Re: [PATCH] devpts: If initialization failed, don't crash when opening /dev/ptmx

josh
2015-05-08T00:37:57Z

On Thu, May 07, 2015 at 03:59:19PM -0700, Andrew Morton wrote:
> On Wed, 6 May 2015 17:35:47 -0700 Josh Triplett <josh@joshtriplett.org> wrote:
>
> > If devpts failed to initialize, it would store an ERR_PTR in the global
> > devpts_mnt. A subsequent open of /dev/ptmx would call devpts_new_index,
> > which would dereference devpts_mnt and crash.
> >
> > Avoid storing invalid values in devpts_mnt; leave it NULL instead.
> > Make both devpts_new_index and devpts_pty_new fail gracefully with
> > ENODEV in that case, which then becomes the return value to the
> > userspace open call on /dev/ptmx.
>
> It looks like the system is pretty crippled if init_devptr_fs() fails.
> Can the user actually get access to consoles and do useful things in
> this situation? Maybe it would be better to just give up and panic?

Mounting devpts doesn't work without it, but you don't *need* to do that
to run a viable system. A full-featured terminal might be unhappy.
init=/bin/sh works, and a console login doesn't strictly require
/dev/pts. A substantial initramfs or rescue system should work without
/dev/pts mounted.

I think this falls under Linus's comments elsewhere about BUG versus
WARN. The system can continue and will function to some degree.
panic() is more suitable for "if I even return from this function,
horrible things will start happening". With this patch, all the
functions provided by devpts gracefully fail if devpts did, so I don't
see a good reason to panic().

> > @@ -676,12 +689,15 @@ static int __init init_devpts_fs(void)
> > struct ctl_table_header *table;
> >
> > if (!err) {
> > + static struct vfsmount *mnt;
>
> static is weird. I assume this was a braino?

Copy/paste issue, yes. Fixed in v2.

- Josh Triplett