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RE: mkdir when target exists and is a broken symlink

From: Avis, Ed
Subject: RE: mkdir when target exists and is a broken symlink
Date: Fri, 20 May 2005 18:49:38 +0100

Paul Eggert wrote:

>>There could be some kind of -f, --follow option so that mkdir will
>>create the directory pointed to.
>There is a potential security problem there, if the symbolic link
>is in a directory writable by an attacker.

I don't agree that this is a security problem, since mkdir is doing
exactly what you asked it to.  But anyway,

>>Is this a sensible thing to put in mkdir or is there some 
>>existing Unix idiom that does what I want?
>Not in POSIX/Unix, but in coreutils 5.3.0 and later: either "mkdir -p
>$(readlink -f file)" or "mkdir -p $(readlink -m file)", depending on
>the exact semantics that you want.

Yes, that would do the trick.

>>I note that 'touch foo' when foo is a broken symlink will create the
>>link destination if possible (though without making any directories,
>POSIX requires this, but it is arguably a misfeature, due to the
>security issues mentioned.

I still don't understand how this is a security issue any more than the
whole concept of symbolic links is a security issue.

I'll wait for a newer coreutils release, and instead of using 'mkdir -p'
as the standard way of trying hard to make a directory, use instead
'mkdir -p `readlink -f`'.

Ed Avis <address@hidden>

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