linux-kernel - Re: [PATCH V2 03/17] asm-generic: fcntl: compat: Remove duplicate definitions

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Date:   Mon, 10 Jan 2022 14:35:19 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>, Arnd Bergmann <arnd@...db.de>,
        Anup Patel <anup.patel@....com>,
        gregkh <gregkh@...uxfoundation.org>,
        liush <liush@...winnertech.com>, Wei Fu <wefu@...hat.com>,
        Drew Fustini <drew@...gleboard.org>,
        Wang Junqiang <wangjunqiang@...as.ac.cn>,
        Christoph Hellwig <hch@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-csky@...r.kernel.org,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        inux-parisc@...r.kernel.org,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V2 03/17] asm-generic: fcntl: compat: Remove duplicate definitions

On Tue, Dec 28, 2021 at 3:39 PM <guoren@...nel.org> wrote:
>
> From: Guo Ren <guoren@...ux.alibaba.com>
>
> Remove duplicate F_GETLK64,F_SETLK64,F_SETLKW64 definitions in
> arch/*/include/asm/compat.h.
>
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>

Unfortunately, this one does not look correct to me:

> @@ -116,7 +116,7 @@
>  #define F_GETSIG       11      /* for sockets. */
>  #endif
>
> -#ifndef CONFIG_64BIT
> +#if !defined(CONFIG_64BIT) || defined(CONFIG_COMPAT)
>  #ifndef F_GETLK64
>  #define F_GETLK64      12      /*  using 'struct flock64' */
>  #define F_SETLK64      13

The problem here is that include/uapi/ headers cannot contain checks for
CONFIG_* symbols because those may have different meanings in user space
compared to kernel.

This is a preexisting problem in the header, but I think the change
makes it worse.

With the current behavior, user space will always see the definitions,
unless it happens to have its own definition for CONFIG_64BIT already.
On 64-bit parisc, this has the effect of defining the macros to the
same values as F_SETOWN/F_SETSIG/F_GETSIG, which is potentially
harmful. On MIPS, it uses values that are different from the 32-bit numbers
but are otherwise unused. Everywhere else, we get the definition from
the 32-bit architecture in user space, which will do nothing in the kernel.

The correct check for a uapi header would be to test for
__BITS_PER_LONG==32. We should probably do that here, but
this won't help you move the definitions, and it is a user-visible change
as the incorrect definition will no longer be visible. [Adding Jeff and Bruce
(the flock mainainers) to Cc for additional feedback on this]

For your series, I would suggest just moving the macro definitions to
include/linux/compat.h along with the 'struct compat_flock64'
definition, and leaving the duplicate one in the uapi header unchanged
until we have decided on a solution.

        Arnd