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

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Date:   Thu, 7 May 2020 17:58:13 -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 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
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).

Reviewed-by: Richard Guy Briggs <rgb@...hat.com>

> ---
>  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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