Re: [Qemu-devel] [PATCH] target-sparc: fix 32-bit truncation in fpackfix, avoid undefined behavior

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 06/11/15 15:50, Paolo Bonzini wrote:

> This is reported by Coverity.  The algorithm description at
> ftp://ftp.icm.edu.pl/packages/ggi/doc/hw/sparc/Sparc.pdf suggests
> that the 32-bit parts of rs2, after the left shift, is treated
> as a 64-bit integer.  Bits 32 and above are used to do the
> saturating truncation.
> 
> Use a cast to unsigned in order to placate ubsan for negative
> inputs, and do so in the other fpack* instructions too.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  target-sparc/vis_helper.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/target-sparc/vis_helper.c b/target-sparc/vis_helper.c
> index 383cc8b..a1c3343 100644
> --- a/target-sparc/vis_helper.c
> +++ b/target-sparc/vis_helper.c
> @@ -404,7 +404,8 @@ uint32_t helper_fpack16(uint64_t gsr, uint64_t rs2)
>      for (byte = 0; byte < 4; byte++) {
>          uint32_t val;
>          int16_t src = rs2 >> (byte * 16);
> -        int32_t scaled = src << scale;
> +        /* The cast to unsigned avoids undefined behavior for negative src.  */
> +        int32_t scaled = (uint32_t)src << scale;
>          int32_t from_fixed = scaled >> 7;
>  
>          val = (from_fixed < 0 ?  0 :
> @@ -426,7 +427,8 @@ uint64_t helper_fpack32(uint64_t gsr, uint64_t rs1, uint64_t rs2)
>      for (word = 0; word < 2; word++) {
>          uint64_t val;
>          int32_t src = rs2 >> (word * 32);
> -        int64_t scaled = (int64_t)src << scale;
> +        /* The cast to unsigned avoids undefined behavior for negative src.  */
> +        int64_t scaled = (uint64_t)src << scale;
>          int64_t from_fixed = scaled >> 23;
>  
>          val = (from_fixed < 0 ? 0 :
> @@ -447,7 +449,8 @@ uint32_t helper_fpackfix(uint64_t gsr, uint64_t rs2)
>      for (word = 0; word < 2; word++) {
>          uint32_t val;
>          int32_t src = rs2 >> (word * 32);
> -        int64_t scaled = src << scale;
> +        /* The cast to unsigned avoids undefined behavior for negative src.  */
> +        int64_t scaled = (uint64_t)src << scale;
>          int64_t from_fixed = scaled >> 16;
>  
>          val = (from_fixed < -32768 ? -32768 :

Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>


ATB,

Mark.