linux-kernel - Re: [PATCH 1/2] uapi: split openat2(2) definitions from fcntl.h

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Date:   Fri, 20 Dec 2019 00:45:25 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Shuah Khan <shuah@...nel.org>,
        David Laight <david.laight@...lab.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        dev@...ncontainers.org, containers@...ts.linux-foundation.org,
        libc-alpha@...rceware.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] uapi: split openat2(2) definitions from fcntl.h

On 2019-12-19, Florian Weimer <fweimer@...hat.com> wrote:
> * Aleksa Sarai:
> 
> > diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h
> > new file mode 100644
> > index 000000000000..19ef775e8e5e
> > --- /dev/null
> > +++ b/include/uapi/linux/openat2.h
> > @@ -0,0 +1,41 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +#ifndef _UAPI_LINUX_OPENAT2_H
> > +#define _UAPI_LINUX_OPENAT2_H
> 
> I think you should include the relevant header for __align_u64
> etc. here.

Right -- no idea why I forgot to include them.

> […]
> > + * Arguments for how openat2(2) should open the target path. If @resolve is
> > + * zero, then openat2(2) operates very similarly to openat(2).
> > + *
> > + * However, unlike openat(2), unknown bits in @flags result in -EINVAL rather
> > + * than being silently ignored. @mode must be zero unless one of {O_CREAT,
> > + * O_TMPFILE} are set.
> > + *
> > + * @flags: O_* flags.
> > + * @mode: O_CREAT/O_TMPFILE file mode.
> > + * @resolve: RESOLVE_* flags.
> > + */
> > +struct open_how {
> > +	__aligned_u64 flags;
> > +	__u16 mode;
> > +	__u16 __padding[3]; /* must be zeroed */
> > +	__aligned_u64 resolve;
> > +};
> > +
> > +#define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
> > +#define OPEN_HOW_SIZE_LATEST	OPEN_HOW_SIZE_VER0
> 
> Are these really useful for the UAPI header?  Is there a situation where
> OPEN_HOW_SIZE_LATEST would be different from sizeof (struct open_how)?
> 
> The header is not compatible with the assembler anyway, so the numeric
> constant does not seem useful.

OPEN_HOW_SIZE_VER0 could conceivably be useful (in the future we may do
size-based checks) but maybe we can just expose it if someone actually
ends up needing it.

I will move them to the in-kernel header (we use them for BUILD_BUG_ONs
to make sure that the sizes are correct).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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