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

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Message-ID: <20200507224955.GA22343@embeddedor>
Date:   Thu, 7 May 2020 17:49:55 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Richard Guy Briggs <rgb@...hat.com>
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 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,

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

> 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;
> > 
> > 
> > --
> > Linux-audit mailing list
> > Linux-audit@...hat.com
> > https://www.redhat.com/mailman/listinfo/linux-audit
> 
> - 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
> 

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