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

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Message-Id: <20150818164425.76b9df40f94bbd2a57d0d518@linux-foundation.org>
Date:	Tue, 18 Aug 2015 16:44:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dongsu Park <dpark@...teo.net>
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

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: {

> +			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;

> +			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?


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.

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.


> -			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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