linux-kernel - Re: [PATCH 4/5] uapi: always define F_GETLK64/F_SETLK64/F

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Date:   Tue, 11 Jan 2022 16:33:30 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Arnd Bergmann <arnd@...db.de>, Guo Ren <guoren@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>
Subject: Re: [PATCH 4/5] uapi: always define F_GETLK64/F_SETLK64/F_SETLKW64 in fcntl.h

On Tue, Jan 11, 2022 at 9:35 AM Christoph Hellwig <hch@....de> wrote:
>
> The fcntl F_GETLK64/F_SETLK64/F_SETLKW64 are only implemented for the
> 32-bit syscall APIs, but we also need them for compat handling on 64-bit
> builds.  Redefining them is error prone (as shown by the example that
> parisc gets it wrong currently), so we should use the same defines for
> both case.  In theory we could try to hide them from userspace, but
> given that only MIPS actually gets that right, while the asm-generic
> version used by most architectures relies on a Kconfig symbol that can't
> be relied on to be set properly by userspace is a clear indicator to not
> bother.
>
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---

> diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> index 98f4ff165b776..43d7c44031be0 100644
> --- a/include/uapi/asm-generic/fcntl.h
> +++ b/include/uapi/asm-generic/fcntl.h
> @@ -116,13 +116,11 @@
>  #define F_GETSIG       11      /* for sockets. */
>  #endif
>
> -#ifndef CONFIG_64BIT
>  #ifndef F_GETLK64
>  #define F_GETLK64      12      /*  using 'struct flock64' */
>  #define F_SETLK64      13
>  #define F_SETLKW64     14
>  #endif
> -#endif
>
>  #ifndef F_SETOWN_EX
>  #define F_SETOWN_EX    15

This is a very subtle change to the exported UAPI header contents:
On 64-bit architectures, the three unusable numbers are now always
shown, rather than depending on a user-controlled symbol.

This is probably what we want here for compatibility reasons, but I think
it should be explained in the changelog text, and I'd like Jeff or Bruce
to comment on it as well: the alternative here would be to make the
uapi definition depend on __BITS_PER_LONG==32, which is
technically the right thing to do but more a of a change.

       Arnd