Re: [RFC PATCH] fs: allow open(dir, O_TMPFILE|..., 0) with mode 0

[Eric Rannaud]

On Wed, Nov 5, 2014 at 12:27 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Mon, Nov 03, 2014 at 11:04:27AM -0800, Linus Torvalds wrote:
>> Oh, so you don't actually need any file contents at all?
>>
>> If that is actually a real usage, then maybe we should just say that
>> "O_TMPFILE|O_RDONLY" is fine, and remove the check that it has to be
>> writable.
>
> Wasn't this disallowed to prevent problems on old kernels that don't use
> O_TMPFILE?  In that case we'd ignore the flag and would just get a file
> handle for the directory instead.

Yes, that was the idea at first, as discussed at
http://article.gmane.org/gmane.linux.file-systems/76273, in commit
bb458c644. But soon after that, O_WRONLY was allowed (ba57ea64cb1),
and O_RDWR was removed from O_TMPFILE. Now only remain the explicit
checks in build_open_flags() that Linus mentioned.

If we allow
    fd = open("/tmp", O_TMPFILE|O_RDONLY, 0600)
it would be seen by an old kernel as
    fd = open("/tmp", O_DIRECTORY|O_RDONLY, 0600)
which will succeed.

But unlike the other cases the creative definition of O_TMPFILE was
meant to prevent, this does not create a security risk for anyone
implementing a secure tmpfile, as they would be asking for a writable
fd.

To implement an atomic open() with O_TMPFILE+flink, if neither
O_WRONLY nor O_RDWR is in flags, you would have to manually check with
fstat that fd is indeed a regular file and not a directory. At least
if you need to run on old kernels.

If such a changes goes in, the man page for open(2) should talk about
what happens on old kernels (it already has an explanation for the
writable case).
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