[PATCH bpf-next v3] libbpf: support weak typed ksyms.

[PATCH bpf-next v3] libbpf: support weak typed ksyms.

[Hao Luo]

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 <haoluo@google.com>
---
Changes since v2:
 - Move special handling and warning from find_ksym_btf_id() to
   bpf_object__resolve_ksym_var_btf_id().
 - Removed bpf_link_fops3 from tests since it's not used.
 - Separated variable declaration and statements.

Changes since v1:
 - Weak typed symbols default to zero, as suggested by Andrii.
 - Use ASSERT_XXX() for tests.

 tools/lib/bpf/libbpf.c                        | 16 +++---
 .../selftests/bpf/prog_tests/ksyms_btf.c      | 31 ++++++++++
 .../selftests/bpf/progs/test_ksyms_weak.c     | 56 +++++++++++++++++++
 3 files changed, 96 insertions(+), 7 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index cb106e8c42cb..ff3c0ee79d85 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -5277,11 +5277,11 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 				}
 				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;
 				}
@@ -6608,11 +6608,8 @@ static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
 				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;
@@ -6629,8 +6626,13 @@ static int bpf_object__resolve_ksym_var_btf_id(struct bpf_object *obj,
 	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;
diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c b/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
index 67bebd324147..cf3acfa5a91d 100644
--- a/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
+++ b/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
@@ -6,6 +6,7 @@
 #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;
 
@@ -81,6 +82,33 @@ static void test_null_check(void)
 	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;
@@ -105,4 +133,7 @@ void test_ksyms_btf(void)
 
 	if (test__start_subtest("null_check"))
 		test_null_check();
+
+	if (test__start_subtest("weak_ksyms"))
+		test_weak_syms();
 }
diff --git a/tools/testing/selftests/bpf/progs/test_ksyms_weak.c b/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
new file mode 100644
index 000000000000..5f8379aadb29
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
@@ -0,0 +1,56 @@
+// 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";
-- 
2.32.0.605.g8dce9f2422-goog

Re: [PATCH bpf-next v3] libbpf: support weak typed ksyms.

[Andrii Nakryiko]

On Wed, Aug 11, 2021 at 5:40 PM Hao Luo <haoluo@google.com> wrote:
>
> 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 <haoluo@google.com>
> ---

Looks good, applied to bpf-next. For the future, please split libbpf
and selftests changes into separate patches, it's nicer to have those
logically separate.

At some point we should probably also improve libbpf error reporting
for such situations, for better user experience. We have a similar
problem with CO-RE relocation, verifier doesn't know about those
concepts, so verifier log is not very helpful, but libbpf can make
sense out of it with some extra BPF verifier log parsing.

> Changes since v2:
>  - Move special handling and warning from find_ksym_btf_id() to
>    bpf_object__resolve_ksym_var_btf_id().
>  - Removed bpf_link_fops3 from tests since it's not used.
>  - Separated variable declaration and statements.
>
> Changes since v1:
>  - Weak typed symbols default to zero, as suggested by Andrii.
>  - Use ASSERT_XXX() for tests.
>
>  tools/lib/bpf/libbpf.c                        | 16 +++---
>  .../selftests/bpf/prog_tests/ksyms_btf.c      | 31 ++++++++++
>  .../selftests/bpf/progs/test_ksyms_weak.c     | 56 +++++++++++++++++++
>  3 files changed, 96 insertions(+), 7 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c
>

[...]

Re: [PATCH bpf-next v3] libbpf: support weak typed ksyms.

[Hao Luo]

On Fri, Aug 13, 2021 at 4:01 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Aug 11, 2021 at 5:40 PM Hao Luo <haoluo@google.com> wrote:
> >
> > 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 <haoluo@google.com>
> > ---
>
> Looks good, applied to bpf-next. For the future, please split libbpf
> and selftests changes into separate patches, it's nicer to have those
> logically separate.
>
> At some point we should probably also improve libbpf error reporting
> for such situations, for better user experience. We have a similar
> problem with CO-RE relocation, verifier doesn't know about those
> concepts, so verifier log is not very helpful, but libbpf can make
> sense out of it with some extra BPF verifier log parsing.
>

Thanks. Will split the tests in future.


> > Changes since v2:
> >  - Move special handling and warning from find_ksym_btf_id() to
> >    bpf_object__resolve_ksym_var_btf_id().
> >  - Removed bpf_link_fops3 from tests since it's not used.
> >  - Separated variable declaration and statements.
> >
> > Changes since v1:
> >  - Weak typed symbols default to zero, as suggested by Andrii.
> >  - Use ASSERT_XXX() for tests.
> >
> >  tools/lib/bpf/libbpf.c                        | 16 +++---
> >  .../selftests/bpf/prog_tests/ksyms_btf.c      | 31 ++++++++++
> >  .../selftests/bpf/progs/test_ksyms_weak.c     | 56 +++++++++++++++++++
> >  3 files changed, 96 insertions(+), 7 deletions(-)
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> >
>
> [...]