netdev - Re: [PATCH net-next] udp_tunnel: Use flex array to simplify code

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Date: Mon, 25 Sep 2023 18:26:50 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Cc: linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH net-next] udp_tunnel: Use flex array to simplify code

Le 24/09/2023 à 18:00, Willem de Bruijn a écrit :
> Christophe JAILLET wrote:
>> 'n_tables' is small, UDP_TUNNEL_NIC_MAX_TABLES	= 4 as a maximum. So there
>> is no real point to allocate the 'entries' pointers array with a dedicate
>> memory allocation.
>>
>> Using a flexible array for struct udp_tunnel_nic->entries avoids the
>> overhead of an additional memory allocation.
>>
>> This also saves an indirection when the array is accessed.
>>
>> Finally, __counted_by() can be used for run-time bounds checking if
>> configured and supported by the compiler.
>>
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> ---
>>   net/ipv4/udp_tunnel_nic.c | 11 ++---------
>>   1 file changed, 2 insertions(+), 9 deletions(-)
>>
>> diff --git a/net/ipv4/udp_tunnel_nic.c b/net/ipv4/udp_tunnel_nic.c
>> index 029219749785..b6d2d16189c0 100644
>> --- a/net/ipv4/udp_tunnel_nic.c
>> +++ b/net/ipv4/udp_tunnel_nic.c
>> @@ -47,7 +47,7 @@ struct udp_tunnel_nic {
>>   
>>   	unsigned int n_tables;
>>   	unsigned long missed;
>> -	struct udp_tunnel_nic_table_entry **entries;
>> +	struct udp_tunnel_nic_table_entry *entries[] __counted_by(n_tables);
>>   };
>>   
>>   /* We ensure all work structs are done using driver state, but not the code.
>> @@ -725,16 +725,12 @@ udp_tunnel_nic_alloc(const struct udp_tunnel_nic_info *info,
>>   	struct udp_tunnel_nic *utn;
>>   	unsigned int i;
>>   
>> -	utn = kzalloc(sizeof(*utn), GFP_KERNEL);
>> +	utn = kzalloc(struct_size(utn, entries, n_tables), GFP_KERNEL);
>>   	if (!utn)
>>   		return NULL;
>>   	utn->n_tables = n_tables;
> 
> Should utn->n_tables be initialized before first use of
> struct_size(utn, entries, n_tables)?
> 

It can't be.
struct_size() is used to compute the memory size to allocate.

Before the kzalloc() call, utn does not exist, so we can't write 
anything to utn->n_tables. It is undefined at this point.

It is initialized the line just after, after the allocation, but before 
any use.

CJ