[PATCH bpf-next v4 3/3] bpf, selftest/bpf: Fix re-attachment branch in bpf_tracing_prog_attach

[Dmitrii Dolgov <9erthalion6@gmail.com>]

It looks like there is an issue in bpf_tracing_prog_attach, in the
"prog->aux->dst_trampoline and tgt_prog is NULL" case. One can construct
a sequence of events when prog->aux->attach_btf will be NULL, and
bpf_trampoline_compute_key will fail.

    BUG: kernel NULL pointer dereference, address: 0000000000000058
    Call Trace:
     <TASK>
     ? __die+0x20/0x70
     ? page_fault_oops+0x15b/0x430
     ? fixup_exception+0x22/0x330
     ? exc_page_fault+0x6f/0x170
     ? asm_exc_page_fault+0x22/0x30
     ? bpf_tracing_prog_attach+0x279/0x560
     ? btf_obj_id+0x5/0x10
     bpf_tracing_prog_attach+0x439/0x560
     __sys_bpf+0x1cf4/0x2de0
     __x64_sys_bpf+0x1c/0x30
     do_syscall_64+0x41/0xf0
     entry_SYSCALL_64_after_hwframe+0x6e/0x76

The issue seems to be not relevant to the previous changes with
recursive tracing prog attach, because the reproducing test doesn't
actually include recursive fentry attaching.

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
---
 kernel/bpf/syscall.c                          |  4 +-
 .../bpf/prog_tests/recursive_attach.c         | 48 +++++++++++++++++++
 .../bpf/progs/fentry_recursive_target.c       | 11 +++++
 3 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a595d7a62dbc..e01a949dfed7 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3197,7 +3197,9 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 			goto out_unlock;
 		}
 		btf_id = prog->aux->attach_btf_id;
-		key = bpf_trampoline_compute_key(NULL, prog->aux->attach_btf, btf_id);
+		if (prog->aux->attach_btf)
+			key = bpf_trampoline_compute_key(NULL, prog->aux->attach_btf,
+											 btf_id);
 	}
 
 	if (!prog->aux->dst_trampoline ||
diff --git a/tools/testing/selftests/bpf/prog_tests/recursive_attach.c b/tools/testing/selftests/bpf/prog_tests/recursive_attach.c
index 9c422dd92c4e..a4abf1745e62 100644
--- a/tools/testing/selftests/bpf/prog_tests/recursive_attach.c
+++ b/tools/testing/selftests/bpf/prog_tests/recursive_attach.c
@@ -83,3 +83,51 @@ void test_recursive_fentry_attach(void)
 			fentry_recursive__destroy(tracing_chain[i]);
 	}
 }
+
+/*
+ * Test that a tracing prog reattachment (when we land in
+ * "prog->aux->dst_trampoline and tgt_prog is NULL" branch in
+ * bpf_tracing_prog_attach) does not lead to a crash due to missing attach_btf
+ */
+void test_fentry_attach_btf_presence(void)
+{
+	struct fentry_recursive_target *target_skel = NULL;
+	struct fentry_recursive *tracing_skel = NULL;
+	struct bpf_program *prog;
+	int err, link_fd, tgt_prog_fd;
+
+	target_skel = fentry_recursive_target__open_and_load();
+	if (!ASSERT_OK_PTR(target_skel, "fentry_recursive_target__open_and_load"))
+		goto close_prog;
+
+	tracing_skel = fentry_recursive__open();
+	if (!ASSERT_OK_PTR(tracing_skel, "fentry_recursive__open"))
+		goto close_prog;
+
+	prog = tracing_skel->progs.recursive_attach;
+	tgt_prog_fd = bpf_program__fd(target_skel->progs.fentry_target);
+	err = bpf_program__set_attach_target(prog, tgt_prog_fd, "fentry_target");
+	if (!ASSERT_OK(err, "bpf_program__set_attach_target"))
+		goto close_prog;
+
+	err = fentry_recursive__load(tracing_skel);
+	if (!ASSERT_OK(err, "fentry_recursive__load"))
+		goto close_prog;
+
+	LIBBPF_OPTS(bpf_link_create_opts, link_opts);
+
+	link_fd = bpf_link_create(bpf_program__fd(tracing_skel->progs.recursive_attach),
+							  0, BPF_TRACE_FENTRY, &link_opts);
+	if (!ASSERT_GE(link_fd, 0, "link_fd"))
+		goto close_prog;
+
+	fentry_recursive__detach(tracing_skel);
+
+	err = fentry_recursive__attach(tracing_skel);
+	if (!ASSERT_ERR(err, "fentry_recursive__attach"))
+		goto close_prog;
+
+close_prog:
+	fentry_recursive_target__destroy(target_skel);
+	fentry_recursive__destroy(tracing_skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/fentry_recursive_target.c b/tools/testing/selftests/bpf/progs/fentry_recursive_target.c
index b6fb8ebd598d..f812d2de0c3c 100644
--- a/tools/testing/selftests/bpf/progs/fentry_recursive_target.c
+++ b/tools/testing/selftests/bpf/progs/fentry_recursive_target.c
@@ -18,3 +18,14 @@ int BPF_PROG(test1, int a)
 	test1_result = a == 1;
 	return 0;
 }
+
+/*
+ * Dummy bpf prog for testing attach_btf presence when attaching an fentry
+ * program.
+ */
+SEC("raw_tp/sys_enter")
+int BPF_PROG(fentry_target, struct pt_regs *regs, long id)
+{
+	test1_result = id == 1;
+	return 0;
+}
-- 
2.41.0