bpf: Create links for BPF struct_ops maps.

Make bpf_link support struct_ops.  Previously, struct_ops were always
used alone without any associated links. Upon updating its value, a
struct_ops would be activated automatically. Yet other BPF program
types required to make a bpf_link with their instances before they
could become active. Now, however, you can create an inactive
struct_ops, and create a link to activate it later.

With bpf_links, struct_ops has a behavior similar to other BPF program
types. You can pin/unpin them from their links and the struct_ops will
be deactivated when its link is removed while previously need someone
to delete the value for it to be deactivated.

bpf_links are responsible for registering their associated
struct_ops. You can only use a struct_ops that has the BPF_F_LINK flag
set to create a bpf_link, while a structs without this flag behaves in
the same manner as before and is registered upon updating its value.

The BPF_LINK_TYPE_STRUCT_OPS serves a dual purpose. Not only is it
used to craft the links for BPF struct_ops programs, but also to
create links for BPF struct_ops them-self.  Since the links of BPF
struct_ops programs are only used to create trampolines internally,
they are never seen in other contexts. Thus, they can be reused for
struct_ops themself.

To maintain a reference to the map supporting this link, we add
bpf_struct_ops_link as an additional type. The pointer of the map is
RCU and won't be necessary until later in the patchset.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Link: https://lore.kernel.org/r/20230323032405.3735486-4-kuifeng@meta.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

1 files changed, 15 insertions, 8 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index cff0348a2871..21f76698875c 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2825,16 +2825,19 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 	const struct bpf_prog *prog = link->prog;
 	char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
 
-	bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
 	seq_printf(m,
 		   "link_type:\t%s\n"
-		   "link_id:\t%u\n"
-		   "prog_tag:\t%s\n"
-		   "prog_id:\t%u\n",
+		   "link_id:\t%u\n",
 		   bpf_link_type_strs[link->type],
-		   link->id,
-		   prog_tag,
-		   prog->aux->id);
+		   link->id);
+	if (prog) {
+		bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
+		seq_printf(m,
+			   "prog_tag:\t%s\n"
+			   "prog_id:\t%u\n",
+			   prog_tag,
+			   prog->aux->id);
+	}
 	if (link->ops->show_fdinfo)
 		link->ops->show_fdinfo(link, m);
 }
@@ -4314,7 +4317,8 @@ static int bpf_link_get_info_by_fd(struct file *file,
 
 	info.type = link->type;
 	info.id = link->id;
-	info.prog_id = link->prog->aux->id;
+	if (link->prog)
+		info.prog_id = link->prog->aux->id;
 
 	if (link->ops->fill_link_info) {
 		err = link->ops->fill_link_info(link, &info);
@@ -4577,6 +4581,9 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 	if (CHECK_ATTR(BPF_LINK_CREATE))
 		return -EINVAL;
 
+	if (attr->link_create.attach_type == BPF_STRUCT_OPS)
+		return bpf_struct_ops_link_create(attr);
+
 	prog = bpf_prog_get(attr->link_create.prog_fd);
 	if (IS_ERR(prog))
 		return PTR_ERR(prog);