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@xxxxxxxxxx>
> > ---
> >   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