Re: [PATCH] batman-adv: Add flex array to struct batadv_tvlv_tt_data

[Sven Eckelmann]

On Tuesday, 2 April 2024 19:23:01 CEST Erick Archer wrote:
> The "struct batadv_tvlv_tt_data" uses a dynamically sized set of
> trailing elements. Specifically, it uses an array of structures of type
> "batadv_tvlv_tt_vlan_data". So, use the preferred way in the kernel
> declaring a flexible array [1].
> 
> The order in which the structure batadv_tvlv_tt_data and the structure
> batadv_tvlv_tt_vlan_data are defined must be swap to avoid an incomplete
> type error.
> 
> Also, avoid the open-coded arithmetic in memory allocator functions [2]
> using the "struct_size" macro and use the "flex_array_size" helper to
> clarify some calculations, when possible.
> 
> Moreover, the new structure member also allow us to avoid the open-coded
> arithmetic on pointers in some situations. Take advantage of this.
> 
> This code was detected with the help of Coccinelle, and audited and
> modified manually.
> 
> Link: 
> https://www.kernel.org/doc/html/next/process/deprecated.html#zero-length-and-one-element-arrays
>  [1]
> Link: 
> https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
>  [2]
> Signed-off-by: Erick Archer <erick.arc...@outlook.com>

> ---
> Hi,
> 
> I would like to add the "__counted_by(num_vlan)" tag to the new flex member
> but I don't know if this line can affect it.
> 
> ntohs(tt_data->num_vlan)


Yes, num_vlan is a __be16. I could only identify the kernel-doc related 
scripts as consumer. But maybe they are more - so I would defer this question 
to kernel-hardening.


And with this change, I get a lot of additional warnings (-Wsparse-all)


cfg: BLA=n DAT=y DEBUG=y TRACING=n NC=y MCAST=n BATMAN_V=n
    net/batman-adv/translation-table.c:574:21: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:859:25: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:859:25: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:938:25: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:938:25: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:2932:16: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:2932:16: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:3378:21: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:3378:21: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:3982:30: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:3986:27: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:4026:30: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:4030:27: warning: using sizeof on a 
flexible structure
    net/batman-adv/translation-table.c:4032:23: warning: cast from restricted 
__be16
    net/batman-adv/translation-table.c:4032:23: warning: restricted __be16 
degrades to integer
    net/batman-adv/translation-table.c:4032:23: warning: incorrect type in 
argument 1 (different base types)
    net/batman-adv/translation-table.c:4032:23:    expected unsigned long 
[usertype] factor1
    net/batman-adv/translation-table.c:4032:23:    got restricted __be16 
[usertype] num_vlan

[...]
>       num_vlan = ntohs(tt_data->num_vlan);
>  
> -     if (tvlv_value_len < sizeof(*tt_vlan) * num_vlan)
> +     flex_size = flex_array_size(tt_data, vlan_data, num_vlan);
> +     if (tvlv_value_len < flex_size)
>               return;

This helper would need an #include of <linux/overflow.h> in 
net/batman-adv/translation-table.c

[....]
>  /**
> @@ -4039,8 +4029,7 @@ static int batadv_tt_tvlv_unicast_handler_v1(struct 
> batadv_priv *bat_priv,
>       tt_data = tvlv_value;
>       tvlv_value_len -= sizeof(*tt_data);
>  
> -     tt_vlan_len = sizeof(struct batadv_tvlv_tt_vlan_data);
> -     tt_vlan_len *= ntohs(tt_data->num_vlan);
> +     tt_vlan_len = flex_array_size(tt_data, vlan_data, tt_data->num_vlan);

This is definitely wrong on little endian systems. You first need to convert 
num_vlan from network (big endian) to host order.

Kind regards,
        Sven

signature.asc
Description: This is a digitally signed message part.