Re: [PATCH V7 03/20] compat: consolidate the compat_flock{,64} definition

[Guo Ren <guoren@kernel.org>]

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/