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Date: Thu, 7 May 2020 16:04:33 -0400
From: Neil Horman <nhorman@...driver.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Vlad Yasevich <vyasevich@...il.com>,
Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
linux-sctp@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sctp: Replace zero-length array with flexible-array
On Thu, May 07, 2020 at 02:21:52PM -0500, 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>
> ---
> include/linux/sctp.h | 36 ++++++++++++++++++------------------
> 1 file changed, 18 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/sctp.h b/include/linux/sctp.h
> index 8ccd82105de8..76731230bbc5 100644
> --- a/include/linux/sctp.h
> +++ b/include/linux/sctp.h
> @@ -221,7 +221,7 @@ struct sctp_datahdr {
> __be16 stream;
> __be16 ssn;
> __u32 ppid;
> - __u8 payload[0];
> + __u8 payload[];
> };
>
> struct sctp_data_chunk {
> @@ -269,7 +269,7 @@ struct sctp_inithdr {
> __be16 num_outbound_streams;
> __be16 num_inbound_streams;
> __be32 initial_tsn;
> - __u8 params[0];
> + __u8 params[];
> };
>
> struct sctp_init_chunk {
> @@ -299,13 +299,13 @@ struct sctp_cookie_preserve_param {
> /* Section 3.3.2.1 Host Name Address (11) */
> struct sctp_hostname_param {
> struct sctp_paramhdr param_hdr;
> - uint8_t hostname[0];
> + uint8_t hostname[];
> };
>
> /* Section 3.3.2.1 Supported Address Types (12) */
> struct sctp_supported_addrs_param {
> struct sctp_paramhdr param_hdr;
> - __be16 types[0];
> + __be16 types[];
> };
>
> /* ADDIP Section 3.2.6 Adaptation Layer Indication */
> @@ -317,25 +317,25 @@ struct sctp_adaptation_ind_param {
> /* ADDIP Section 4.2.7 Supported Extensions Parameter */
> struct sctp_supported_ext_param {
> struct sctp_paramhdr param_hdr;
> - __u8 chunks[0];
> + __u8 chunks[];
> };
>
> /* AUTH Section 3.1 Random */
> struct sctp_random_param {
> struct sctp_paramhdr param_hdr;
> - __u8 random_val[0];
> + __u8 random_val[];
> };
>
> /* AUTH Section 3.2 Chunk List */
> struct sctp_chunks_param {
> struct sctp_paramhdr param_hdr;
> - __u8 chunks[0];
> + __u8 chunks[];
> };
>
> /* AUTH Section 3.3 HMAC Algorithm */
> struct sctp_hmac_algo_param {
> struct sctp_paramhdr param_hdr;
> - __be16 hmac_ids[0];
> + __be16 hmac_ids[];
> };
>
> /* RFC 2960. Section 3.3.3 Initiation Acknowledgement (INIT ACK) (2):
> @@ -350,7 +350,7 @@ struct sctp_initack_chunk {
> /* Section 3.3.3.1 State Cookie (7) */
> struct sctp_cookie_param {
> struct sctp_paramhdr p;
> - __u8 body[0];
> + __u8 body[];
> };
>
> /* Section 3.3.3.1 Unrecognized Parameters (8) */
> @@ -384,7 +384,7 @@ struct sctp_sackhdr {
> __be32 a_rwnd;
> __be16 num_gap_ack_blocks;
> __be16 num_dup_tsns;
> - union sctp_sack_variable variable[0];
> + union sctp_sack_variable variable[];
> };
>
> struct sctp_sack_chunk {
> @@ -436,7 +436,7 @@ struct sctp_shutdown_chunk {
> struct sctp_errhdr {
> __be16 cause;
> __be16 length;
> - __u8 variable[0];
> + __u8 variable[];
> };
>
> struct sctp_operr_chunk {
> @@ -594,7 +594,7 @@ struct sctp_fwdtsn_skip {
>
> struct sctp_fwdtsn_hdr {
> __be32 new_cum_tsn;
> - struct sctp_fwdtsn_skip skip[0];
> + struct sctp_fwdtsn_skip skip[];
> };
>
> struct sctp_fwdtsn_chunk {
> @@ -611,7 +611,7 @@ struct sctp_ifwdtsn_skip {
>
> struct sctp_ifwdtsn_hdr {
> __be32 new_cum_tsn;
> - struct sctp_ifwdtsn_skip skip[0];
> + struct sctp_ifwdtsn_skip skip[];
> };
>
> struct sctp_ifwdtsn_chunk {
> @@ -658,7 +658,7 @@ struct sctp_addip_param {
>
> struct sctp_addiphdr {
> __be32 serial;
> - __u8 params[0];
> + __u8 params[];
> };
>
> struct sctp_addip_chunk {
> @@ -718,7 +718,7 @@ struct sctp_addip_chunk {
> struct sctp_authhdr {
> __be16 shkey_id;
> __be16 hmac_id;
> - __u8 hmac[0];
> + __u8 hmac[];
> };
>
> struct sctp_auth_chunk {
> @@ -733,7 +733,7 @@ struct sctp_infox {
>
> struct sctp_reconf_chunk {
> struct sctp_chunkhdr chunk_hdr;
> - __u8 params[0];
> + __u8 params[];
> };
>
> struct sctp_strreset_outreq {
> @@ -741,13 +741,13 @@ struct sctp_strreset_outreq {
> __be32 request_seq;
> __be32 response_seq;
> __be32 send_reset_at_tsn;
> - __be16 list_of_streams[0];
> + __be16 list_of_streams[];
> };
>
> struct sctp_strreset_inreq {
> struct sctp_paramhdr param_hdr;
> __be32 request_seq;
> - __be16 list_of_streams[0];
> + __be16 list_of_streams[];
> };
>
> struct sctp_strreset_tsnreq {
>
>
Acked-by: Neil Horman <nhorman@...hat.com>
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