linux-kernel - Re: [PATCH] ovl: Handle ENOSYS when fileattr support is missing in lower/upper fs

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Date:   Mon, 18 Jul 2022 14:21:16 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Christian Kohlschütter 
        <christian@...lschutter.com>
Cc:     overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ovl: Handle ENOSYS when fileattr support is missing in
 lower/upper fs

On Mon, 18 Jul 2022 at 12:56, Christian Kohlschütter
<christian@...lschutter.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