libbpf: Support weak typed ksyms.

Currently weak typeless ksyms have default value zero, when they don't
exist in the kernel. However, weak typed ksyms are rejected by libbpf
if they can not be resolved. This means that if a bpf object contains
the declaration of a nonexistent weak typed ksym, it will be rejected
even if there is no program that references the symbol.

Nonexistent weak typed ksyms can also default to zero just like
typeless ones. This allows programs that access weak typed ksyms to be
accepted by verifier, if the accesses are guarded. For example,

extern const int bpf_link_fops3 __ksym __weak;

/* then in BPF program */

if (&bpf_link_fops3) {
/* use bpf_link_fops3 */
}

If actual use of nonexistent typed ksym is not guarded properly,
verifier would see that register is not PTR_TO_BTF_ID and wouldn't
allow to use it for direct memory reads or passing it to BPF helpers.

Signed-off-by: Hao Luo <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/libbpf.c

···
 				}
 				insn[1].imm = ext->kcfg.data_off;
 			} else /* EXT_KSYM */ {
-				if (ext->ksym.type_id) { /* typed ksyms */
+				if (ext->ksym.type_id && ext->is_set) { /* typed ksyms */
 					insn[0].src_reg = BPF_PSEUDO_BTF_ID;
 					insn[0].imm = ext->ksym.kernel_btf_id;
 					insn[1].imm = ext->ksym.kernel_btf_obj_fd;
-				} else { /* typeless ksyms */
+				} else { /* typeless ksyms or unresolved typed ksyms */
 					insn[0].imm = (__u32)ext->ksym.addr;
 					insn[1].imm = ext->ksym.addr >> 32;
 				}
···
 				break;
 		}
 	}
-	if (id <= 0) {
-		pr_warn("extern (%s ksym) '%s': failed to find BTF ID in kernel BTF(s).\n",
-			__btf_kind_str(kind), ksym_name);
+	if (id <= 0)
 		return -ESRCH;
-	}
 
 	*res_btf = btf;
 	*res_btf_fd = btf_fd;
···
 	struct btf *btf = NULL;
 
 	id = find_ksym_btf_id(obj, ext->name, BTF_KIND_VAR, &btf, &btf_fd);
-	if (id < 0)
+	if (id == -ESRCH && ext->is_weak) {
+		return 0;
+	} else if (id < 0) {
+		pr_warn("extern (var ksym) '%s': not found in kernel BTF\n",
+			ext->name);
 		return id;
+	}
 
 	/* find local type_id */
 	local_type_id = ext->ksym.type_id;

tools/testing/selftests/bpf/prog_tests/ksyms_btf.c

···
 #include <bpf/btf.h>
 #include "test_ksyms_btf.skel.h"
 #include "test_ksyms_btf_null_check.skel.h"
+#include "test_ksyms_weak.skel.h"
 
 static int duration;
 
···
 	test_ksyms_btf_null_check__destroy(skel);
 }
 
+static void test_weak_syms(void)
+{
+	struct test_ksyms_weak *skel;
+	struct test_ksyms_weak__data *data;
+	int err;
+
+	skel = test_ksyms_weak__open_and_load();
+	if (CHECK(!skel, "test_ksyms_weak__open_and_load", "failed\n"))
+		return;
+
+	err = test_ksyms_weak__attach(skel);
+	if (CHECK(err, "test_ksyms_weak__attach", "skeleton attach failed: %d\n", err))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	usleep(1);
+
+	data = skel->data;
+	ASSERT_EQ(data->out__existing_typed, 0, "existing typed ksym");
+	ASSERT_NEQ(data->out__existing_typeless, -1, "existing typeless ksym");
+	ASSERT_EQ(data->out__non_existent_typeless, 0, "nonexistent typeless ksym");
+	ASSERT_EQ(data->out__non_existent_typed, 0, "nonexistent typed ksym");
+
+cleanup:
+	test_ksyms_weak__destroy(skel);
+}
+
 void test_ksyms_btf(void)
 {
 	int percpu_datasec;
···
 
 	if (test__start_subtest("null_check"))
 		test_null_check();
+
+	if (test__start_subtest("weak_ksyms"))
+		test_weak_syms();
 }

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

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test weak ksyms.
+ *
+ * Copyright (c) 2021 Google
+ */
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+
+int out__existing_typed = -1;
+__u64 out__existing_typeless = -1;
+
+__u64 out__non_existent_typeless = -1;
+__u64 out__non_existent_typed = -1;
+
+/* existing weak symbols */
+
+/* test existing weak symbols can be resolved. */
+extern const struct rq runqueues __ksym __weak; /* typed */
+extern const void bpf_prog_active __ksym __weak; /* typeless */
+
+
+/* non-existent weak symbols. */
+
+/* typeless symbols, default to zero. */
+extern const void bpf_link_fops1 __ksym __weak;
+
+/* typed symbols, default to zero. */
+extern const int bpf_link_fops2 __ksym __weak;
+
+SEC("raw_tp/sys_enter")
+int pass_handler(const void *ctx)
+{
+	struct rq *rq;
+
+	/* tests existing symbols. */
+	rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, 0);
+	if (rq)
+		out__existing_typed = rq->cpu;
+	out__existing_typeless = (__u64)&bpf_prog_active;
+
+	/* tests non-existent symbols. */
+	out__non_existent_typeless = (__u64)&bpf_link_fops1;
+
+	/* tests non-existent symbols. */
+	out__non_existent_typed = (__u64)&bpf_link_fops2;
+
+	if (&bpf_link_fops2) /* can't happen */
+		out__non_existent_typed = (__u64)bpf_per_cpu_ptr(&bpf_link_fops2, 0);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";