null bytes in file names?

null bytes in file names?

[Dominik Vogt]

As far as I know, UNIX file systems allow null bytes in file
names.  Out of curiosity I tried generating such a file.  I edited
a file fn in a hex editor and put a single null byte into it.

Take I (with "touch"):

  $ touch $(< fn)
  touch: creating `': No such file or directory
  touch: creating `': No such file or directory
  $ touch $(< fn)
  touch: creating `': No such file or directory
  $ touch s$(< fn)t
  $ touch u"$(< fn)"v
  $ ls
  fn s t u

"u" might actually be named "u^@v" but displayed incorrectly by ls,
so let's try to access a file named "u".

  $ ls u
  u

Nope, the name got cut off.

  $ rm s t y

Take II (shell builtins only):

  $ echo foo > $(< fn)
  zsh: no such file or directory: 
  $ echo foo > "$(< fn)"
  zsh: no such file or directory: ^@
  $ echo foo > w$(< fn)x
  $ echo foo > y$(< fn)z
  $ ls
  fn w  x  y  z
  $ ls w y
  w  y

So, no luck.  I wonder where the problem comes from:

 - zsh? (4.0.4)
 - libc?
 - kernel? (linux-2.4.20)

Bye

Dominik ^_^  ^_^

Re: null bytes in file names?

[Erik Trulsson]

On Mon, Apr 07, 2003 at 10:32:21AM +0200, Dominik Vogt wrote:
> As far as I know, UNIX file systems allow null bytes in file
> names.  Out of curiosity I tried generating such a file.  I edited
> a file fn in a hex editor and put a single null byte into it.

Your knowledge is somewhat faulty.
There are exactly two characters that Unix does not allow in filenames.
Those are NUL (ASCII code 0) and / (ASCII code 47).
The former is used to indicate the end of a filename, while the latter
is used to separate directory paths.

So, you cant't have null bytes in a filename under Unix.

> 
> Take I (with "touch"):
> 
>   $ touch $(< fn)
>   touch: creating `': No such file or directory
>   touch: creating `': No such file or directory
>   $ touch $(< fn)
>   touch: creating `': No such file or directory
>   $ touch s$(< fn)t
>   $ touch u"$(< fn)"v
>   $ ls
>   fn s t u
> 
> "u" might actually be named "u^@v" but displayed incorrectly by ls,
> so let's try to access a file named "u".
> 
>   $ ls u
>   u
> 
> Nope, the name got cut off.
> 
>   $ rm s t y
> 
> Take II (shell builtins only):
> 
>   $ echo foo > $(< fn)
>   zsh: no such file or directory: 
>   $ echo foo > "$(< fn)"
>   zsh: no such file or directory: ^@
>   $ echo foo > w$(< fn)x
>   $ echo foo > y$(< fn)z
>   $ ls
>   fn w  x  y  z
>   $ ls w y
>   w  y
> 
> So, no luck.  I wonder where the problem comes from:
> 
>  - zsh? (4.0.4)
>  - libc?
>  - kernel? (linux-2.4.20)
> 
> Bye
> 
> Dominik ^_^  ^_^

-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013@student.uu.se

Re: null bytes in file names?

[Peter Stephenson]

Erik Trulsson wrote:
> On Mon, Apr 07, 2003 at 10:32:21AM +0200, Dominik Vogt wrote:
> > As far as I know, UNIX file systems allow null bytes in file
> > names.  Out of curiosity I tried generating such a file.  I edited
> > a file fn in a hex editor and put a single null byte into it.
> 
> Your knowledge is somewhat faulty.
> There are exactly two characters that Unix does not allow in filenames.
> Those are NUL (ASCII code 0) and / (ASCII code 47).
> The former is used to indicate the end of a filename, while the latter
> is used to separate directory paths.

Just to be particular...

> > Take I (with "touch"):
> > 
> >   $ touch $(< fn)
> >   touch: creating `': No such file or directory
> >   touch: creating `': No such file or directory

...zsh actually handles the NUL internally, but there's no way to pass
it down as an argument to `touch'.  That gets passed a string `NUL
NUL'.  However, since as you no doubt know all the command receives is a
count of arguments and a series of null-terminated strings, touch simply
sees the first NUL.  The same would happen with an `open()' call from
within zsh.  That's where the rule Erik mentioned comes from.

-- 
Peter Stephenson <pws@csr.com>                  Software Engineer
CSR Ltd., Science Park, Milton Road,
Cambridge, CB4 0WH, UK                          Tel: +44 (0)1223 692070


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