[RFC PATCH 1/4] bpf, cgroup: fix cgroup struct_ops query for a second attach type

[Shakeel Butt <shakeel.butt@linux.dev>]

Two things in __cgroup_bpf_query() work only because CGROUP_TCP_SOCK_OPS is
the only one struct_ops attach type.

It calls cgroup_bpf_enabled(atype) with an atype that
find_atype_by_struct_ops_id() works out at runtime.  That macro is an asm
goto and needs a constant.  Today the compiler can see there is only one
value; add a second type and the build breaks with "impossible constraint in
'asm'".  Add cgroup_bpf_enabled_runtime(), which reads the key instead, and
use it here.  This is a syscall path, so the cost does not matter.

And find_atype_by_struct_ops_id() matches on type_id alone.  An attach type
whose subsystem is not built keeps type_id 0, so a query for type 0 finds it
and returns success with nothing instead of -ENOENT.  Skip such slots.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 include/linux/bpf-cgroup.h | 9 +++++++++
 kernel/bpf/cgroup.c        | 3 ++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index 8a75a6cd7309..3b2c127d401d 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -76,6 +76,14 @@ to_cgroup_bpf_attach_type(enum bpf_attach_type attach_type)
 
 extern struct static_key_false cgroup_bpf_enabled_key[MAX_CGROUP_BPF_ATTACH_TYPE];
 #define cgroup_bpf_enabled(atype) static_branch_unlikely(&cgroup_bpf_enabled_key[atype])
+/*
+ * Same test when @atype is not a constant.  cgroup_bpf_enabled() uses
+ * static_branch_unlikely which creates jump-label site and requires statically
+ * selected key. Since key is selected dynamically, use static_key_enabled here
+ * and keep it off fast paths.
+ */
+#define cgroup_bpf_enabled_runtime(atype) \
+	static_key_enabled(&cgroup_bpf_enabled_key[atype])
 
 struct bpf_cgroup_storage_map;
 
@@ -508,6 +516,7 @@ static inline int cgroup_bpf_struct_ops_attach(struct bpf_map *map,
 }
 
 #define cgroup_bpf_enabled(atype) (0)
+#define cgroup_bpf_enabled_runtime(atype) (0)
 #define BPF_CGROUP_RUN_SA_PROG_LOCK(sk, uaddr, uaddrlen, atype, t_ctx) ({ 0; })
 #define BPF_CGROUP_RUN_SA_PROG(sk, uaddr, uaddrlen, atype) ({ 0; })
 #define BPF_CGROUP_PRE_CONNECT_ENABLED(sk) (0)
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 696b27383974..99b4e96f5db7 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -45,6 +45,7 @@ static enum cgroup_bpf_attach_type find_atype_by_struct_ops_id(u32 type_id)
 
 	for (atype = 0; atype < MAX_CGROUP_BPF_ATTACH_TYPE; atype++) {
 		if (cgroup_bpf_is_struct_ops_atype(atype) &&
+		    cgroup_struct_ops[atype].type_id &&
 		    cgroup_struct_ops[atype].type_id == type_id)
 			return atype;
 	}
@@ -1448,7 +1449,7 @@ static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 			return -ENOENT;
 		from_atype = to_atype = atype;
 		flags = 0;
-		if (!cgroup_bpf_enabled(atype))
+		if (!cgroup_bpf_enabled_runtime(atype))
 			goto skip_count;
 	} else if (type == BPF_LSM_CGROUP) {
 		if (!effective_query && attr->query.prog_cnt &&
-- 
2.53.0-Meta