bpf: Support BPF ksym variables in kernel modules

Add support for directly accessing kernel module variables from BPF programs
using special ldimm64 instructions. This functionality builds upon vmlinux
ksym support, but extends ldimm64 with src_reg=BPF_PSEUDO_BTF_ID to allow
specifying kernel module BTF's FD in insn[1].imm field.

During BPF program load time, verifier will resolve FD to BTF object and will
take reference on BTF object itself and, for module BTFs, corresponding module
as well, to make sure it won't be unloaded from under running BPF program. The
mechanism used is similar to how bpf_prog keeps track of used bpf_maps.

One interesting change is also in how per-CPU variable is determined. The
logic is to find .data..percpu data section in provided BTF, but both vmlinux
and module each have their own .data..percpu entries in BTF. So for module's
case, the search for DATASEC record needs to look at only module's added BTF
types. This is implemented with custom search function.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20210112075520.4103414-6-andrii@kernel.org

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 6e585dbc10df..1aac2af12fed 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -761,9 +761,15 @@ struct bpf_ctx_arg_aux {
 	u32 btf_id;
 };
 
+struct btf_mod_pair {
+	struct btf *btf;
+	struct module *module;
+};
+
 struct bpf_prog_aux {
 	atomic64_t refcnt;
 	u32 used_map_cnt;
+	u32 used_btf_cnt;
 	u32 max_ctx_offset;
 	u32 max_pkt_offset;
 	u32 max_tp_access;
@@ -802,6 +808,7 @@ struct bpf_prog_aux {
 	const struct bpf_prog_ops *ops;
 	struct bpf_map **used_maps;
 	struct mutex used_maps_mutex; /* mutex for used_maps and used_map_cnt */
+	struct btf_mod_pair *used_btfs;
 	struct bpf_prog *prog;
 	struct user_struct *user;
 	u64 load_time; /* ns since boottime */
@@ -1668,6 +1675,9 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 }
 #endif /* CONFIG_BPF_SYSCALL */
 
+void __bpf_free_used_btfs(struct bpf_prog_aux *aux,
+			  struct btf_mod_pair *used_btfs, u32 len);
+
 static inline struct bpf_prog *bpf_prog_get_type(u32 ufd,
 						 enum bpf_prog_type type)
 {