Merge branch 'bpf: Add detection of kfuncs.'

Alexei Starovoitov says:

====================

From: Alexei Starovoitov <ast@kernel.org>

Allow BPF programs detect at load time whether particular kfunc exists.

Patch 1: Allow ld_imm64 to point to kfunc in the kernel.
Patch 2: Fix relocation of kfunc in ld_imm64 insn when kfunc is in kernel module.
Patch 3: Introduce bpf_ksym_exists() macro.
Patch 4: selftest.

NOTE: detection of kfuncs from light skeleton is not supported yet.
====================
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

kernel/bpf/verifier.c

@@ -15952,8 +15952,8 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,
 		goto err_put;
 	}
 
-	if (!btf_type_is_var(t)) {
-		verbose(env, "pseudo btf_id %d in ldimm64 isn't KIND_VAR.\n", id);
+	if (!btf_type_is_var(t) && !btf_type_is_func(t)) {
+		verbose(env, "pseudo btf_id %d in ldimm64 isn't KIND_VAR or KIND_FUNC\n", id);
 		err = -EINVAL;
 		goto err_put;
 	}
@@ -15966,6 +15966,14 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,
 		err = -ENOENT;
 		goto err_put;
 	}
+	insn[0].imm = (u32)addr;
+	insn[1].imm = addr >> 32;
+
+	if (btf_type_is_func(t)) {
+		aux->btf_var.reg_type = PTR_TO_MEM | MEM_RDONLY;
+		aux->btf_var.mem_size = 0;
+		goto check_btf;
+	}
 
 	datasec_id = find_btf_percpu_datasec(btf);
 	if (datasec_id > 0) {
@@ -15978,9 +15986,6 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,
 		}
 	}
 
-	insn[0].imm = (u32)addr;
-	insn[1].imm = addr >> 32;
-
 	type = t->type;
 	t = btf_type_skip_modifiers(btf, type, NULL);
 	if (percpu) {
@@ -16008,7 +16013,7 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,
 		aux->btf_var.btf = btf;
 		aux->btf_var.btf_id = type;
 	}
-
+check_btf:
 	/* check whether we recorded this BTF (and maybe module) already */
 	for (i = 0; i < env->used_btf_cnt; i++) {
 		if (env->used_btfs[i].btf == btf) {

tools/lib/bpf/bpf_helpers.h

@@ -177,6 +177,11 @@ enum libbpf_tristate {
 #define __kptr_untrusted __attribute__((btf_type_tag("kptr_untrusted")))
 #define __kptr __attribute__((btf_type_tag("kptr")))
 
+#define bpf_ksym_exists(sym) ({									\
+	_Static_assert(!__builtin_constant_p(!!sym), #sym " should be marked as __weak");	\
+	!!sym;											\
+})
+
 #ifndef ___bpf_concat
 #define ___bpf_concat(a, b) a ## b
 #endif

tools/lib/bpf/libbpf.c

@@ -7533,6 +7533,12 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
 	ext->is_set = true;
 	ext->ksym.kernel_btf_id = kfunc_id;
 	ext->ksym.btf_fd_idx = mod_btf ? mod_btf->fd_array_idx : 0;
+	/* Also set kernel_btf_obj_fd to make sure that bpf_object__relocate_data()
+	 * populates FD into ld_imm64 insn when it's used to point to kfunc.
+	 * {kernel_btf_id, btf_fd_idx} -> fixup bpf_call.
+	 * {kernel_btf_id, kernel_btf_obj_fd} -> fixup ld_imm64.
+	 */
+	ext->ksym.kernel_btf_obj_fd = mod_btf ? mod_btf->fd : 0;
 	pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
 		 ext->name, kfunc_id);
 

tools/testing/selftests/bpf/progs/task_kfunc_success.c

@@ -17,6 +17,10 @@ int err, pid;
  *         TP_PROTO(struct task_struct *p, u64 clone_flags)
  */
 
+struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym __weak;
+void invalid_kfunc(void) __ksym __weak;
+void bpf_testmod_test_mod_kfunc(int i) __ksym __weak;
+
 static bool is_test_kfunc_task(void)
 {
 	int cur_pid = bpf_get_current_pid_tgid() >> 32;
@@ -26,7 +30,21 @@ static bool is_test_kfunc_task(void)
 
 static int test_acquire_release(struct task_struct *task)
 {
-	struct task_struct *acquired;
+	struct task_struct *acquired = NULL;
+
+	if (!bpf_ksym_exists(bpf_task_acquire)) {
+		err = 3;
+		return 0;
+	}
+	if (!bpf_ksym_exists(bpf_testmod_test_mod_kfunc)) {
+		err = 4;
+		return 0;
+	}
+	if (bpf_ksym_exists(invalid_kfunc)) {
+		/* the verifier's dead code elimination should remove this */
+		err = 5;
+		asm volatile ("goto -1"); /* for (;;); */
+	}
 
 	acquired = bpf_task_acquire(task);
 	bpf_task_release(acquired);