linux-kernel - RE: [PATCH V7 03/20] compat: consolidate the compat

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Message-ID: <b8e765910e274c0fb574ff23f88b881c@AcuMS.aculab.com>
Date:   Mon, 28 Feb 2022 06:40:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'guoren@...nel.org'" <guoren@...nel.org>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "anup@...infault.org" <anup@...infault.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "liush@...winnertech.com" <liush@...winnertech.com>,
        "wefu@...hat.com" <wefu@...hat.com>,
        "drew@...gleboard.org" <drew@...gleboard.org>,
        "wangjunqiang@...as.ac.cn" <wangjunqiang@...as.ac.cn>,
        "hch@....de" <hch@....de>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH V7 03/20] compat: consolidate the compat_flock{,64}
 definition

From: guoren@...nel.org
> Sent: 27 February 2022 16:28
> 
> From: Christoph Hellwig <hch@....de>
> 
> Provide a single common definition for the compat_flock and
> compat_flock64 structures using the same tricks as for the native
> variants.  Another extra define is added for the packing required on
> x86.
...
> diff --git a/arch/x86/include/asm/compat.h b/arch/x86/include/asm/compat.h
...
>  /*
> - * IA32 uses 4 byte alignment for 64 bit quantities,
> - * so we need to pack this structure.
> + * IA32 uses 4 byte alignment for 64 bit quantities, so we need to pack the
> + * compat flock64 structure.
>   */
...
> +#define __ARCH_NEED_COMPAT_FLOCK64_PACKED
> 
>  struct compat_statfs {
>  	int		f_type;
> diff --git a/include/linux/compat.h b/include/linux/compat.h
> index 1c758b0e0359..a0481fe6c5d5 100644
> --- a/include/linux/compat.h
> +++ b/include/linux/compat.h
> @@ -258,6 +258,37 @@ struct compat_rlimit {
>  	compat_ulong_t	rlim_max;
>  };
> 
> +#ifdef __ARCH_NEED_COMPAT_FLOCK64_PACKED
> +#define __ARCH_COMPAT_FLOCK64_PACK	__attribute__((packed))
> +#else
> +#define __ARCH_COMPAT_FLOCK64_PACK
> +#endif
...
> +struct compat_flock64 {
> +	short		l_type;
> +	short		l_whence;
> +	compat_loff_t	l_start;
> +	compat_loff_t	l_len;
> +	compat_pid_t	l_pid;
> +#ifdef __ARCH_COMPAT_FLOCK64_PAD
> +	__ARCH_COMPAT_FLOCK64_PAD
> +#endif
> +} __ARCH_COMPAT_FLOCK64_PACK;
> +

Provided compat_loff_t are correctly defined with __aligned__(4)
marking the structure packed isn't needed.
I believe compat_u64 and compat_s64 both have aligned(4).
It is also wrong, consider:

struct foo {
	char x;
	struct compat_flock64 fl64;
};

There should be 3 bytes of padding after 'x'.
But you've removed it.

	David

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