[RFC][PATCH 2/4] bpf: Allow BPF_JIT with CONFIG_MODULES=n

[Calvin Owens]

No BPF code has to change, except in struct_ops (for module refs).

This conflicts with bpf-next because of this (relevant) series:

    https://lore.kernel.org/all/20240119225005.668602-1-thinker...@gmail.com/

If something like this is merged down the road, it can go through
bpf-next at leisure once the module_alloc change is in: it's a one-way
dependency.

Signed-off-by: Calvin Owens <jcalvinow...@gmail.com>
---
 kernel/bpf/Kconfig          |  2 +-
 kernel/bpf/bpf_struct_ops.c | 28 ++++++++++++++++++++++++----
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
index 6a906ff93006..77df483a8925 100644
--- a/kernel/bpf/Kconfig
+++ b/kernel/bpf/Kconfig
@@ -42,7 +42,7 @@ config BPF_JIT
        bool "Enable BPF Just In Time compiler"
        depends on BPF
        depends on HAVE_CBPF_JIT || HAVE_EBPF_JIT
-       depends on MODULES
+       select MODULE_ALLOC
        help
          BPF programs are normally handled by a BPF interpreter. This option
          allows the kernel to generate native code when a program is loaded
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 02068bd0e4d9..fbf08a1bb00c 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -108,11 +108,30 @@ const struct bpf_prog_ops bpf_struct_ops_prog_ops = {
 #endif
 };
 
+#if IS_ENABLED(CONFIG_MODULES)
 static const struct btf_type *module_type;
 
+static int bpf_struct_module_type_init(struct btf *btf)
+{
+       s32 module_id;
+
+       module_id = btf_find_by_name_kind(btf, "module", BTF_KIND_STRUCT);
+       if (module_id < 0)
+               return 1;
+
+       module_type = btf_type_by_id(btf, module_id);
+       return 0;
+}
+#else
+static int bpf_struct_module_type_init(struct btf *btf)
+{
+       return 0;
+}
+#endif
+
 void bpf_struct_ops_init(struct btf *btf, struct bpf_verifier_log *log)
 {
-       s32 type_id, value_id, module_id;
+       s32 type_id, value_id;
        const struct btf_member *member;
        struct bpf_struct_ops *st_ops;
        const struct btf_type *t;
@@ -125,12 +144,10 @@ void bpf_struct_ops_init(struct btf *btf, struct 
bpf_verifier_log *log)
 #include "bpf_struct_ops_types.h"
 #undef BPF_STRUCT_OPS_TYPE
 
-       module_id = btf_find_by_name_kind(btf, "module", BTF_KIND_STRUCT);
-       if (module_id < 0) {
+       if (bpf_struct_module_type_init(btf)) {
                pr_warn("Cannot find struct module in btf_vmlinux\n");
                return;
        }
-       module_type = btf_type_by_id(btf, module_id);
 
        for (i = 0; i < ARRAY_SIZE(bpf_struct_ops); i++) {
                st_ops = bpf_struct_ops[i];
@@ -433,12 +450,15 @@ static long bpf_struct_ops_map_update_elem(struct bpf_map 
*map, void *key,
 
                moff = __btf_member_bit_offset(t, member) / 8;
                ptype = btf_type_resolve_ptr(btf_vmlinux, member->type, NULL);
+
+#if IS_ENABLED(CONFIG_MODULES)
                if (ptype == module_type) {
                        if (*(void **)(udata + moff))
                                goto reset_unlock;
                        *(void **)(kdata + moff) = BPF_MODULE_OWNER;
                        continue;
                }
+#endif
 
                err = st_ops->init_member(t, member, kdata, udata);
                if (err < 0)
-- 
2.43.0