/file2

/tmp/file1 must be readable by me
/tmp/file2 must be writable by me

Both sides of that pipe are performed as my user, regardless of the 
command being executed.

Hope that helps.

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