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Subject: Re: [PATCH v12 11/12] open: openat2(2) syscall
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAHk-=whZx97Nm-gUK0ppofj2RA2LLz2vmaDUTKSSV-+yYB9q_Q@mail.gmail.com>
Date:   Sat, 7 Sep 2019 10:42:23 -0700
Cc:     Jeff Layton <jlayton@kernel.org>, Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Christian Brauner <christian@brauner.io>,
        Eric Biederman <ebiederm@xmission.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Kees Cook <keescook@chromium.org>,
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References: <20190904201933.10736-1-cyphar@cyphar.com> <20190904201933.10736-12-cyphar@cyphar.com> <7236f382d72130f2afbbe8940e72cc67e5c6dce0.camel@kernel.org> <CAHk-=whZx97Nm-gUK0ppofj2RA2LLz2vmaDUTKSSV-+yYB9q_Q@mail.gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
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> On Sep 7, 2019, at 9:58 AM, Linus Torvalds <torvalds@linux-foundation.org>=
 wrote:
>=20
>> On Sat, Sep 7, 2019 at 5:40 AM Jeff Layton <jlayton@kernel.org> wrote:
>>=20
>> After thinking about this a bit, I wonder if we might be better served
>> with a new set of OA2_* flags instead of repurposing the O_* flags?
>=20
> I'd hate to have yet _another_ set of translation functions, and
> another chance of people just getting it wrong either in user space or
> the kernel.
>=20
> So no. Let's not make another set of flags that has no sane way to
> have type-safety to avoid more confusion.
>=20
> The new flags that _only_ work with openat2() might be named with a
> prefix/suffix to mark that, but I'm not sure it's a huge deal.
>=20
>           =20

I agree with the philosophy, but I think it doesn=E2=80=99t apply in this ca=
se.  Here are the flags:

O_RDONLY, O_WRONLY, O_RDWR: not even a proper bitmask. The kernel already ha=
s the FMODE_ bits to make this make sense. How about we make the openat2 per=
mission bits consistent with the internal representation and let the O_ perm=
ission bits remain as an awful translation.  The kernel already translates l=
ike this, and it already sucks.

O_CREAT, O_TMPFILE, O_NOCTTY, O_TRUNC: not modes on the fd at all.  These af=
fect the meaning of open().  Heck, for openat2, NOCTTY should be this defaul=
t.

O_EXCL: hopelessly overloaded.

O_APPEND, O_DIRECT, O_SYNC, O_DSYNC, O_LARGEFILE, O_NOATIME, O_PATH, O_NONBL=
OCK: genuine mode bits

O_CLOEXEC: special because it affects the fd, not the struct file.

Linus, you rejected resolveat() because you wanted a *nice* API that people w=
ould use and that might even be adopted by other OSes. Let=E2=80=99s please n=
ot make openat2() be a giant pile of crap in the name of consistency with op=
en().  open(), frankly, is horrible.