linux-kernel - Re: [PATCH] mm/secretmem: fix panic when growing a memfd

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Date:   Thu, 24 Mar 2022 14:44:20 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] mm/secretmem: fix panic when growing a memfd_secret

On Thu, Mar 24, 2022 at 2:33 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Mar 24, 2022 at 02:09:09PM -0700, Axel Rasmussen wrote:
> > This patch avoids the panic by implementing a custom setattr for
> > memfd_secret, which detects resizes specifically (setting the size for
> > the first time works just fine, since there are no existing pages to try
> > to zero), and rejects them as not supported (ENOTSUP).
>
> Isn't ENOTTY the normal return value for this?  Or even ENOSYS?

I'm unsure.

Since errno(3) says ENOTTY means "Inappropriate I/O control operation"
that makes me think it's meant to be used only for ioctls?

I tried ENOSYS, but checkpatch warns me it's meant to be used for
"invalid syscall nr" and nothing else.

ENOTSUP / ENOTSUPP / EOPNOTSUPP all have their own share of
weirdnesses too, though. There's the whole ENOTSUP / ENOTSUPP mess,
and then also the fact that glibc says ENOTSUP == EOPNOTSUPP, whereas
POSIX says EOPNOTSUPP should be distinct and used specifically for
sockets...