Re: [PATCH] Fix PTRACE_GETREGSET failure for compat inferiors on arm64

[Yao Qi <qiyaoltc@gmail.com>]

On 16-12-02 13:46:13, Kees Cook wrote:
> When running a 32-bit ARM inferior on a 64-bit ARM host, only the hardware
> floating-point registers (NT_ARM_VFP) are available. If the inferior
> uses hard-float, do not request soft-float registers (NT_PRFPREG) and
> run the risk of failing with EINVAL. This is most noticeably exposed

"soft-float" is not accurate.  FPA is coprocessor.  Both VFP and FPA is
implemented in the combination of software and hardware.  I'd like to
rewrite the commit log like this,

"When running a 32-bit ARM inferior with a 32-bit ARM GDB on 64-bit
AArch64 host, only VFP registers (NT_ARM_VFP) are available.  The FPA
registers (NT_PRFPREG) is not available."

> when running "generate-core-file":
> 
> (gdb) generate-core-file myprog.core
> Unable to fetch the floating point registers.: Invalid argument.
> 
> ptrace(PTRACE_GETREGSET, 27642, NT_FPREGSET, 0xffcc67f0) = -1 EINVAL (Invalid argument)
> 
> gdb/ChangeLog:
> 
> 2016-12-02  Kees Cook  <keescook@google.com>

You don't have FSF copyright assignment.

> 
> 	* gdb/arm-linux-nat.c: Skip soft-float registers when using hard-float.
> 
> ---
>  gdb/arm-linux-nat.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/gdb/arm-linux-nat.c b/gdb/arm-linux-nat.c
> index d11bdc6..2126cd7 100644
> --- a/gdb/arm-linux-nat.c
> +++ b/gdb/arm-linux-nat.c
> @@ -384,17 +384,19 @@ arm_linux_fetch_inferior_registers (struct target_ops *ops,
>    if (-1 == regno)
>      {
>        fetch_regs (regcache);
> -      fetch_fpregs (regcache);

We should only call fetch_fpregs if tdep->have_fpa_registers is true.

>        if (tdep->have_wmmx_registers)
>  	fetch_wmmx_regs (regcache);
>        if (tdep->vfp_register_count > 0)
>  	fetch_vfp_regs (regcache);
> +      else
> +	fetch_fpregs (regcache);
>      }
> -  else 
> +  else
>      {
>        if (regno < ARM_F0_REGNUM || regno == ARM_PS_REGNUM)
>  	fetch_regs (regcache);
> -      else if (regno >= ARM_F0_REGNUM && regno <= ARM_FPS_REGNUM)
> +      else if (tdep->vfp_register_count == 0
> +	       && regno >= ARM_F0_REGNUM && regno <= ARM_FPS_REGNUM)
>  	fetch_fpregs (regcache);

Do we really need this change?  If FPA registers are not available,
REGNO can't fall in this range (ARM_F0_REGNUM, ARM_FPS_REGNUM).

These two comments above also apply to store registers.

-- 
Yao