linux-kernel - Re: [PATCH] vfs: add faccessat2 syscall

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Message-ID: <3dce8811-a54e-1f74-c7ed-715b97a4652c@samba.org>
Date:   Sat, 18 Apr 2020 22:22:40 +0200
From:   Stefan Metzmacher <metze@...ba.org>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Eric Sandeen <sandeen@...deen.net>
Subject: Re: [PATCH] vfs: add faccessat2 syscall

Am 18.04.20 um 21:00 schrieb Miklos Szeredi:
> On Sat, Apr 18, 2020 at 8:36 PM Stefan Metzmacher <metze@...ba.org> wrote:
>>
>> Hi Miklos,
>>
>>> POSIX defines faccessat() as having a fourth "flags" argument, while the
>>> linux syscall doesn't have it.  Glibc tries to emulate AT_EACCESS and
>>> AT_SYMLINK_NOFOLLOW, but AT_EACCESS emulation is broken.
>>>
>>> Add a new faccessat(2) syscall with the added flags argument and implement
>>> both flags.
>>>
>>> The value of AT_EACCESS is defined in glibc headers to be the same as
>>> AT_REMOVEDIR.  Use this value for the kernel interface as well, together
>>> with the explanatory comment.
>>
>> It would be nice if resolv_flags would also be passed in addition to the
>> at flags.
>> See:https://lore.kernel.org/linux-api/CAHk-=wiaL6zznNtCHKg6+MJuCqDxO=yVfms3qR9A0czjKuSSiA@mail.gmail.com/
>>
>> We should avoid expecting yet another syscall in near future.
> 
> What is the objection against
> 
> openat(... O_PATH)
> foobarat(fd, AT_EMPTY_PATH, ...)

openat2(), foobarat(), close() are 3 syscalls vs. just one.

As we have the new features available, I think it would be
good to expose them to userspace for all new syscalls, so
that applications can avoid boiler plate stuff around each syscall
and get better performance in a world where context switches are not for
free.

metze



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