Re: A little bit wondering about how a syscall works

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?

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.
>
> 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
>