linux-kernel - Re: [PATCH] audit: Replace zero-length array with flexible-array

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Date:   Fri, 8 May 2020 07:17:00 -0400
From:   Richard Guy Briggs <rgb@...hat.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Paul Moore <paul@...l-moore.com>, Eric Paris <eparis@...hat.com>,
        linux-audit@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] audit: Replace zero-length array with flexible-array

On 2020-05-07 17:49, Gustavo A. R. Silva wrote:
> On Thu, May 07, 2020 at 05:58:13PM -0400, Richard Guy Briggs wrote:
> > On 2020-05-07 13:50, Gustavo A. R. Silva wrote:
> > > The current codebase makes use of the zero-length array language
> > > extension to the C90 standard, but the preferred mechanism to declare
> > > variable-length types such as these ones is a flexible array member[1][2],
> > > introduced in C99:
> > > 
> > > struct foo {
> > >         int stuff;
> > >         struct boo array[];
> > > };
> > > 
> > > By making use of the mechanism above, we will get a compiler warning
> > > in case the flexible array does not occur last in the structure, which
> > > will help us prevent some kind of undefined behavior bugs from being
> > > inadvertently introduced[3] to the codebase from now on.
> > > 
> > > Also, notice that, dynamic memory allocations won't be affected by
> > > this change:
> > > 
> > > "Flexible array members have incomplete type, and so the sizeof operator
> > > may not be applied. As a quirk of the original implementation of
> > > zero-length arrays, sizeof evaluates to zero."[1]
> > > 
> > > sizeof(flexible-array-member) triggers a warning because flexible array
> > > members have incomplete type[1]. There are some instances of code in
> > > which the sizeof operator is being incorrectly/erroneously applied to
> > > zero-length arrays and the result is zero. Such instances may be hiding
> > > some bugs. So, this work (flexible-array member conversions) will also
> > > help to get completely rid of those sorts of issues.
> > > 
> > > This issue was found with the help of Coccinelle.
> > > 
> > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > > [2] https://github.com/KSPP/linux/issues/21
> > > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > > 
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > 
> > Sounds reasonable to me.  There's another in include/uapi/linux/audit.h
> 
> Hi,

Hello Gustavo,

> I wouldn't advise to make any of these conversions in include/uapi/
> [1][2].

Ah-hah.  Thanks for the contra-indicating supporting references.

> > in struct audit_rule_data buf[0].  This alert also helped me fix another
> > one in a patchset I'm about to post (and will probably cause a merge
> > conflict but we can figure that out).
> 
> Awesome. :)
> 
> > Reviewed-by: Richard Guy Briggs <rgb@...hat.com>
> 
> Thanks
> --
> Gustavo
> 
> [1] https://lore.kernel.org/lkml/20200424121553.GE26002@ziepe.ca/
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1e6e9d0f4859ec698d55381ea26f4136eff3afe1
> 
> > > ---
> > >  include/linux/audit.h |    2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > > index f9ceae57ca8d..2b63aee6e9fa 100644
> > > --- a/include/linux/audit.h
> > > +++ b/include/linux/audit.h
> > > @@ -19,7 +19,7 @@
> > >  struct audit_sig_info {
> > >  	uid_t		uid;
> > >  	pid_t		pid;
> > > -	char		ctx[0];
> > > +	char		ctx[];
> > >  };
> > >  
> > >  struct audit_buffer;
> > 
> > - RGB

- RGB

--
Richard Guy Briggs <rgb@...hat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635