[PATCH bpf-next v2 20/23] selftests/bpf: Add precision backtracking test for stack arguments

[Yonghong Song <yonghong.song@linux.dev>]

Add a test that verifies precision backtracking works correctly
across BPF-to-BPF calls when stack arguments are involved.

The test passes a size value as incoming stack arg (arg6) to a
subprog, which bounds-checks it and forwards it as the mem__sz
parameter (outgoing arg7) to bpf_kfunc_call_stack_arg_mem. The
expected __msg annotations verify that precision propagates from
the kfunc's mem__sz argument back through the subprog frame to the
caller's outgoing stack arg store.

A companion BTF file (btf__stack_arg_precision.c) provides named
parameter BTF for the __naked subprog via __btf_func_path.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 .../bpf/prog_tests/stack_arg_precision.c      |  10 ++
 .../bpf/progs/btf__stack_arg_precision.c      |  23 +++
 .../selftests/bpf/progs/stack_arg_precision.c | 137 ++++++++++++++++++
 3 files changed, 170 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/stack_arg_precision.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__stack_arg_precision.c
 create mode 100644 tools/testing/selftests/bpf/progs/stack_arg_precision.c

diff --git a/tools/testing/selftests/bpf/prog_tests/stack_arg_precision.c b/tools/testing/selftests/bpf/prog_tests/stack_arg_precision.c
new file mode 100644
index 000000000000..1ab041d66de3
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/stack_arg_precision.c
@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include "stack_arg_precision.skel.h"
+
+void test_stack_arg_precision(void)
+{
+	RUN_TESTS(stack_arg_precision);
+}
diff --git a/tools/testing/selftests/bpf/progs/btf__stack_arg_precision.c b/tools/testing/selftests/bpf/progs/btf__stack_arg_precision.c
new file mode 100644
index 000000000000..296fddfe6804
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/btf__stack_arg_precision.c
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "../test_kmods/bpf_testmod_kfunc.h"
+
+#if defined(__TARGET_ARCH_x86) && defined(__BPF_FEATURE_STACK_ARGUMENT)
+
+long subprog_call_mem_kfunc(long a, long b, long c, long d, long e, long size)
+{
+	char buf[8] = {};
+
+	return bpf_kfunc_call_stack_arg_mem(a, b, c, d, e, buf, size);
+}
+
+#else
+
+long subprog_call_mem_kfunc(void)
+{
+	return 0;
+}
+
+#endif
diff --git a/tools/testing/selftests/bpf/progs/stack_arg_precision.c b/tools/testing/selftests/bpf/progs/stack_arg_precision.c
new file mode 100644
index 000000000000..c94905f07dcc
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/stack_arg_precision.c
@@ -0,0 +1,137 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "../test_kmods/bpf_testmod_kfunc.h"
+#include "bpf_misc.h"
+
+#if defined(__TARGET_ARCH_x86) && defined(__BPF_FEATURE_STACK_ARGUMENT)
+
+/* Force kfunc extern BTF generation for inline asm call below.
+ * Uses its own SEC so it's not included as a .text subprog.
+ * The '?' prefix sets autoload=false so libbpf won't load it.
+ */
+SEC("?tc")
+int __btf_kfunc_gen(struct __sk_buff *ctx)
+{
+	char buf[8] = {};
+
+	return bpf_kfunc_call_stack_arg_mem(0, 0, 0, 0, 0, buf, sizeof(buf));
+}
+
+/*
+ * Test precision backtracking across bpf-to-bpf call for kfunc stack arg.
+ * subprog_call_mem_kfunc receives a size as incoming stack arg (arg6),
+ * bounds-checks it, then passes it as mem__sz (arg7) to
+ * bpf_kfunc_call_stack_arg_mem.
+ *
+ * 1+2+3+4+5+(1+2+3+4) = 25
+ */
+__naked __noinline __used
+static long subprog_call_mem_kfunc(long a, long b, long c, long d, long e, long size)
+{
+	asm volatile (
+		"r1 = *(u64 *)(r11 + 8);"	/* r1 = incoming arg6 (size) */
+		"r2 = 0x0807060504030201 ll;"	/* r2 = buf contents */
+		"*(u64 *)(r10 - 8) = r2;"	/* store buf to stack */
+		"r2 = r10;"
+		"r2 += -8;"			/* r2 = &buf */
+		"*(u64 *)(r11 - 8) = r2;"	/* outgoing arg6 = buf */
+		"*(u64 *)(r11 - 16) = r1;"	/* outgoing arg7 = size */
+		"r1 = 1;"
+		"r2 = 2;"
+		"r3 = 3;"
+		"r4 = 4;"
+		"r5 = 5;"
+		"call %[bpf_kfunc_call_stack_arg_mem];"
