Re: A little bit wondering about how a syscall works

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.

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