Re: [PATCH v7 3/8] Use xml-syscall to compare syscall numbers in arm_linux_sigreturn_return-addr.

[Pedro Alves <palves@redhat.com>]

On 12/11/2015 01:04 PM, Antoine Tremblay wrote:
> 
> 
> On 12/11/2015 07:47 AM, Pedro Alves wrote:
>> On 12/11/2015 12:31 PM, Antoine Tremblay wrote:
>>>
>>>
>>> On 12/11/2015 07:05 AM, Pedro Alves wrote:
>>>> On 12/11/2015 12:02 PM, Yao Qi wrote:
>>>>> On 11/12/15 11:59, Pedro Alves wrote:
>>>>>> but then you might as well just do:
>>>>>>
>>>>>>     #define ARM_SIGRETURN 119
>>>>>
>>>>> How about using __NR_sigreturn and __NR_rt_sigreturn directly? like
>>>>> what patch #6 does.
>>>>
>>>> Those are host/native macros.  Only make sense for the running host.
>>>> Can't use that in a tdep file, like arm-linux-tdep.c.  Otherwise, e.g.,
>>>> a x86-hosted cross debugger would be using the x86 __NR_sigreturn, etc.
>>>>
>>>
>>> Exactly that's why I use those only in GDBServer.
>>
>> Hmm, actually, that sounds wrong.
>>
>> What about Aarch64 gdbserver debugging Aarch32?
>>
>> I see that the new code in question in patch #6 is in gdbserver/linux-arm-low.c,
>> which is not built on Aarch64, but, shouldn't that new arm_sigreturn_next_pc
>> code be used in the biarch scenario as well?
>>
> 
> This is only used if we need software single stepping, I think in that 
> case hardware single stepping will be used on aarch64 even if stepping 
> over an arm32 program, Yao can confirm ?

Ah, I think you're right.

> 
> 
>> If you just made that 32-bit-specific code compile on Aarch64 it would
>> be compiling against Aarch64's __NR_sigreturn, which I'd assume is wrong for Aarch32.
>>
>>>
> 
> Looking at the kernel code in linux/arch/arm64 I see :
> 
> include/asm/unistd.h:#define __NR_compat_sigreturn		119
> include/asm/unistd.h:#define __NR_compat_rt_sigreturn	173
> 
> include/asm/unistd32.h:#define __NR_sigreturn 119
> include/asm/unistd32.h:__SYSCALL(__NR_sigreturn, 
> compat_sys_sigreturn_wrapper)
> 
> include/asm/unistd32.h:#define __NR_rt_sigreturn 173
> include/asm/unistd32.h:__SYSCALL(__NR_rt_sigreturn, 
> compat_sys_rt_sigreturn_wrapper)
> 
> So I think it's ok... but I can't compile on aarch64 at the moment.

From that, it seems to be that Aarch64 64-bit doesn't even
have __NR_sigreturn.  The one we see above is in unistd32.h,
which should mean it's only included when compiling 32-bit code.
So it seems like if you tried to reference __NR_sigreturn on
Aarch64, you'd get a build failure.

Note there are Aarch64 Ubuntu machines on the gcc compile farm.

But, it's moot if we do hardware stepping.

Thanks,
Pedro Alves