linux-kernel - Re: RFD: x32 ABI system call numbers

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Date:	Fri, 2 Sep 2011 09:42:06 +0100
From:	Pedro Alves <pedro@...esourcery.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Avi Kivity <avi@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"H.J. Lu" <hjl.tools@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Kuo <rkuo@...eaurora.org>,
	Mark Salter <msalter@...hat.com>,
	Jonas Bonn <jonas@...thpole.se>,
	Tobias Klauser <tklauser@...tanz.ch>
Subject: Re: RFD: x32 ABI system call numbers

On Friday 02 September 2011 02:51:35, H. Peter Anvin wrote:
> On 09/01/2011 05:49 PM, Pedro Alves wrote:
> >>>
> >>>       struct iovec
> >>>       {
> >>>           void __user *iov_base;    /* BSD uses caddr_t (1003.1g requires
> >>> void *) */
> >>>           __kernel_size_t iov_len; /* Must be size_t (1003.1g) */
> >>>       } __attribute__((x32_abi_64));
> >>>
> >>>       typedef long time_t __attribute__((x32_abi_64));
> >>>
> >>> The x32_abi_64 attribute converts pointers and longs back to 64-bit and
> >>> adjusts the alignment accordingly.  If we tag all userspace visible
> >>> structures with this attribute, we can use the 64-bit ABI without changes.
> >
> > I would expect no new gcc extension to be needed for that -- there's the
> > mode attribute (you can read DI as 64-bit):
> >
> >   typedef void * __kernel_ptr64 __attribute ((mode(DI)));
> >
> >   struct iovec
> >   {
> >     __kernel_ptr64 iov_base;
> >     ...
> >   };
> >
> 
> Does that work for *writing*, too?  That might be a very useful little 
> escape hatch for some particularly tight corners.

With a gcc trunk from earlier this month (that supports -mx32):

typedef void * ptr64 __attribute ((mode(DI)));

struct foo
{
  ptr64 p64;
};

int
foofunc (void)
{
  struct foo foo;
  int i;
  void *p32;
  ptr64 p64;

  p32 = &i;
  p32 = &foofunc;

  p64 = p32;
  p64 = &i;
  p64 = foofunc;

  foo.p64 = p32;

  void **ptrp64 = &p64;  /* gives "warning: initialization from incompatible pointer type [enabled by default]". */

  return (int) (void*) ptrp64;
}

With:

./cc1 ~/mode.c -o mode.o -mx32 -fverbose-asm

gives me:

/home/pedro/mode.c: In function ‘foofunc’:
/home/pedro/mode.c:25:19: warning: initialization from incompatible pointer type [enabled by default]

and produced (note, -O0):

foofunc:
.LFB0:
        .cfi_startproc
        pushq   %rbp    #
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp      #,
        .cfi_def_cfa_register 6
        leaq    -20(%rbp), %rax #, tmp61
        movl    %eax, -4(%rbp)  # tmp61, p32
        movl    $foofunc, -4(%rbp)      #, p32
        mov     -4(%rbp), %eax  # p32,
        movq    %rax, -32(%rbp) #, p64
        leaq    -20(%rbp), %rax #, tmp62
        movq    %rax, -32(%rbp) # tmp62, p64
        movq    $foofunc, -32(%rbp)     #, p64
        mov     -4(%rbp), %eax  # p32, tmp63
        movq    %rax, -16(%rbp) # tmp63, foo.p64
        leaq    -32(%rbp), %rax #, tmp64
        movl    %eax, -8(%rbp)  # tmp64, ptrp64
        movl    -8(%rbp), %eax  # ptrp64, D.2693
        popq    %rbp    #
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc

-- 
Pedro Alves
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