linux-kernel - Re: [PATCH 2/5] bpf: convert queue and stack map to map_copy

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Date:   Wed, 11 Mar 2020 15:31:41 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        kernel-team@...udflare.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] bpf: convert queue and stack map to map_copy_value

Jakub Sitnicki wrote:
> On Tue, Mar 10, 2020 at 06:47 PM CET, Lorenz Bauer wrote:
> > Migrate BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK to map_copy_value,
> > by introducing small wrappers that discard the (unused) key argument.
> >
> > Signed-off-by: Lorenz Bauer <lmb@...udflare.com>
> > ---
> >  kernel/bpf/queue_stack_maps.c | 18 ++++++++++++++++++
> >  kernel/bpf/syscall.c          |  5 +----
> >  2 files changed, 19 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c
> > index f697647ceb54..5c89b7583cd2 100644
> > --- a/kernel/bpf/queue_stack_maps.c
> > +++ b/kernel/bpf/queue_stack_maps.c
> > @@ -262,11 +262,28 @@ static int queue_stack_map_get_next_key(struct bpf_map *map, void *key,
> >  	return -EINVAL;
> >  }
> >
> > +/* Called from syscall */
> > +static int queue_map_copy_value(struct bpf_map *map, void *key, void *value)
> > +{
> > +	(void)key;
> 
> Alternatively, there's is the __always_unused compiler attribute from
> include/linux/compiler_attributes.h that seems to be in wide use.
> 

+1 use the attribute its much nicer imo.

> > +
> > +	return queue_map_peek_elem(map, value);
> > +}
> > +
> > +/* Called from syscall */
> > +static int stack_map_copy_value(struct bpf_map *map, void *key, void *value)
> > +{
> > +	(void)key;
> > +
> > +	return stack_map_peek_elem(map, value);
> > +}
> > +
> >  const struct bpf_map_ops queue_map_ops = {
> >  	.map_alloc_check = queue_stack_map_alloc_check,
> >  	.map_alloc = queue_stack_map_alloc,
> >  	.map_free = queue_stack_map_free,
> >  	.map_lookup_elem = queue_stack_map_lookup_elem,
> > +	.map_copy_value = queue_map_copy_value,
> >  	.map_update_elem = queue_stack_map_update_elem,
> >  	.map_delete_elem = queue_stack_map_delete_elem,
> >  	.map_push_elem = queue_stack_map_push_elem,
> > @@ -280,6 +297,7 @@ const struct bpf_map_ops stack_map_ops = {
> >  	.map_alloc = queue_stack_map_alloc,
> >  	.map_free = queue_stack_map_free,
> >  	.map_lookup_elem = queue_stack_map_lookup_elem,
> > +	.map_copy_value = stack_map_copy_value,
> >  	.map_update_elem = queue_stack_map_update_elem,
> >  	.map_delete_elem = queue_stack_map_delete_elem,
> >  	.map_push_elem = queue_stack_map_push_elem,
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 6503824e81e9..20c6cdace275 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -218,10 +218,7 @@ static int bpf_map_copy_value(struct bpf_map *map, void *key, void *value,
> >  		return bpf_map_offload_lookup_elem(map, key, value);
> >
> >  	bpf_disable_instrumentation();
> > -	if (map->map_type == BPF_MAP_TYPE_QUEUE ||
> > -	    map->map_type == BPF_MAP_TYPE_STACK) {
> > -		err = map->ops->map_peek_elem(map, value);
> > -	} else if (map->ops->map_copy_value) {
> > +	if (map->ops->map_copy_value) {
> >  		err = map->ops->map_copy_value(map, key, value);
> >  	} else {
> >  		rcu_read_lock();