+		"exit;"
+		:
+		: __imm(bpf_kfunc_call_stack_arg_mem)
+		: __clobber_all
+	);
+}
+
+SEC("tc")
+__description("stack_arg: precision backtracking across bpf2bpf call for kfunc")
+__success __retval(25)
+__log_level(2)
+__flag(BPF_F_TEST_STATE_FREQ)
+__btf_func_path("btf__stack_arg_precision.bpf.o")
+__msg("mark_precise: frame1: last_idx 26 first_idx 13 subseq_idx -1")
+__msg("mark_precise: frame1: regs= stack= before 25: (b7) r5 = 5")
+__msg("mark_precise: frame1: regs= stack= before 24: (b7) r4 = 4")
+__msg("mark_precise: frame1: regs= stack= before 23: (b7) r3 = 3")
+__msg("mark_precise: frame1: regs= stack= before 22: (b7) r2 = 2")
+__msg("mark_precise: frame1: regs= stack= before 21: (b7) r1 = 1")
+__msg("mark_precise: frame1: regs= stack= before 20: (7b) *(u64 *)(r11 -16) = r1")
+__msg("mark_precise: frame1: regs=r1 stack= before 19: (7b) *(u64 *)(r11 -8) = r2")
+__msg("mark_precise: frame1: regs=r1 stack= before 18: (07) r2 += -8")
+__msg("mark_precise: frame1: regs=r1 stack= before 17: (bf) r2 = r10")
+__msg("mark_precise: frame1: regs=r1 stack= before 16: (7b) *(u64 *)(r10 -8) = r2")
+__msg("mark_precise: frame1: regs=r1 stack= before 14: (18) r2 = 0x807060504030201")
+__msg("mark_precise: frame1: regs=r1 stack= before 13: (79) r1 = *(u64 *)(r11 +8)")
+__msg("mark_precise: frame1: parent state regs= stack=:  frame1: R10=fp0")
+__msg("mark_precise: frame0: parent state regs= stack=:  R10=fp0")
+__msg("mark_precise: frame1: last_idx 11 first_idx 11 subseq_idx 13")
+__msg("mark_precise: frame1: regs= stack= before 11: (85) call pc+1")
+__msg("mark_precise: frame0: parent state regs= stack=:  R1=1 R2=2 R3=3 R4=4 R5=5 R10=fp0")
+__msg("mark_precise: frame0: last_idx 9 first_idx 7 subseq_idx 11")
+__msg("mark_precise: frame0: regs= stack= before 9: (05) goto pc+1")
+__msg("mark_precise: frame0: regs= stack= before 8: (7a) *(u64 *)(r11 -8) = 4")
+__msg("mark_precise: frame1: last_idx 26 first_idx 13 subseq_idx -1 ")
+__msg("mark_precise: frame1: regs= stack= before 25: (b7) r5 = 5")
+__msg("mark_precise: frame1: regs= stack= before 24: (b7) r4 = 4")
+__msg("mark_precise: frame1: regs= stack= before 23: (b7) r3 = 3")
+__msg("mark_precise: frame1: regs= stack= before 22: (b7) r2 = 2")
+__msg("mark_precise: frame1: regs= stack= before 21: (b7) r1 = 1")
+__msg("mark_precise: frame1: regs= stack= before 20: (7b) *(u64 *)(r11 -16) = r1")
+__msg("mark_precise: frame1: regs=r1 stack= before 19: (7b) *(u64 *)(r11 -8) = r2")
+__msg("mark_precise: frame1: regs=r1 stack= before 18: (07) r2 += -8")
+__msg("mark_precise: frame1: regs=r1 stack= before 17: (bf) r2 = r10")
+__msg("mark_precise: frame1: regs=r1 stack= before 16: (7b) *(u64 *)(r10 -8) = r2")
+__msg("mark_precise: frame1: regs=r1 stack= before 14: (18) r2 = 0x807060504030201")
+__msg("mark_precise: frame1: regs=r1 stack= before 13: (79) r1 = *(u64 *)(r11 +8)")
+__msg("mark_precise: frame1: parent state regs= stack=:  frame1: R10=fp0")
+__msg("mark_precise: frame0: parent state regs= stack=:  R10=fp0")
+__msg("mark_precise: frame1: last_idx 11 first_idx 11 subseq_idx 13 ")
+__msg("mark_precise: frame1: regs= stack= before 11: (85) call pc+1")
+__msg("mark_precise: frame0: parent state regs= stack=:  R1=1 R2=2 R3=3 R4=4 R5=5 R10=fp0")
+__msg("mark_precise: frame0: last_idx 10 first_idx 10 subseq_idx 11 ")
+__msg("mark_precise: frame0: regs= stack= before 10: (7a) *(u64 *)(r11 -8) = 6")
+__naked void stack_arg_precision_bpf2bpf(void)
+{
+	asm volatile (
+		"call %[bpf_get_prandom_u32];"
+		"r6 = r0;"
+		"r1 = 1;"
+		"r2 = 2;"
+		"r3 = 3;"
+		"r4 = 4;"
+		"r5 = 5;"
+		"if r6 < 2 goto l0_%=;"
+		"*(u64 *)(r11 - 8) = 4;"
+		"goto l1_%=;"
+	"l0_%=:"
+		"*(u64 *)(r11 - 8) = 6;"
+	"l1_%=:"
+		"call subprog_call_mem_kfunc;"
+		"exit;"
+		:: __imm(bpf_get_prandom_u32)
+		: __clobber_all
+	);
+}
+
+#else
+
+SEC("socket")
+__description("stack_arg_precision: not supported, dummy test")
+__success
+int dummy_test(void)
+{
+	return 0;
+}
+
+#endif
+
+char _license[] SEC("license") = "GPL";
-- 
2.53.0-Meta