[PATCH bpf V2 1/2] bpf: devmap adjust uapi for attach bpf program

Jesper Dangaard Brouer
2020-06-09T13:32:08Z

V2:
- Defer changing BPF-syscall to start at file-descriptor 1
- Use {} to zero initialise struct.

The recent commit fbee97feed9b ("bpf: Add support to attach bpf program to a
devmap entry"), introduced ability to attach (and run) a separate XDP
bpf_prog for each devmap entry. A bpf_prog is added via a file-descriptor.
As zero were a valid FD, not using the feature requires using value minus-1.
The UAPI is extended via tail-extending struct bpf_devmap_val and using
map->value_size to determine the feature set.

This will break older userspace applications not using the bpf_prog feature.
Consider an old userspace app that is compiled against newer kernel
uapi/bpf.h, it will not know that it need to initialise the member
bpf_prog.fd to minus-1. Thus, users will be forced to update source code to
get program running on newer kernels.

This patch remove the minus-1 checks, and have zero mean feature isn't used.

Followup patches either for kernel or libbpf should handle and avoid
returning file-descriptor zero in the first place.

Fixes: fbee97feed9b ("bpf: Add support to attach bpf program to a devmap entry")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
include/uapi/linux/bpf.h | 13 +++++++++++++
kernel/bpf/devmap.c | 17 ++++-------------
2 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index c65b374a5090..19684813faae 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3761,6 +3761,19 @@ struct xdp_md {
__u32 egress_ifindex; /* txq->dev->ifindex */
};

+/* DEVMAP map-value layout
+ *
+ * The struct data-layout of map-value is a configuration interface.
+ * New members can only be added to the end of this structure.
+ */
+struct bpf_devmap_val {
+ __u32 ifindex; /* device index */
+ union {
+ int fd; /* prog fd on map write */
+ __u32 id; /* prog id on map read */
+ } bpf_prog;
+};
+
enum sk_action {
SK_DROP = 0,
SK_PASS,
diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 854b09beb16b..899a30a67cc1 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -60,15 +60,6 @@ struct xdp_dev_bulk_queue {
unsigned int count;
};

-/* DEVMAP values */
-struct bpf_devmap_val {
- u32 ifindex; /* device index */
- union {
- int fd; /* prog fd on map write */
- u32 id; /* prog id on map read */
- } bpf_prog;
-};
-
struct bpf_dtab_netdev {
struct net_device *dev; /* must be first member, due to tracepoint */
struct hlist_node index_hlist;
@@ -618,7 +609,7 @@ static struct bpf_dtab_netdev *__dev_map_alloc_node(struct net *net,
if (!dev->dev)
goto err_out;

- if (val->bpf_prog.fd >= 0) {
+ if (val->bpf_prog.fd > 0) {
prog = bpf_prog_get_type_dev(val->bpf_prog.fd,
BPF_PROG_TYPE_XDP, false);
if (IS_ERR(prog))
@@ -652,8 +643,8 @@ static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
void *key, void *value, u64 map_flags)
{
struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
- struct bpf_devmap_val val = { .bpf_prog.fd = -1 };
struct bpf_dtab_netdev *dev, *old_dev;
+ struct bpf_devmap_val val = {};
u32 i = *(u32 *)key;

if (unlikely(map_flags > BPF_EXIST))
@@ -669,7 +660,7 @@ static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
if (!val.ifindex) {
dev = NULL;
/* can not specify fd if ifindex is 0 */
- if (val.bpf_prog.fd != -1)
+ if (val.bpf_prog.fd > 0)
return -EINVAL;
} else {
dev = __dev_map_alloc_node(net, dtab, &val, i);
@@ -699,8 +690,8 @@ static int __dev_map_hash_update_elem(struct net *net, struct bpf_map *map,
void *key, void *value, u64 map_flags)
{
struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
- struct bpf_devmap_val val = { .bpf_prog.fd = -1 };
struct bpf_dtab_netdev *dev, *old_dev;
+ struct bpf_devmap_val val = {};
u32 idx = *(u32 *)key;
unsigned long flags;
int err = -EEXIST;