[PATCH bpf-next v1 6/9] selftests/bpf: Add JIT-sequence tests for __arena kfunc arguments

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

From: Tejun Heo <tj@kernel.org>

Pin the exact rebase sequences the JITs emit for __arena and
__arena_nullable kfunc arguments with __jited assertions on x86-64: the
unconditional truncate-and-add, the nullable test-and-skip variant, and
all five argument registers in one call, which also covers the
REX-prefixed encoding of r8 on x86. The capture kfuncs take the argument
without dereferencing, so only the emitted code is under test.  The
tests skip without LLVM disassembler support.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../bpf/prog_tests/arena_kfunc_jit.c          | 13 +++
 .../selftests/bpf/progs/arena_kfunc_jit.c     | 96 +++++++++++++++++++
 2 files changed, 109 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_kfunc_jit.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_kfunc_jit.c

diff --git a/tools/testing/selftests/bpf/prog_tests/arena_kfunc_jit.c b/tools/testing/selftests/bpf/prog_tests/arena_kfunc_jit.c
new file mode 100644
index 000000000000..2359cde24c45
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/arena_kfunc_jit.c
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <test_progs.h>
+#include "arena_kfunc_jit.skel.h"
+
+/*
+ * Runs with full capabilities: resolving module kfunc ksyms requires
+ * CAP_SYS_ADMIN, which rules out the capability-restricted runner.
+ */
+void test_arena_kfunc_jit(void)
+{
+	RUN_TESTS(arena_kfunc_jit);
+}
diff --git a/tools/testing/selftests/bpf/progs/arena_kfunc_jit.c b/tools/testing/selftests/bpf/progs/arena_kfunc_jit.c
new file mode 100644
index 000000000000..84747611725f
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/arena_kfunc_jit.c
@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+/*
+ * Verify the JIT-emitted rebase sequences for __arena and __arena_nullable
+ * kfunc arguments. The capture kfuncs take the argument without
+ * dereferencing it, so these tests pin only the emitted code.
+ */
+#define BPF_NO_KFUNC_PROTOTYPES
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+#include "bpf_experimental.h"
+#include <bpf_arena_common.h>
+#include "../test_kmods/bpf_testmod_kfunc.h"
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARENA);
+	__uint(map_flags, BPF_F_MMAPABLE);
+	__uint(max_entries, 1);
+} arena SEC(".maps");
+
+/* volatile to force the scalar reloads below */
+volatile u64 stash;
+
+#if defined(__BPF_FEATURE_ADDR_SPACE_CAST)
+
+SEC("syscall")
+__arch_x86_64
+__jited("...")
+__jited("	movl	%edi, %edi")
+__jited("	addq	%r12, %rdi")
+__jited("...")
+__jited("	callq	{{.*}}")
+__success
+int arena_arg_jit_rebase(void *ctx)
+{
+	stash = (u64)bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	bpf_kfunc_arena_cap_test((u64 *)stash);
+	return 0;
+}
+
+SEC("syscall")
+__arch_x86_64
+__jited("...")
+__jited("	movl	%edi, %edi")
+__jited("	testl	%edi, %edi")
+__jited("	je	{{.*}}")
+__jited("	addq	%r12, %rdi")
+__success
+int arena_arg_jit_nullable(void *ctx)
+{
+	stash = (u64)bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	bpf_kfunc_arena_cap_nullable_test((u64 *)stash);
+	return 0;
+}
+
+SEC("syscall")
+__arch_x86_64
+__jited("...")
+__jited("	movl	%edi, %edi")
+__jited("	addq	%r12, %rdi")
+__jited("	movl	%esi, %esi")
+__jited("	addq	%r12, %rsi")
+__jited("	movl	%edx, %edx")
+__jited("	addq	%r12, %rdx")
+__jited("	movl	%ecx, %ecx")
+__jited("	addq	%r12, %rcx")
+__jited("	movl	%r8d, %r8d")
+__jited("	testl	%r8d, %r8d")
+__jited("	je	{{.*}}")
+__jited("	addq	%r12, %r8")
+__success
+int arena_arg_jit_args5(void *ctx)
+{
+	u64 __arena *val;
+
+	val = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	if (!val)
+		return 1;
+
+	val[0] = 1;
+	val[1] = 2;
+	val[2] = 4;
+	val[3] = 8;
+	val[4] = 16;
+
+	bpf_kfunc_arena_args5_test((u64 *)&val[0], (u64 *)&val[1],
+				   (u64 *)&val[2], (u64 *)&val[3],
+				   (u64 *)&val[4]);
+	return 0;
+}
+
+#endif /* __BPF_FEATURE_ADDR_SPACE_CAST */
+
+char _license[] SEC("license") = "GPL";
-- 
2.53.0