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

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Date:	Thu, 12 Nov 2009 15:57:14 +0100 (CET)
From:	Julia Lawall <julia@...u.dk>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	James Morris <jmorris@...ei.org>,
	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

On Thu, 12 Nov 2009, Serge E. Hallyn wrote:

> 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).

If strcmp is what is wanted, I can make a patch for that.

julia
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