Re: [PATCH] vfs: relax linkat() AT_EMPTY_PATH - aka flink() - requirements

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Charles Mirabile <cmirabil@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>,
	 linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	 Andrew Lutomirski <luto@kernel.org>, Peter Anvin <hpa@zytor.com>
Subject: Re: [PATCH] vfs: relax linkat() AT_EMPTY_PATH - aka flink() - requirements
Date: Thu, 11 Apr 2024 11:13:53 -0700	[thread overview]
Message-ID: <CAHk-=wjuzUTH0ZiPe0dAZ4rcVeNoJxhK8Hh_WRBY-ZqM-pGBqg@mail.gmail.com> (raw)
In-Reply-To: <CABe3_aE_quPE0zKe-p11DF1rBx-+ecJKORY=96WyJ_b+dbxL9A@mail.gmail.com>

On Thu, 11 Apr 2024 at 10:35, Charles Mirabile <cmirabil@redhat.com> wrote:
>
> And a slightly dubious addition to bypass these checks for tmpfiles
> across the board.

Does this make sense?

I 100% agree that one of the primary reasons why people want flink()
is that "open tmpfile, finalize contents and permissions, then link
the final result into the filesystem".

But I would expect that the "same credentials as open" check is the
one that really matters.

And __O_TMPFILE is just a special case that might not even be used -
it's entirely possible to just do the same with a real file (ie
non-O_TMPFILE) and link it in place and remove the original.

Not to mention that ->tmpfile() isn't necessarily even available, so
the whole concept of "use O_TMPFILE and then linkat" is actually
broken. It *has* to be able to fall back to a regular file to work at
all on NFS.

So while I understand your motivation, I actually think it's actively
wrong to special-case __O_TMPFILE, because it encourages a pattern
that is bad.

                    Linus