Re: [Qemu-devel] [PATCH] linux-user: correctly align types in thunking code

[Peter Maydell]

On 10 May 2018 at 21:59, Laurent Vivier <address@hidden> wrote:
> This is a follow up
> of patch:
>
>         commit c2e3dee6e03527baf8698698cce76b1a3174969a
>         Author: Laurent Vivier <address@hidden>
>         Date:   Sun Feb 13 23:37:34 2011 +0100
>
>             linux-user: Define target alignment size
>
> In my case m68k aligns "int" on 2 not 4. You can check this with the
> following program:
>
> int main(void)
> {
>         struct rtentry rt;
>         printf("rt_pad1 %ld %zd\n", offsetof(struct rtentry, rt_pad1),
>                 sizeof(rt.rt_pad1));
>         printf("rt_dst %ld %zd\n", offsetof(struct rtentry, rt_dst),
>                 sizeof(rt.rt_dst));
>         printf("rt_gateway %ld %zd\n", offsetof(struct rtentry, rt_gateway),
>                 sizeof(rt.rt_gateway));
>         printf("rt_genmask %ld %zd\n", offsetof(struct rtentry, rt_genmask),
>                 sizeof(rt.rt_genmask));
>         printf("rt_flags %ld %zd\n", offsetof(struct rtentry, rt_flags),
>                 sizeof(rt.rt_flags));
>         printf("rt_pad2 %ld %zd\n", offsetof(struct rtentry, rt_pad2),
>                 sizeof(rt.rt_pad2));
>         printf("rt_pad3 %ld %zd\n", offsetof(struct rtentry, rt_pad3),
>                 sizeof(rt.rt_pad3));
>         printf("rt_pad4 %ld %zd\n", offsetof(struct rtentry, rt_pad4),
>                 sizeof(rt.rt_pad4));
>         printf("rt_metric %ld %zd\n", offsetof(struct rtentry, rt_metric),
>                 sizeof(rt.rt_metric));
>         printf("rt_dev %ld %zd\n", offsetof(struct rtentry, rt_dev),
>                 sizeof(rt.rt_dev));
>         printf("rt_mtu %ld %zd\n", offsetof(struct rtentry, rt_mtu),
>                 sizeof(rt.rt_mtu));
>         printf("rt_window %ld %zd\n", offsetof(struct rtentry, rt_window),
>                 sizeof(rt.rt_window));
>         printf("rt_irtt %ld %zd\n", offsetof(struct rtentry, rt_irtt),
>                 sizeof(rt.rt_irtt));
> }
>
> And result is :
>
> i386
>
> rt_pad1 0 4
> rt_dst 4 16
> rt_gateway 20 16
> rt_genmask 36 16
> rt_flags 52 2
> rt_pad2 54 2
> rt_pad3 56 4
> rt_pad4 62 2
> rt_metric 64 2
> rt_dev 68 4
> rt_mtu 72 4
> rt_window 76 4
> rt_irtt 80 2
>
> m68k
>
> rt_pad1 0 4
> rt_dst 4 16
> rt_gateway 20 16
> rt_genmask 36 16
> rt_flags 52 2
> rt_pad2 54 2
> rt_pad3 56 4
> rt_pad4 62 2
> rt_metric 64 2
> rt_dev 66 4
> rt_mtu 70 4
> rt_window 74 4
> rt_irtt 78 2
>
> This affects the "route" command :
>
> WITHOUT this patch:
>
> $ sudo route add -net default gw 10.0.3.1 window 1024 irtt 2 eth0
> $ netstat -nr
> Kernel IP routing table
> Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
> 0.0.0.0         10.0.3.1        0.0.0.0         UG        0 67108866  32768 
> eth0
> 10.0.3.0        0.0.0.0         255.255.255.0   U         0 0          0 eth0
>
> WITH this patch:
>
> $ sudo route add -net default gw 10.0.3.1 window 1024 irtt 2 eth0
> $ netstat -nr
> Kernel IP routing table
> Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
> 0.0.0.0         10.0.3.1        0.0.0.0         UG        0 1024       2 eth0
> 10.0.3.0        0.0.0.0         255.255.255.0   U         0 0          0 eth0
>
> Signed-off-by: Laurent Vivier <address@hidden>
> ---
>  include/exec/user/thunk.h | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/include/exec/user/thunk.h b/include/exec/user/thunk.h
> index f19ef4b230..8f55b233b3 100644
> --- a/include/exec/user/thunk.h
> +++ b/include/exec/user/thunk.h
> @@ -149,20 +149,32 @@ static inline int thunk_type_align(const argtype 
> *type_ptr, int is_host)
>      case TYPE_CHAR:
>          return 1;
>      case TYPE_SHORT:
> -        return 2;
> +        if (is_host) {
> +            return __alignof__(short);
> +        } else {
> +            return ABI_SHORT_ALIGNMENT;
> +        }
>      case TYPE_INT:
> -        return 4;
> +        if (is_host) {
> +            return __alignof__(int);
> +        } else {
> +            return ABI_INT_ALIGNMENT;
> +        }
>      case TYPE_LONGLONG:
>      case TYPE_ULONGLONG:
> -        return 8;
> +        if (is_host) {
> +            return __alignof__(long long);
> +        } else {
> +            return ABI_LLONG_ALIGNMENT;
> +        }
>      case TYPE_LONG:
>      case TYPE_ULONG:
>      case TYPE_PTRVOID:
>      case TYPE_PTR:
>          if (is_host) {
> -            return sizeof(void *);
> +            return __alignof__(long);
>          } else {
> -            return TARGET_ABI_BITS / 8;
> +            return ABI_LONG_ALIGNMENT;
>          }
>          break;
>      case TYPE_OLDDEVT:
> --

This definitely looks like the right thing. As well as m68k,
this will also affect sh4 and i386 (where long long is 4
aligned but we were saying it should be 8 aligned) -- I wonder
if that fixes any bugs...

Reviewed-by: Peter Maydell <address@hidden>

thanks
-- PMM