creating temporary files in a secure way

[Bruno Haible]

Hi Alexandre,

Followup to a discussion from 2004-10-26: I think it's worth describing how
to portably create a temporary file. If this had existed in the autoconf
documentation, Debian bug #278283 (affecting autopoint and gettextize)
would not have existed. I don't see another canonical place where this
could be documented. Can you please put this in for me?


2005-01-09  Bruno Haible  <address@hidden>

        * doc/autoconf.texi (Limitations of Usual Tools): Add a paragraph
        about mktemp.

*** autoconf/doc/autoconf.texi.bak      2004-10-12 13:50:46.000000000 +0200
--- autoconf/doc/autoconf.texi  2004-10-26 20:30:58.000000000 +0200
***************
*** 11581,11586 ****
--- 11581,11624 ----
  older versions are not thread-safe either).
  
  
+ @item @command{mktemp}
+ @c -------------------
+ @prindex @command{mktemp}
+ @cindex Creating temporary files
+ The command @command{mktemp} exists only on a few systems.  A portable
+ way to create a temporary file name into which it is safe to write,
+ protecting against symlink attacks, is to create a temporary directory
+ with mode 700 and use a file inside this directory.
+ 
+ Here is sample code to securely create a temporary directory:
+ 
+ @example
+ # Use the environment variable TMPDIR, falling back to /tmp. This allows
+ # users to specify a different temporary directory, for example, if their
+ # /tmp is filled up or too small.
+ : address@hidden/address@hidden
+ @{
+   # Use the mktemp program if available. If not available, hide the error
+   # message.
+   tmp=`(umask 077 && mktemp -d -q "$TMPDIR/fooXXXXXX") 2>/dev/null` &&
+   test -n "$tmp" && test -d "$tmp"
+ @} ||
+ @{
+   # Use a simple mkdir command. It is guaranteed to fail if the directory
+   # already exists.  $RANDOM is bash specific and expands to empty in shells
+   # other than bash, ksh and zsh.  Its use does not increase security;
+   # rather, it minimizes the probability of failure in a very cluttered /tmp
+   # directory.
+   tmp=$TMPDIR/foo$$-$RANDOM
+   (umask 077 && mkdir "$tmp")
+ @} ||
+ @{
+   echo "$0: cannot create a temporary directory in $TMPDIR" >&2
+   @{ (exit 1); exit 1; @}
+ @}
+ @end example
+ 
+ 
  @item @command{mv}
  @c ---------------
  @prindex @command{mv}

Bruno