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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 18 Jul 2022 14:21:16 +0200
Message-ID: <CAJfpeguDAJpLMABsomBFQ=w6Li0=sBW0bFyALv4EJrAmR2BkpQ@mail.gmail.com>
Subject: Re: [PATCH] ovl: Handle ENOSYS when fileattr support is missing in
 lower/upper fs
To:     =?UTF-8?Q?Christian_Kohlsch=C3=BCtter?= 
        <christian@kohlschutter.com>
Cc:     overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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On Mon, 18 Jul 2022 at 12:56, Christian Kohlsch=C3=BCtter
<christian@kohlschutter.com> wrote:

> However, users of fuse that have no business with overlayfs suddenly see =
their ioctl return ENOTTY instead of ENOSYS.

And returning ENOTTY is the correct behavior.  See this comment in
<asm-generic/errrno.h>:

/*
 * This error code is special: arch syscall entry code will return
 * -ENOSYS if users try to call a syscall that doesn't exist.  To keep
 * failures of syscalls that really do exist distinguishable from
 * failures due to attempts to use a nonexistent syscall, syscall
 * implementations should refrain from returning -ENOSYS.
 */
#define ENOSYS 38 /* Invalid system call number */

Thanks,
Miklos