linux-kernel - Re: [PATCH 3/4] security/selinux: decrement sizeof size in strncmp

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Date:	Thu, 12 Nov 2009 08:53:14 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	James Morris <jmorris@...ei.org>
Cc:	Julia Lawall <julia@...u.dk>, Stephen Smalley <sds@...ho.nsa.gov>,
	Eric Paris <eparis@...isplace.org>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 3/4] security/selinux: decrement sizeof size in strncmp

Quoting James Morris (jmorris@...ei.org):
> On Thu, 12 Nov 2009, Julia Lawall wrote:
> 
> > From: Julia Lawall <julia@...u.dk>
> > 
> > As observed by Joe Perches, sizeof of a constant string includes the
> > trailing 0.  If what is wanted is to check the initial characters of
> > another string, this trailing 0 should not be taken into account.  If an
> > exact match is wanted, strcmp should be used instead.
> 
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -448,7 +448,7 @@ static int sb_finish_set_opts(struct sup
> >  		sbsec->flags &= ~SE_SBLABELSUPP;
> >  
> >  	/* Special handling for sysfs. Is genfs but also has setxattr handler*/
> > -	if (strncmp(sb->s_type->name, "sysfs", sizeof("sysfs")) == 0)
> > +	if (strncmp(sb->s_type->name, "sysfs", sizeof("sysfs") - 1) == 0)
> >  		sbsec->flags |= SE_SBLABELSUPP;
> 
> Shouldn't this be a simple strcmp() ?

Yes I think so.

Julia seems to be arguing that if a module introduces a new fs with
name 'sysfs_foo' then this check should match that fs too (since
for sysfs, sb->s_type->name = "sysfs" which also has a trailing \0,
so for the regular sysfs her patch makes no practical difference).

-serge
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