selftests/bpf: Cover negative buffer pointer offsets

Add verifier coverage for constant negative offsets on PTR_TO_TP_BUFFER
and PTR_TO_BUF pointers. Both programs adjust the buffer pointer by -8
and access it at offset zero, so the negative effective start must be
rejected at load time.

Switch the raw tracepoint writable attach checks from nbd_send_request
to bpf_testmod_test_writable_bare_tp, avoiding a dependency on the NBD
tracepoint. Keep the existing past-end case and add a case with a
negative var_off compensated by a positive instruction offset. The
effective start remains non-negative, so the program loads, but its
access end exceeds the writable context size and
bpf_raw_tracepoint_open() must return -EINVAL.

Cc: stable@vger.kernel.org # 5.2.0
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://patch.msgid.link/20260714093846.18159-3-sun.jian.kdev@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

Author: ahjf-07

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

@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "test_kmods/bpf_testmod.h"
+#include "bpf_util.h"
+
+static void check_attach_reject(const struct bpf_insn *program, size_t prog_len)
+{
+	LIBBPF_OPTS(bpf_prog_load_opts, opts);
+	char error[4096];
+	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("bpf_testmod_test_writable_bare_tp", bpf_fd);
+	ASSERT_EQ(tp_fd, -EINVAL, "bpf_raw_tracepoint_open");
+	if (tp_fd >= 0)
+		close(tp_fd);
+
+	close(bpf_fd);
+}
+
+void test_raw_tp_writable_reject_bad_access(void)
+{
+	const struct bpf_insn program[] = {
+		/* r6 is our tp buffer */
+		BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 0),
+		/* one byte beyond the end of the writable context */
+		BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_6,
+			    sizeof(struct bpf_testmod_test_writable_ctx)),
+		BPF_EXIT_INSN(),
+	};
+
+	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 writable context */
+		BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_6,
+			    sizeof(struct bpf_testmod_test_writable_ctx) + 8),
+		BPF_EXIT_INSN(),
+	};
+
+	if (test__start_subtest("past_end"))
+		check_attach_reject(program, ARRAY_SIZE(program));
+
+	if (test__start_subtest("negative_var_off_past_end"))
+		check_attach_reject(negative_var_off_program,
+				    ARRAY_SIZE(negative_var_off_program));
+}

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

@@ -78,6 +78,7 @@
 #include "verifier_precision.skel.h"
 #include "verifier_prevent_map_lookup.skel.h"
 #include "verifier_private_stack.skel.h"
+#include "verifier_ptr_to_buf.skel.h"
 #include "verifier_raw_stack.skel.h"
 #include "verifier_raw_tp_writable.skel.h"
 #include "verifier_reg_equal.skel.h"
@@ -230,6 +231,7 @@ void test_verifier_or_jmp32_k(void)           { RUN(verifier_or_jmp32_k); }
 void test_verifier_precision(void)            { RUN(verifier_precision); }
 void test_verifier_prevent_map_lookup(void)   { RUN(verifier_prevent_map_lookup); }
 void test_verifier_private_stack(void)        { RUN(verifier_private_stack); }
+void test_verifier_ptr_to_buf(void)           { RUN(verifier_ptr_to_buf); }
 void test_verifier_raw_stack(void)            { RUN(verifier_raw_stack); }
 void test_verifier_raw_tp_writable(void)      { RUN(verifier_raw_tp_writable); }
 void test_verifier_reg_equal(void)            { RUN(verifier_reg_equal); }

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

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+SEC("iter/bpf_map_elem")
+__description("PTR_TO_BUF: reject negative const offset")
+__failure
+__msg("invalid negative rdwr buffer offset")
+__naked void ptr_to_buf_reject_negative_const_offset(void)
+{
+	asm volatile ("r0 = 0;					\
+	 r2 = *(u64 *)(r1 + %[value_off]);			\
+	 if r2 == 0 goto l0_%=;					\
+	 r2 += -8;						\
+	 r0 = *(u64 *)(r2 + 0);					\
+l0_%=:								\
+	 exit;							\
+	"
+	:
+	: __imm_const(value_off,
+		      offsetof(struct bpf_iter__bpf_map_elem, value))
+	: __clobber_all);
+}
+
+char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/verifier_ptr_to_buf.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";