linux-kernel - Why is the bit size different between a syscall and its wrapper?

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Date:   Fri, 12 Mar 2021 11:48:11 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org
Subject: Why is the bit size different between a syscall and its wrapper?

Hi.

I think I am missing something, but
is there any particular reason to
use a different bit size between
a syscall and its userspace wrapper?

For example, for the unshare syscall,

unshare(2) says the parameter is int.

SYNOPSIS
       #define _GNU_SOURCE
       #include <sched.h>

       int unshare(int flags);

In the kernel, it is unsigned long.

SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
{
        return ksys_unshare(unshare_flags);
}

I guess the upper 32-bit will be
zeroed out in the c library when
sizeof(int) != sizeof(unsigned long)
(i.e. 64-bit system), but I'd like to know
why we do it this way.

-- 
Best Regards
Masahiro Yamada