Re: [PATCH] do_open(): Fix O_DIRECTORY | O_CREAT behavior

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] do_open(): Fix O_DIRECTORY | O_CREAT behavior
Date: Mon, 20 Mar 2023 13:24:52 -0700	[thread overview]
Message-ID: <CAHk-=wgc9qYOtuyW_Tik0AqMrQJK00n-LKWvcBifLyNFUdohDw@mail.gmail.com> (raw)
In-Reply-To: <CAKbZUD2Y2F=3+jf+0dRvenNKk=SsYPxKwLuPty_5-ppBPsoUeQ@mail.gmail.com>

On Mon, Mar 20, 2023 at 12:27 PM Pedro Falcato <pedro.falcato@gmail.com> wrote:
>
> 1) Pre v5.7 Linux did the open-dir-if-exists-else-create-regular-file
> we all know and """love""".

So I think we should fall back to this as a last resort, as a "well,
it's our historical behavior".

> 2) Post 5.7, we started returning this buggy -ENOTDIR error, even when
> successfully creating a file.

Yeah, I think this is the worst of the bunch and has no excuse (unless
some crazy program has started depending on it, which sounds really
*really* unlikely).

> 3) NetBSD just straight up returns EINVAL on open(O_DIRECTORY | O_CREAT)
> 4) FreeBSD's open(O_CREAT | O_DIRECTORY) succeeds if the file exists
> and is a directory. Fails with -ENOENT if it falls onto the "O_CREAT"
> path (i.e it doesn't try to create the file at all, just ENOENT's;
> this changed relatively recently, in 2015)

Either of these sound sensible to me.

I suspect (3) is the clearest case.

And (4) might be warranted just because it's closer to what we used to
do, and it's *possible* that somebody happens to use O_DIRECTORY |
O_CREAT on directories that exist, and never noticed how broken that
was.

And (4) has another special case: O_EXCL. Because I'm really hoping
that O_DIRECTORY | O_EXCL will always fail.

Is the proper patch something along the lines of this?

   --- a/fs/open.c
   +++ b/fs/open.c
   @@ -1186,6 +1186,8 @@ inline int build_open_flags(const struct
open_how *how, struct open_flags *op)

        /* Deal with the mode. */
        if (WILL_CREATE(flags)) {
   +            if (flags & O_DIRECTORY)
   +                    return -EINVAL;
                if (how->mode & ~S_IALLUGO)
                        return -EINVAL;
                op->mode = how->mode | S_IFREG;

I dunno. Not tested, not thought about very much.

What about O_PATH? I guess it's fine to create a file and only get a
path fd to the result?

             Linus