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    SubjectRe: [PATCH V7 03/20] compat: consolidate the compat_flock{,64} definition
    On Mon, Feb 28, 2022 at 2:40 PM David Laight <David.Laight@aculab.com> wrote:
    >
    > From: guoren@kernel.org
    > > Sent: 27 February 2022 16:28
    > >
    > > From: Christoph Hellwig <hch@lst.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)
    See include/asm-generic/compat.h

    typedef s64 compat_loff_t;

    Only:
    #ifdef CONFIG_COMPAT_FOR_U64_ALIGNMENT
    typedef s64 __attribute__((aligned(4))) compat_s64;

    So how do you think compat_loff_t could be 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
    >
    > -
    > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
    > Registration No: 1397386 (Wales)
    >


    --
    Best Regards
    Guo Ren

    ML: https://lore.kernel.org/linux-csky/

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