Re: A little bit wondering about how a syscall works

On 2/1/24 00:47, Lin Lee wrote:
> Hi,
> 
> So, if I understand correctly,
> 
> Each thread's sv_fetch_syscall_args hook function is initialized as 
> cpu_fetch_syscall_args(),
> 
> And when it enter syscallenter, it first use `error = 
> (p->p_sysent->sv_fetch_syscall_args)(td);` to read the system call 
> number, then use `error = (se->sy_call)(td, sa->args)` to execute the 
> system call.
> 
> Do I understand corrected?
> 

That's right.

> Thank you very much.
> 
> Best Regards,
> Lin Lee
> On Feb 1, 2024 at 12:27 AM +0800, Mitchell Horne <mhorne@freebsd.org>, 
> wrote:
>> On 1/31/24 01:03, Lin Lee wrote:
>>> Hello Mitchell,
>>>
>>> Thank you for your kindly responding.
>>>
>>> Now I have still a question, when does the function
>>> cpu_fetch_syscall_args be called?
>>>
>>> As the previous letter mentions, I traced the code and entered the
>>> elf_machdep.c.
>>>
>>> I have no idea if there are something to do between elf_machdep.c and
>>> system calll.
>>>
>>
>> The short answer is yes, it is related. In syscallenter() we have:
>>
>> error = (p->p_sysent->sv_fetch_syscall_args)(td);
>>
>> And as you saw, the sv_fetch_syscall_args hook is set to 
>> cpu_fetch_syscall_args() for elf64_freebsd_sysvec. Similarly, there is 
>> an sv_set_syscall_retval hook, called by syscallret() when we are done 
>> executing the system call.
>>

One correction: the sv_set_syscall_retval hook is actually called at the 
very end of syscallenter(), after the execution of the syscall has 
completed.

>> Each process 'p' has a corresponding sysentvec (p_sysent). On the 
>> riscv architecture there is currently only one registered systentvec, 
>> elf64_freebsd_sysvec, because we can only execute 64-bit FreeBSD ELF 
>> binaries on this platform.
>>
>> By contrast, on amd64 there are several registered sysentvecs. This 
>> allows it to execute, for example, 32-bit FreeBSD ELF binaries, or 
>> 64-bit Linux ELF binaries. The sysentvec enables different handling 
>> for these different types of executables, e.g. the system call table 
>> is different for Linux processes (.sv_table = linux_sysent).
>>
>> You will see also that Linux processes have a different function for 
>> sv_fetch_syscall_args, take a look in sys/amd64/linux/linux_sysvec.c.
>>
>> Mitchell
>>
>>> If not, when(how) the cpu_fetch_syscall_args is called?
>>>
>>> Thank you very much.
>>>
>>> Best Regards,
>>> Lin Lee
>>> On Jan 31, 2024 at 1:17 AM +0800, Mitchell Horne <mhorne@freebsd.org>,
>>> wrote:
>>>>
>>>> Mitchell
>>