[PATCH libXi] Fix bus error on MIPS N32 for bug #38331.

[Mark Kettenis]

> From: =?UTF-8?q?Micha=C5=82=20Mas=C5=82owski?= <mtjm at mtjm.eu>
> Date: Mon, 20 Feb 2012 20:09:30 +0100
> 
> XIValuatorClassInfo might have an address of 4 bytes modulo 8, while
> it contains a double which needs 8Â byte alignment.  This is fixed by
> adding extra padding before instances of this structure in
> size_classes and copy_classes.
> 
> Padding added by other commits probably prevents unaligned accesses on
> all architectures with 8Â byte longs and 8Â byte double alignment
> requirement, so this error occurs only on ABIs with 4Â byte longs and
> 8Â byte required double alignment.  I don't know any other such ABI
> than MIPSÂ N32, so the change uses conditionals on ABI-specific macro.

This is issue is by no means MIPS N32-specific.  Most 32-bit
architectures will suffer from the same issue.  A very similar issue
popped up in the xserver/test/xinput.c, where we eventually settled
for assuming double-alignment on all platforms except __i386__ and
__sh__.

It probably is best to use "sizeof(double)" instead of "8" since that
is a bit more xpressive.

> Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=38331
> Signed-off-by: Michał Masłowski <mtjm at mtjm.eu>
> ---
>  src/XExtInt.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/src/XExtInt.c b/src/XExtInt.c
> index b12886d..297b95c 100644
> --- a/src/XExtInt.c
> +++ b/src/XExtInt.c
> @@ -1495,6 +1495,11 @@ size_classes(xXIAnyInfo* from, int nclasses)
>                          ((xXIKeyInfo*)any_wire)->num_keycodes);
>                  break;
>              case XIValuatorClass:
> +#ifdef _ABIN32
> +              /* Avoid a bus error on MIPS N32. */
> +              if (len % 8 != 0)
> +                len += 8 - len % 8;
> +#endif
>                  l = sizeDeviceClassType(XIValuatorClass, 0);
>                  break;
>              case XIScrollClass:
> @@ -1598,6 +1603,14 @@ copy_classes(XIDeviceInfo* to, xXIAnyInfo* from, int *nclasses)
>                      XIValuatorClassInfo *cls_lib;
>                      xXIValuatorInfo *cls_wire;
>  
> +#ifdef _ABIN32
> +                    /* On MIPS n32 doubles must be 8Â byte aligned, but
> +                       longs take 4Â bytes, so ptr_lib might be
> +                       unaligned, causing a bus error when setting
> +                       cls_lib->min. */
> +                    if (((size_t) ptr_lib) % 8 != 0)
> +                      *(unsigned char**) &ptr_lib += 8 - ((size_t) ptr_lib) % 8;
> +#endif
>                      cls_lib = next_block(&ptr_lib, sizeof(XIValuatorClassInfo));
>                      cls_wire = (xXIValuatorInfo*)any_wire;
>  
> -- 
> 1.7.9.1
> 
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