Subject: Re: [PATCH] net: wwan: iosm: Convert single instance struct member to flexible array

From: "Gustavo A. R. Silva" <[email-masked]>
Date: Tue, 20 Jun 2023 15:17:41 -0600

On Tue, Jun 20, 2023 at 12:42:38PM -0700, Kees Cook wrote:
> struct mux_adth actually ends with multiple struct mux_adth_dg members.
> This is seen both in the comments about the member:
> 
> /**
>  * struct mux_adth - Structure of the Aggregated Datagram Table Header.
>  ...
>  * @dg:		datagramm table with variable length
>  */
> 
> and in the preparation for populating it:
> 
>                         adth_dg_size = offsetof(struct mux_adth, dg) +
>                                         ul_adb->dg_count[i] * sizeof(*dg);
> 			...
>                         adth_dg_size -= offsetof(struct mux_adth, dg);
>                         memcpy(&adth->dg, ul_adb->dg[i], adth_dg_size);
> 
> This was reported as a run-time false positive warning:
> 
> memcpy: detected field-spanning write (size 16) of single field "&adth->dg" at drivers/net/wwan/iosm/iosm_ipc_mux_codec.c:852 (size 8)
> 
> Adjust the struct mux_adth definition and associated sizeof() math; no binary
> output differences are observed in the resulting object file.
> 
> Reported-by: Florian Klink <[email-masked]>
> Closes: https://lore.kernel.org/lkml/[email-masked]/
> Fixes: 1f52d7b62285 ("net: wwan: iosm: Enable M.2 7360 WWAN card support")
> Cc: M Chetan Kumar <[email-masked]>
> Cc: Bagas Sanjaya <[email-masked]>
> Cc: Intel Corporation <[email-masked]>
> Cc: Loic Poulain <[email-masked]>
> Cc: Sergey Ryazanov <[email-masked]>
> Cc: Johannes Berg <[email-masked]>
> Cc: "David S. Miller" <[email-masked]>
> Cc: Eric Dumazet <[email-masked]>
> Cc: Jakub Kicinski <[email-masked]>
> Cc: Paolo Abeni <[email-masked]>
> Cc: "Gustavo A. R. Silva" <[email-masked]>
> Cc: [email-masked]
> Signed-off-by: Kees Cook <[email-masked]>

Reviewed-by: Gustavo A. R. Silva <[email-masked]>

Thank god they used offsetof(struct mux_adth, dg) everywhere. :P

--
Gustavo

> ---
>  drivers/net/wwan/iosm/iosm_ipc_mux_codec.c | 15 ++++++---------
>  drivers/net/wwan/iosm/iosm_ipc_mux_codec.h |  2 +-
>  2 files changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.c b/drivers/net/wwan/iosm/iosm_ipc_mux_codec.c
> index d6b166fc5c0e..bff46f7ca59f 100644
> --- a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.c
> +++ b/drivers/net/wwan/iosm/iosm_ipc_mux_codec.c
> @@ -626,14 +626,12 @@ static void mux_dl_adb_decode(struct iosm_mux *ipc_mux,
>  		if (adth->signature != cpu_to_le32(IOSM_AGGR_MUX_SIG_ADTH))
>  			goto adb_decode_err;
>  
> -		if (le16_to_cpu(adth->table_length) < (sizeof(struct mux_adth) -
> -				sizeof(struct mux_adth_dg)))
> +		if (le16_to_cpu(adth->table_length) < sizeof(struct mux_adth))
>  			goto adb_decode_err;
>  
>  		/* Calculate the number of datagrams. */
>  		nr_of_dg = (le16_to_cpu(adth->table_length) -
> -					sizeof(struct mux_adth) +
> -					sizeof(struct mux_adth_dg)) /
> +					sizeof(struct mux_adth)) /
>  					sizeof(struct mux_adth_dg);
>  
>  		/* Is the datagram table empty ? */
> @@ -649,7 +647,7 @@ static void mux_dl_adb_decode(struct iosm_mux *ipc_mux,
>  		}
>  
>  		/* New aggregated datagram table. */
> -		dg = &adth->dg;
> +		dg = adth->dg;
>  		if (mux_dl_process_dg(ipc_mux, adbh, dg, skb, if_id,
>  				      nr_of_dg) < 0)
>  			goto adb_decode_err;
> @@ -849,7 +847,7 @@ static void ipc_mux_ul_encode_adth(struct iosm_mux *ipc_mux,
>  			adth->if_id = i;
>  			adth->table_length = cpu_to_le16(adth_dg_size);
>  			adth_dg_size -= offsetof(struct mux_adth, dg);
> -			memcpy(&adth->dg, ul_adb->dg[i], adth_dg_size);
> +			memcpy(adth->dg, ul_adb->dg[i], adth_dg_size);
>  			ul_adb->if_cnt++;
>  		}
>  
> @@ -1426,14 +1424,13 @@ static int ipc_mux_get_payload_from_adb(struct iosm_mux *ipc_mux,
>  
>  		if (adth->signature == cpu_to_le32(IOSM_AGGR_MUX_SIG_ADTH)) {
>  			nr_of_dg = (le16_to_cpu(adth->table_length) -
> -					sizeof(struct mux_adth) +
> -					sizeof(struct mux_adth_dg)) /
> +					sizeof(struct mux_adth)) /
>  					sizeof(struct mux_adth_dg);
>  
>  			if (nr_of_dg <= 0)
>  				return payload_size;
>  
> -			dg = &adth->dg;
> +			dg = adth->dg;
>  
>  			for (i = 0; i < nr_of_dg; i++, dg++) {
>  				if (le32_to_cpu(dg->datagram_index) <
> diff --git a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.h b/drivers/net/wwan/iosm/iosm_ipc_mux_codec.h
> index 5d4e3b89542c..f8df88f816c4 100644
> --- a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.h
> +++ b/drivers/net/wwan/iosm/iosm_ipc_mux_codec.h
> @@ -161,7 +161,7 @@ struct mux_adth {
>  	u8 opt_ipv4v6;
>  	__le32 next_table_index;
>  	__le32 reserved2;
> -	struct mux_adth_dg dg;
> +	struct mux_adth_dg dg[];
>  };
>  
>  /**
> -- 
> 2.34.1
>