[PATCH bpf v2 2/2] selftests/bpf: Cover negative raw_tp writable buffer offsets

[Sun Jian <sun.jian.kdev@gmail.com>]

Add raw tracepoint writable coverage for buffer accesses involving
negative constant pointer adjustments.

The verifier case checks that a negative effective offset is rejected.
The attach-time case adds incremental coverage beyond the existing nbd
test and the verifier rejection case: it uses a negative var_off and a
positive instruction offset whose effective offset remains non-negative.
This exercises the checked access_end accounting path and verifies that
the resulting max_tp_access is still checked against nbd_send_request's
writable size at attach time.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 .../raw_tp_writable_reject_nbd_invalid.c      | 58 +++++++++++--------
 .../bpf/progs/verifier_raw_tp_writable.c      | 16 +++++
 2 files changed, 51 insertions(+), 23 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/raw_tp_writable_reject_nbd_invalid.c b/tools/testing/selftests/bpf/prog_tests/raw_tp_writable_reject_nbd_invalid.c
index 216b0dfac0fe..efe4cad47b28 100644
--- a/tools/testing/selftests/bpf/prog_tests/raw_tp_writable_reject_nbd_invalid.c
+++ b/tools/testing/selftests/bpf/prog_tests/raw_tp_writable_reject_nbd_invalid.c
@@ -4,12 +4,31 @@
 #include <linux/nbd.h>
 #include "bpf_util.h"
 
-void test_raw_tp_writable_reject_nbd_invalid(void)
+static void check_nbd_attach_reject(const char *name,
+				    const struct bpf_insn *program, size_t prog_len)
 {
-	__u32 duration = 0;
+	LIBBPF_OPTS(bpf_prog_load_opts, opts);
 	char error[4096];
-	int bpf_fd = -1, tp_fd = -1;
+	int bpf_fd, tp_fd;
+
+	opts.log_level = 2;
+	opts.log_buf = error;
+	opts.log_size = sizeof(error);
+
+	bpf_fd = bpf_prog_load(BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE, NULL, "GPL v2",
+			       program, prog_len, &opts);
+	if (!ASSERT_GE(bpf_fd, 0, "prog_load"))
+		return;
+
+	tp_fd = bpf_raw_tracepoint_open("nbd_send_request", bpf_fd);
+	if (!ASSERT_LT(tp_fd, 0, name))
+		close(tp_fd);
+
+	close(bpf_fd);
+}
 
+void test_raw_tp_writable_reject_nbd_invalid(void)
+{
 	const struct bpf_insn program[] = {
 		/* r6 is our tp buffer */
 		BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 0),
@@ -19,25 +38,18 @@ void test_raw_tp_writable_reject_nbd_invalid(void)
 		BPF_EXIT_INSN(),
 	};
 
-	LIBBPF_OPTS(bpf_prog_load_opts, opts,
-		.log_level = 2,
-		.log_buf = error,
-		.log_size = sizeof(error),
-	);
-
-	bpf_fd = bpf_prog_load(BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE, NULL, "GPL v2",
-			       program, ARRAY_SIZE(program),
-			       &opts);
-	if (CHECK(bpf_fd < 0, "bpf_raw_tracepoint_writable load",
-		  "failed: %d errno %d\n", bpf_fd, errno))
-		return;
-
-	tp_fd = bpf_raw_tracepoint_open("nbd_send_request", bpf_fd);
-	if (CHECK(tp_fd >= 0, "bpf_raw_tracepoint_writable open",
-		  "erroneously succeeded\n"))
-		goto out_bpffd;
+	const struct bpf_insn negative_var_off_program[] = {
+		BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 0),
+		/* make var_off negative, but keep the effective access offset non-negative */
+		BPF_ALU64_IMM(BPF_ADD, BPF_REG_6, -8),
+		/* one byte beyond the end of the nbd_request struct */
+		BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_6,
+			    sizeof(struct nbd_request) + 8),
+		BPF_EXIT_INSN(),
+	};
 
-	close(tp_fd);
-out_bpffd:
-	close(bpf_fd);
+	check_nbd_attach_reject("nbd_invalid", program, ARRAY_SIZE(program));
+	check_nbd_attach_reject("nbd_invalid_negative_var_off",
+				negative_var_off_program,
+				ARRAY_SIZE(negative_var_off_program));
 }
diff --git a/tools/testing/selftests/bpf/progs/verifier_raw_tp_writable.c b/tools/testing/selftests/bpf/progs/verifier_raw_tp_writable.c
index 14a0172e2141..4055a6443bc2 100644
--- a/tools/testing/selftests/bpf/progs/verifier_raw_tp_writable.c
+++ b/tools/testing/selftests/bpf/progs/verifier_raw_tp_writable.c
@@ -47,4 +47,20 @@ l0_%=:	/* shift the buffer pointer to a variable location */\
 	: __clobber_all);
 }
 
+SEC("raw_tracepoint.w")
+__description("raw_tracepoint_writable: reject negative const offset")
+__failure
+__msg("invalid negative tracepoint buffer offset")
+__naked void tracepoint_writable_reject_negative_const_offset(void)
+{
+	asm volatile ("					\
+	r6 = *(u64 *)(r1 + 0);				\
+	r6 += -8;					\
+	r0 = *(u64 *)(r6 + 0);				\
+	exit;						\
+"	:
+	:
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0