linux-kernel - Re: [PATCH v2] devpts: allow mounting with uid/gid of uint32

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Date:	Wed, 19 Aug 2015 09:24:31 +0200
From:	Dongsu Park <dpark@...teo.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>,
	Josh Triplett <josh@...htriplett.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Alban Crequy <alban@...ocode.com>
Subject: Re: [PATCH v2] devpts: allow mounting with uid/gid of uint32_t

Hi,

thanks for the review.

On 18.08.2015 16:44, Andrew Morton wrote:
> On Tue, 18 Aug 2015 17:18:19 +0200 Dongsu Park <dpark@...teo.net> wrote:
> 
> > To allow devpts to be mounted with options of uid/gid of uint32_t,
> > use kstrtouint() instead of match_int(). Doing that, mounting devpts
> > with uid or gid > (2^31 - 1) will work as expected, e.g.:
> > 
> >  # mount -t devpts devpts /tmp/devptsdir -o \
> >    newinstance,ptmxmode=0666,mode=620,uid=3598450688,gid=3598450693
> > 
> > It was originally by reported on systemd github issues:
> > https://github.com/systemd/systemd/issues/956
> > 
> > --- a/fs/devpts/inode.c
> > +++ b/fs/devpts/inode.c
> > @@ -188,23 +188,35 @@ static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
> >  		token = match_token(p, tokens, args);
> >  		switch (token) {
> >  		case Opt_uid:
> > -			if (match_int(&args[0], &option))
> > +		{
> 
> It might be neater to lay this out as
> 
>   		case Opt_uid: {

I'll do it.

> > +			char *uidstr = args[0].from;
> > +			uid_t uidval;
> > +			int rc = kstrtouint(uidstr, 0, &uidval);
> 
> This assumes that the architecture/config uses a uint for uid_t.  We
> have no business assuming this - it's an opaque type for a reason.  It
> would be safer to do
> 
> 	unsigned long uidl;
> 
> 	rc = kstrtoul(uidstr, 0, &uidl);
> 	uidval = uidl;

That's a good point. I'll do it.

> > +			if (rc)
> >  				return -EINVAL;
> 
> I don't get it.  From my reading, kstrtouint->parse_integer() returns
> "number of characters parsed or -E".  So this code won't work.  But
> presumably it *does* work, so why?

It's probably because kstrtouint() returns just 0 on success.
That's what functions in the call chain of kstrtouint() -> kstrtoull() ->
_kstrtoull() -> _parse_integer() are actually doing.
_parse_integer() actually returns rv, i.e. number of characters parsed.
But after that, if there's no error, _kstrtoull() simply returns 0.

> Also, we should probably return `rc' here if it's negative, to
> propagate the error which kstrtouint() detected.  That's a minor
> non-back-compatible change but it shouldn't matter.

Okay, I also think that we should return rc. I'll do it.

> otoh, kstrtouint() likes to return -ERANGE when things go wrong. 
> ERANGE means "Math result not representable", which is a nonsenscal
> error code in this context.  Sigh, why do people keep doing this.

Hmm, good to know.

Thanks,
Dongsu

> > -			uid = make_kuid(current_user_ns(), option);
> > +			uid = make_kuid(current_user_ns(), uidval);
> >  			if (!uid_valid(uid))
> >  				return -EINVAL;
> >  			opts->uid = uid;
> >  			opts->setuid = 1;
> >  			break;
> >
> > ...
> >
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