[PATCH 0/2] arm64: ftrace: enable single ftrace_ops for direct calls

[PATCH 0/2] arm64: ftrace: enable single ftrace_ops for direct calls

[Leon Hwang]

Beyond the DIRECT_CALLS support on arm64 [1], select
HAVE_SINGLE_FTRACE_DIRECT_OPS for arm64, which enables the BPF
tracing_multi link on arm64 [2].

All tracing_multi link selftests are passed on arm64.

 cd tools/testing/selftests/bpf
 ./test_progs -a 'tracing_multi_*'
 #545/1   tracing_multi_attach_rollback/put:OK
 #545/2   tracing_multi_attach_rollback/unlink:OK
 #545     tracing_multi_attach_rollback:OK
 #546     tracing_multi_bench_attach:OK
 #547/1   tracing_multi_test/skel_api:OK
 #547/2   tracing_multi_test/link_api_pattern:OK
 #547/3   tracing_multi_test/link_api_ids:OK
 #547/4   tracing_multi_test/module_skel_api:OK
 #547/5   tracing_multi_test/module_link_api_pattern:OK
 #547/6   tracing_multi_test/module_link_api_ids:OK
 #547/7   tracing_multi_test/intersect:OK
 #547/8   tracing_multi_test/cookies:OK
 #547/9   tracing_multi_test/session:OK
 #547/10  tracing_multi_test/attach_api_fails:OK
 #547/11  tracing_multi_test/fentry_direct_access:OK
 #547/12  tracing_multi_test/fexit_direct_access:OK
 #547/13  tracing_multi_test/fsession_direct_access:OK
 #547/14  tracing_multi_test/fentry_after_multi:OK
 #547     tracing_multi_test:OK
 Summary: 3/16 PASSED, 0 SKIPPED, 0 FAILED

 ./test_progs -a 'fill_link_info/tracing_multi_*'
 #132/11  fill_link_info/tracing_multi_link_info:OK
 #132/12  fill_link_info/tracing_multi_invalid_ubuff:OK
 #132     fill_link_info:OK
 Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

Links:
[1] https://lore.kernel.org/all/20260609-arm64-ftrace-direct-calls-v1-0-4a46f266697f@linux.dev/
[2] https://github.com/kernel-patches/bpf/pull/12986

Leon Hwang (2):
  arm64: ftrace: enable single ftrace_ops for direct calls
  selftests/bpf: Enable tracing_multi tests on arm64

 arch/arm64/Kconfig                                     | 2 ++
 arch/arm64/kernel/ftrace.c                             | 3 +--
 tools/testing/selftests/bpf/prog_tests/tracing_multi.c | 9 +++++++--
 tools/testing/selftests/bpf/test_progs.h               | 6 ++++++
 4 files changed, 16 insertions(+), 4 deletions(-)

-- 
2.55.0

[PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls

[Leon Hwang]

The BPF tracing multi link updates several direct-call sites through one
ftrace_ops. Its implementation is therefore gated by
HAVE_SINGLE_FTRACE_DIRECT_OPS in addition to
DYNAMIC_FTRACE_WITH_DIRECT_CALLS.

Select HAVE_SINGLE_FTRACE_DIRECT_OPS whenever arm64 enables dynamic ftrace
direct calls. This enables BPF tracing multi links on arm64. Also
generalize the unreachable-trampoline comment because the single-ops path
does not use ops->direct_call.

Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 arch/arm64/Kconfig         | 2 ++
 arch/arm64/kernel/ftrace.c | 3 +--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 0de419ed780f..c98dca76859b 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -188,6 +188,8 @@ config ARM64
 		    CLANG_SUPPORTS_DYNAMIC_FTRACE_WITH_ARGS)
 	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS \
 		if DYNAMIC_FTRACE_WITH_ARGS
+	select HAVE_SINGLE_FTRACE_DIRECT_OPS \
+		if DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 	select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS \
 		if (DYNAMIC_FTRACE_WITH_ARGS && !CFI && \
 		    (CC_IS_CLANG || !CC_OPTIMIZE_FOR_SIZE))
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index e1a3c0b3a051..56ba72a87dfa 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -301,8 +301,7 @@ static bool ftrace_find_callable_addr(struct dyn_ftrace *rec,
 
 	/*
 	 * If a custom trampoline is unreachable, rely on the ftrace_caller
-	 * trampoline which knows how to indirectly reach that trampoline
-	 * through ops->direct_call.
+	 * trampoline which knows how to indirectly reach that trampoline.
 	 */
 	if (*addr != FTRACE_ADDR && !reachable_by_bl(*addr, pc))
 		*addr = FTRACE_ADDR;
-- 
2.55.0

Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls

[Jiri Olsa]

On Mon, Jul 27, 2026 at 10:28:43PM +0800, Leon Hwang wrote:
> The BPF tracing multi link updates several direct-call sites through one
> ftrace_ops. Its implementation is therefore gated by
> HAVE_SINGLE_FTRACE_DIRECT_OPS in addition to
> DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
> 
> Select HAVE_SINGLE_FTRACE_DIRECT_OPS whenever arm64 enables dynamic ftrace
> direct calls. This enables BPF tracing multi links on arm64. Also
> generalize the unreachable-trampoline comment because the single-ops path
> does not use ops->direct_call.

hi,
iirc arm needs ops->direct_call for trampolines to work properly,
that's the reason we enabled single ftrace_ops on x86 only:

  424f6a361096 bpf,x86: Use single ftrace_ops for direct calls

I don't understand the arm usage of op->direct_call, but it looks like
(codex thinks) that the fast path won't always work without op->direct_call
being set

from arch/arm64/kernel/entry-ftrace.S:

	#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
		/*
		 * If the op has a direct call, handle it immediately without
		 * saving/restoring registers.
		 */
		ldr     x17, [x11, #FTRACE_OPS_DIRECT_CALL]             // op->direct_call
		cbnz    x17, ftrace_caller_direct
	#endif
	#endif

jirka

Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls

[Leon Hwang]

On 29/7/26 19:56, Jiri Olsa wrote:
> On Mon, Jul 27, 2026 at 10:28:43PM +0800, Leon Hwang wrote:
>> The BPF tracing multi link updates several direct-call sites through one
>> ftrace_ops. Its implementation is therefore gated by
>> HAVE_SINGLE_FTRACE_DIRECT_OPS in addition to
>> DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
>>
>> Select HAVE_SINGLE_FTRACE_DIRECT_OPS whenever arm64 enables dynamic ftrace
>> direct calls. This enables BPF tracing multi links on arm64. Also
>> generalize the unreachable-trampoline comment because the single-ops path
>> does not use ops->direct_call.
> 
> hi,
> iirc arm needs ops->direct_call for trampolines to work properly,
> that's the reason we enabled single ftrace_ops on x86 only:
> 
>   424f6a361096 bpf,x86: Use single ftrace_ops for direct calls
> 
> I don't understand the arm usage of op->direct_call, but it looks like
> (codex thinks) that the fast path won't always work without op->direct_call
> being set


In theory, op->direct_call is the fallback for the case:
HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS=y, out-of-range target, and non-BPF
direct ops. This is what Codex thinks about.

However, after searching the call sites of
register_ftrace_direct()/__modify_ftrace_direct() in kernel, which would
set ops->direct_call, the actual callers are in bpf/trampoline.c, which
have been gated with HAVE_SINGLE_FTRACE_DIRECT_OPS. That said, for
normal usage of HAVE_SINGLE_FTRACE_DIRECT_OPS=y, op->direct_call is unused.

If someone uses register_ftrace_direct() or
modify_ftrace_direct/_nolock() outside BPF in the future,
op->direct_call will be used.

So, I'd like to drop the comment change in the next revision.

Thanks,
Leon

> 
> from arch/arm64/kernel/entry-ftrace.S:
> 
> 	#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> 		/*
> 		 * If the op has a direct call, handle it immediately without
> 		 * saving/restoring registers.
> 		 */
> 		ldr     x17, [x11, #FTRACE_OPS_DIRECT_CALL]             // op->direct_call
> 		cbnz    x17, ftrace_caller_direct
> 	#endif
> 	#endif
> 
> jirka

Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls

[Jiri Olsa]

