Re: A little bit wondering about how a syscall works

On 2 Feb 2024, at 05:49, Lin Lee <leelin2602@gmail.com> wrote:
> 
> Hi,
> 
> But what I see in https://github.com/freebsd/freebsd-src/blob/main/sys/kern/subr_syscall.c is(after removing some condition branching):
> 
> 77 error = (p->p_sysent->sv_fetch_syscall_args)(td);
> 78 se = sa->callp;
> 156  error = (se->sy_call)(td, sa->args);
> 
> It seems that `sv_set_syscall_retval hook` is called earlier than syscall is executed.

And sv_set_syscall_retval is called on line 204 after all of that.
What’s making you think otherwise?

Jess

> Thank you so much for your replying. 
> 
> Best Regards, 
> Lin Lee
> On Feb 2, 2024 at 12:19 AM +0800, Mitchell Horne <mhorne@freebsd.org>, wrote:
>> 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
>>>> 
>>