Re: [PATCH] mm/secretmem: fix panic when growing a memfd_secret

[Axel Rasmussen <axelrasmussen@google.com>]

On Thu, Mar 24, 2022 at 2:33 PM Matthew Wilcox <willy@infradead.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...