Re: A little bit wondering about how a syscall works

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.

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
> > >
>