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

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

[Christophe JAILLET]

'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@wanadoo.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;
 	INIT_WORK(&utn->work, udp_tunnel_nic_device_sync_work);
 
-	utn->entries = kmalloc_array(n_tables, sizeof(void *), GFP_KERNEL);
-	if (!utn->entries)
-		goto err_free_utn;
-
 	for (i = 0; i < n_tables; i++) {
 		utn->entries[i] = kcalloc(info->tables[i].n_entries,
 					  sizeof(*utn->entries[i]), GFP_KERNEL);
@@ -747,8 +743,6 @@ udp_tunnel_nic_alloc(const struct udp_tunnel_nic_info *info,
 err_free_prev_entries:
 	while (i--)
 		kfree(utn->entries[i]);
-	kfree(utn->entries);
-err_free_utn:
 	kfree(utn);
 	return NULL;
 }
@@ -759,7 +753,6 @@ static void udp_tunnel_nic_free(struct udp_tunnel_nic *utn)
 
 	for (i = 0; i < utn->n_tables; i++)
 		kfree(utn->entries[i]);
-	kfree(utn->entries);
 	kfree(utn);
 }
 
-- 
2.34.1

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

[Willem de Bruijn]

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@wanadoo.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)?

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

[Christophe JAILLET]

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@wanadoo.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

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

[Willem de Bruijn]

On Mon, Sep 25, 2023 at 6:26 PM Christophe JAILLET
<christophe.jaillet@wanadoo.fr> wrote:
>
> 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@wanadoo.fr>

Reviewed-by: Willem de Bruijn <willemb@google.com>

> >> ---
> >>   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.

Of course, sorry. I confused __counted_by's reference to a field
member in the struct, with the normal argument passing of struct_size
and flex_array_size.

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Sun, 24 Sep 2023 10:03:07 +0200 you 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.
> 
> [...]

Here is the summary with links:
  - [net-next] udp_tunnel: Use flex array to simplify code
    https://git.kernel.org/netdev/net-next/c/ef35bed6fad6

You are awesome, thank you!
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