svn commit: r350315 - in head/sys: kern sys

[Konstantin Belousov]

On Sat, Jul 27, 2019 at 01:35:06AM +0000, Rick Macklem wrote:
> Konstantin Belousov wrote:
> >On Thu, Jul 25, 2019 at 05:46:17AM +0000, Rick Macklem wrote:
> >> Author: rmacklem
> >> Date: Thu Jul 25 05:46:16 2019
> >> New Revision: 350315
> >> URL: https://svnweb.freebsd.org/changeset/base/350315
> >>
> >> Log:
> >>   Add kernel support for a Linux compatible copy_file_range(2) syscall.
> >
> >> Modified: head/sys/kern/syscalls.master
> >> >=====================================================================>=========
> >> --- head/sys/kern/syscalls.master     Thu Jul 25 03:55:05 2019        (r350314)
> >> +++ head/sys/kern/syscalls.master     Thu Jul 25 05:46:16 2019        (r350315)
> >> @@ -3175,6 +3175,16 @@
> >>                   int flag
> >>               );
> >>       }
> >> +569  AUE_NULL        STD {
> >> +             ssize_t copy_file_range(
> >> +                 int infd,
> >> +                 _Inout_opt_ off_t *inoffp,
> >> +                 int outfd,
> >> +                 _Inout_opt_ off_t *outoffp,
> >> +                 size_t len,
> >> +                 unsigned int flags
> >> +             );
> >> +     }
> >
> >I sat to write the compat32 shims, and only then noted that len has size_t
> >type.  Why is it size_t and not off_t ?
> Well, that's what Linux did.
> 
> Also, since it returns ssize_t, it can't do more than SSIZE_MAX
> (generally 1/2 of SIZE_T_MAX). Returning ssize_t is also what Linux
> does and is consistent with read(2)/write(2).

If changing the length argument type to off_t, it is reasonable to change
the return type to off_t as well.  We already have the lseek(2) syscall that
requires two return registers on 32bit.

Note that it is reasonable for read(2) to take length as size_t-typed
parameter, because size_t is the type for object sizes. There is no
object in user address space for copy_file_range(2) API, so potentially
wider off_t is acceptable and is in fact useful there. It is useful on
32bit machines where FreeBSD size_t is 32bit, while off_t is 64bit.