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

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Date:   Mon, 20 Apr 2020 10:58:40 +0200
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Stefan Metzmacher <metze@...ba.org>
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

On Sat, Apr 18, 2020 at 10:23 PM Stefan Metzmacher <metze@...ba.org> wrote:
>
> 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.

That's not a good argument.  We could have a million specialized
syscalls that all do very useful things.  Except it would be a
nightmare in terms of maintenance...

"do one thing and do it well"

> 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.

The io-uring guys are working on that problem, AFAIK.

Thanks,
Miklos