linux-kernel - Re: [PATCH] do_open(): Fix O_DIRECTORY

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Date:   Wed, 22 Mar 2023 11:17:10 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Pedro Falcato <pedro.falcato@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Aleksa Sarai <cyphar@...har.com>
Subject: Re: [PATCH] do_open(): Fix O_DIRECTORY | O_CREAT behavior

On Tue, Mar 21, 2023 at 02:47:55PM -0700, Linus Torvalds wrote:
> On Tue, Mar 21, 2023 at 1:16 PM Christian Brauner <brauner@...nel.org> wrote:
> >
> > But yes, that is a valid complaint so - without having tested - sm like:
> 
> I'd actually go a bit further, and really spell all the bits out explicitly.
> 
> I mean, I was *literally* involved in that original O_TMPFILE_MASK thing:
> 
>    https://lore.kernel.org/all/CA+55aFxA3qoM5wpMUya7gEA8SZyJep7kMBRjrPOsOm_OudD8aQ@mail.gmail.com/
> 
> with the whole O_DIRECOTY games to make O_TMPFILE safer, but despite
> that I didn't remember this at all, and my suggested "maybe something
> like this" patch was broken for the O_TMPFILE case.
> 
> So while we do have all this documented in our history (both git
> commit logs and lore.kernel.org), I actually think it would be lovely
> to just make build_open_flags() be very explicit about all the exact
> O_xyz flags, and really write out the logic fully.
> 
> For example, even your clarified version that gets rid of the
> "O_TMPFILE_MASK" thing still eends up doing
> 
>         if (flags & __O_TMPFILE) {
>                 if ((flags & O_TMPFILE) != O_TMPFILE)
>                         return -EINVAL;
> 
> and so when you look at that code, you don't actually realize that
> O_TMPFILE _cotnains_ that __O_TMPFILE bit, and what the above really
> means is "also check O_DIRECTORY".
> 
> So considering how I couldn't remember this mess myself, despite
> having been involved with it personally (a decade ago..), I really do
> think that maybe this shoudl be open-coded with a comment, and the
> above code should instead be
> 
>         if (flags & __O_TMPFILE) {
>                 if (!(flags & O_DIRECTORY))
>                         return -EINVAL;
> 
> together with an explicit comment about how O_TMPFILE is the
> *combination* of __O_TMPFILE and O_DIRECTORY, along with a short
> explanation as to why.
> 
> Now, I agree that that test for O_DIRECTORY then _looks_ odd, but the
> thing is, it then makes the reality of this all much more explicit.
> 
> In contrast, doing that
> 
>                 if ((flags & O_TMPFILE) != O_TMPFILE)
> 
> may *look* more natural in that context, but if you actually start
> thinking about it, that check makes no sense unless you then look up
> what O_TMPFILE is, and the history behind it.
> 
> So I'd rather have code that looks a bit odd, but that explains itself
> and is explicit about what it does, than code that _tries_ to look
> natural but actually hides the reason for what it is doing.
> 
> And then next time somebody looks at that O_DIRECTORY | O_CREAT
> combination, suddenly the __O_TMPFILE interaction is there, and very
> explicit.
> 
> Hmm?
> 
> I don't feel *hugely* strongly about this, so in the end I'll bow to
> your decision, but considering that my initial patch looked sane but
> was buggy because I had forgotten about O_TMPFILE, I really think we
> should make this more explicit at a source level..

I don't feel strongly about this either and your points are valid. So I
incorporated that and updated the comments in the code. In case you'd like to
take another look I've now put this up at:

The following changes since commit e8d018dd0257f744ca50a729e3d042cf2ec9da65:

  Linux 6.3-rc3 (2023-03-19 13:27:55 -0700)

are available in the Git repository at:

  ssh://git@...olite.kernel.org/pub/scm/linux/kernel/git/vfs/idmapping.git tags/vfs.open.directory.creat.einval

for you to fetch changes up to 43b450632676fb60e9faeddff285d9fac94a4f58:

  open: return EINVAL for O_DIRECTORY | O_CREAT (2023-03-22 11:06:55 +0100)

----------------------------------------------------------------
vfs.open.directory.creat.einval

----------------------------------------------------------------
Christian Brauner (1):
      open: return EINVAL for O_DIRECTORY | O_CREAT

 fs/open.c                              | 18 +++++++++++++-----
 include/uapi/asm-generic/fcntl.h       |  1 -
 tools/include/uapi/asm-generic/fcntl.h |  1 -
 3 files changed, 13 insertions(+), 7 deletions(-)