On Thu, Jul 30, 2026 at 11:05:42AM +0800, Leon Hwang wrote:
> On 29/7/26 19:56, Jiri Olsa wrote:
> > On Mon, Jul 27, 2026 at 10:28:43PM +0800, Leon Hwang wrote:
> >> The BPF tracing multi link updates several direct-call sites through one
> >> ftrace_ops. Its implementation is therefore gated by
> >> HAVE_SINGLE_FTRACE_DIRECT_OPS in addition to
> >> DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
> >>
> >> Select HAVE_SINGLE_FTRACE_DIRECT_OPS whenever arm64 enables dynamic ftrace
> >> direct calls. This enables BPF tracing multi links on arm64. Also
> >> generalize the unreachable-trampoline comment because the single-ops path
> >> does not use ops->direct_call.
> > 
> > hi,
> > iirc arm needs ops->direct_call for trampolines to work properly,
> > that's the reason we enabled single ftrace_ops on x86 only:
> > 
> >   424f6a361096 bpf,x86: Use single ftrace_ops for direct calls
> > 
> > I don't understand the arm usage of op->direct_call, but it looks like
> > (codex thinks) that the fast path won't always work without op->direct_call
> > being set
> 
> 
> In theory, op->direct_call is the fallback for the case:
> HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS=y, out-of-range target, and non-BPF
> direct ops. This is what Codex thinks about.
> 
> However, after searching the call sites of
> register_ftrace_direct()/__modify_ftrace_direct() in kernel, which would
> set ops->direct_call, the actual callers are in bpf/trampoline.c, which
> have been gated with HAVE_SINGLE_FTRACE_DIRECT_OPS. That said, for
> normal usage of HAVE_SINGLE_FTRACE_DIRECT_OPS=y, op->direct_call is unused.
> 
> If someone uses register_ftrace_direct() or
> modify_ftrace_direct/_nolock() outside BPF in the future,
> op->direct_call will be used.

so any 2nd user on the same function triggers the slow path,
and that's fine, the single_ops handles slow path correctly

but IIUC the out-of-range target will now always go through slow path,
which in current code it doesn't, right?

jirka

[PATCH 2/2] selftests/bpf: Enable tracing_multi tests on arm64

[Leon Hwang]

arm64 now supports dynamic ftrace direct calls backed by a single
ftrace_ops, which enables BPF tracing multi links.

Define ARCH_SUPPORTS_TRACING_MULTI_LINK for x86-64 and arm64, and use the
shared capability to gate the tracing_multi functional, rollback,
and attach benchmark tests.

This allows the tests to run on arm64 while continuing to skip
architectures without tracing multi link support. It also prevents the
rollback test from treating an unsupported-link error as its expected
attachment failure.

Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 tools/testing/selftests/bpf/prog_tests/tracing_multi.c | 9 +++++++--
 tools/testing/selftests/bpf/test_progs.h               | 6 ++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/tracing_multi.c b/tools/testing/selftests/bpf/prog_tests/tracing_multi.c
index f02ffc7f41d7..5aa48e090340 100644
--- a/tools/testing/selftests/bpf/prog_tests/tracing_multi.c
+++ b/tools/testing/selftests/bpf/prog_tests/tracing_multi.c
@@ -612,7 +612,7 @@ void serial_test_tracing_multi_bench_attach(void)
 	struct btf *btf;
 	int err;
 
-#ifndef __x86_64__
+#if !ARCH_SUPPORTS_TRACING_MULTI_LINK
 	test__skip();
 	return;
 #endif
@@ -923,6 +923,11 @@ static void test_rollback_unlink(void)
 
 void serial_test_tracing_multi_attach_rollback(void)
 {
+#if !ARCH_SUPPORTS_TRACING_MULTI_LINK
+	test__skip();
+	return;
+#endif
+
 	if (test__start_subtest("put"))
 		test_rollback_put();
 	if (test__start_subtest("unlink"))
@@ -931,7 +936,7 @@ void serial_test_tracing_multi_attach_rollback(void)
 
 void test_tracing_multi_test(void)
 {
-#ifndef __x86_64__
+#if !ARCH_SUPPORTS_TRACING_MULTI_LINK
 	test__skip();
 	return;
 #endif
diff --git a/tools/testing/selftests/bpf/test_progs.h b/tools/testing/selftests/bpf/test_progs.h
index 2cf950afcd85..b1498e84773c 100644
--- a/tools/testing/selftests/bpf/test_progs.h
+++ b/tools/testing/selftests/bpf/test_progs.h
@@ -567,4 +567,10 @@ void validate_msgs(const char *log_buf, struct expected_msgs *msgs,
 void free_msgs(struct expected_msgs *msgs);
 void verify_test_stderr(struct bpf_object *obj, struct bpf_program *prog);
 
+#if defined(__x86_64__) || defined(__aarch64__)
+#define ARCH_SUPPORTS_TRACING_MULTI_LINK 1
+#else
+#define ARCH_SUPPORTS_TRACING_MULTI_LINK 0
+#endif
+
 #endif /* __TEST_PROGS_H */
-- 
2.55.0