bpf: btf: Add pretty print support to the basic arraymap

This patch adds pretty print support to the basic arraymap.
Support for other bpf maps can be added later.

This patch adds new attrs to the BPF_MAP_CREATE command to allow
specifying the btf_fd, btf_key_id and btf_value_id.  The
BPF_MAP_CREATE can then associate the btf to the map if
the creating map supports BTF.

A BTF supported map needs to implement two new map ops,
map_seq_show_elem() and map_check_btf().  This patch has
implemented these new map ops for the basic arraymap.

It also adds file_operations, bpffs_map_fops, to the pinned
map such that the pinned map can be opened and read.
After that, the user has an intuitive way to do
"cat bpffs/pathto/a-pinned-map" instead of getting
an error.

bpffs_map_fops should not be extended further to support
other operations.  Other operations (e.g. write/key-lookup...)
should be realized by the userspace tools (e.g. bpftool) through
the BPF_OBJ_GET_INFO_BY_FD, map's lookup/update interface...etc.
Follow up patches will allow the userspace to obtain
the BTF from a map-fd.

Here is a sample output when reading a pinned arraymap
with the following map's value:

struct map_value {
	int count_a;
	int count_b;
};

cat /sys/fs/bpf/pinned_array_map:

0: {1,2}
1: {3,4}
2: {5,6}
...

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 31 insertions, 1 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 0a4924a0a8da..fe23dc5a3ec4 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -27,6 +27,7 @@
 #include <linux/cred.h>
 #include <linux/timekeeping.h>
 #include <linux/ctype.h>
+#include <linux/btf.h>
 
 #define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PROG_ARRAY || \
                            (map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
@@ -251,6 +252,7 @@ static void bpf_map_free_deferred(struct work_struct *work)
 
         bpf_map_uncharge_memlock(map);
         security_bpf_map_free(map);
+        btf_put(map->btf);
         /* implementation dependent freeing */
         map->ops->map_free(map);
 }
@@ -416,7 +418,7 @@ static int bpf_obj_name_cpy(char *dst, const char *src)
         return 0;
 }
 
-#define BPF_MAP_CREATE_LAST_FIELD map_ifindex
+#define BPF_MAP_CREATE_LAST_FIELD btf_value_id
 /* called via syscall */
 static int map_create(union bpf_attr *attr)
 {
@@ -450,6 +452,33 @@ static int map_create(union bpf_attr *attr)
         atomic_set(&map->refcnt, 1);
         atomic_set(&map->usercnt, 1);
 
+        if (bpf_map_support_seq_show(map) &&
+            (attr->btf_key_id || attr->btf_value_id)) {
+                struct btf *btf;
+
+                if (!attr->btf_key_id || !attr->btf_value_id) {
+                        err = -EINVAL;
+                        goto free_map_nouncharge;
+                }
+
+                btf = btf_get_by_fd(attr->btf_fd);
+                if (IS_ERR(btf)) {
+                        err = PTR_ERR(btf);
+                        goto free_map_nouncharge;
+                }
+
+                err = map->ops->map_check_btf(map, btf, attr->btf_key_id,
+                                              attr->btf_value_id);
+                if (err) {
+                        btf_put(btf);
+                        goto free_map_nouncharge;
+                }
+
+                map->btf = btf;
+                map->btf_key_id = attr->btf_key_id;
+                map->btf_value_id = attr->btf_value_id;
+        }
+
         err = security_bpf_map_alloc(map);
         if (err)
                 goto free_map_nouncharge;
@@ -482,6 +511,7 @@ free_map:
 free_map_sec:
         security_bpf_map_free(map);
 free_map_nouncharge:
+        btf_put(map->btf);
         map->ops->map_free(map);
         return err;
 }