Re: Temporary file policy

[Juan Cespedes <cespedes@debian.org>]

On 12 Sep 1997, Guy Maor wrote:

> A new command, tempfile(1), was added to debianutils 1.6.  tempfile
> creates a temporary file in a safe manner.  It uses tempnam(3) and
> allows you to specify the directory, prefix, and mode.  For example,
> you can use it like this:
> 
> set -e
> tf=$(tempfile -m 600)
> echo "super secret something" > $tf
> mv $tf /etc/whatever

	I see a problem here... suppose /etc and $TMPDIR (/tmp) aren't
in the same filesystem.  In this case, the `mv' isn't atomic and the
file in /etc may be truncated at some time.

	For example:

# strace mv /tmp/something /etc/something
	...
rename("/tmp/something", "/etc/something") = -1 EXDEV (Invalid cross-device link)
unlink("/etc/something")                = -1 ENOENT (No such file or directory)
open("/tmp/something", O_RDONLY)        = 3
open("/etc/something", O_WRONLY|O_CREAT|O_TRUNC, 0600) = 4
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 8192) = 8192
write(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 8192) = 8192
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 8192) = 2048
write(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 2048) = 2048
read(3, "", 8192)                       = 0
close(3)                                = 0
close(4)                                = 0
	...

	I think it's a big problem... specially if the file to be
written is as important as /etc/passwd can be.

	Suggested fix: add an argument with the directory where you
want the temp file created, in order to create temporate uniq files in
/etc, for example.

--
Juan Cespedes



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