Re: [PATCH 2/2] aarch64: Set the syscall register right before doing the syscall.

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 11/04/23 13:03, Florian Weimer wrote:
> * Adhemerval Zanella Netto:
> 
>> On 11/04/23 10:50, Florian Weimer via Libc-alpha wrote:
>>> * Joe Simmons-Talbott via Libc-alpha:
>>>
>>>>    ({ long _sys_result;						\
>>>>       {								\
>>>>         LOAD_ARGS_##nr (args)					\
>>>>         register long _x8 asm ("x8") = (name);			\
>>>> +       if (__builtin_constant_p(name))				\
>>>> +         asm volatile ("mov	x8, " MSTR(name) ";"		\
>>>> +                       : /* no output */ : "i" (name) : "x8");	\
>>>>         asm volatile ("svc	0	// syscall " # name     \
>>>>  		     : "=r" (_x0) : "r"(_x8) ASM_ARGS_##nr : "memory");	\
>>>>         _sys_result = _x0;					\
>>>
>>> I think you should do this in a single assembler statement, load the
>>> constant only once.
>>
>> Is this required because compiler is free to reorganize the argument
>> list? I think it should me it clear on the commit message.
> 
> Yes, that's the reason.  It's a bit tricky to recover the system call
> number using static analysis otherwise.  I suggested to Joe that we
> should put something into glibc, rather than improving that static
> analysis tool so that it's fully reliable.

Direct syscalls are done by different projects, like sanitizer, libgomp,
etc; so imho improving the static analysis tool could potentially catch
a wide range of usages than trying to fix only on glibc.