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SubjectRe: RFD: x32 ABI system call numbers
Date
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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