[PATCHv3 bpf-next 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

[PATCHv3 bpf-next 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

[Jiri Olsa]

Mahe reported issue with bpf_override_return helper not working when
executed from kprobe.multi bpf program on arm.

The problem is that on arm we use alternate storage for pt_regs object
that is passed to bpf_prog_run and if any register is changed (which
is the case of bpf_override_return) it's not propagated back to actual
pt_regs object.

Fixing this by introducing and calling ftrace_partial_regs_update function
to propagate the values of changed registers (ip and stack).

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Will Deacon <will@kernel.org>
Reported-by: Mahe Tardy <mahe.tardy@gmail.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
v3 changes:
- drop const from fregs argument [ci]
- added acks

 include/linux/ftrace_regs.h | 25 +++++++++++++++++++++++++
 kernel/trace/bpf_trace.c    |  1 +
 2 files changed, 26 insertions(+)

diff --git a/include/linux/ftrace_regs.h b/include/linux/ftrace_regs.h
index 15627ceea9bc..386fa48c4a95 100644
--- a/include/linux/ftrace_regs.h
+++ b/include/linux/ftrace_regs.h
@@ -33,6 +33,31 @@ struct ftrace_regs;
 #define ftrace_regs_get_frame_pointer(fregs) \
 	frame_pointer(&arch_ftrace_regs(fregs)->regs)
 
+static __always_inline void
+ftrace_partial_regs_update(struct ftrace_regs *fregs, struct pt_regs *regs) { }
+
+#else
+
+/*
+ * ftrace_partial_regs_update - update the original ftrace_regs from regs
+ * @fregs: The ftrace_regs to update from @regs
+ * @regs: The partial regs from ftrace_partial_regs() that was updated
+ *
+ * Some architectures have the partial regs living in the ftrace_regs
+ * structure, whereas other architectures need to make a different copy
+ * of the @regs. If a partial @regs is retrieved by ftrace_partial_regs() and
+ * if the code using @regs updates a field (like the instruction pointer or
+ * stack pointer) it may need to propagate that change to the original @fregs
+ * it retrieved the partial @regs from. Use this function to guarantee that
+ * update happens.
+ */
+static __always_inline void
+ftrace_partial_regs_update(struct ftrace_regs *fregs, struct pt_regs *regs)
+{
+	ftrace_regs_set_instruction_pointer(fregs, instruction_pointer(regs));
+	ftrace_regs_set_return_value(fregs, regs_return_value(regs));
+}
+
 #endif /* HAVE_ARCH_FTRACE_REGS */
 
 /* This can be overridden by the architectures */
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 6e076485bf70..3a17f79b20c2 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2564,6 +2564,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
 	err = bpf_prog_run(link->link.prog, regs);
 	bpf_reset_run_ctx(old_run_ctx);
+	ftrace_partial_regs_update(fregs, bpf_kprobe_multi_pt_regs_ptr());
 	rcu_read_unlock();
 
  out:
-- 
2.52.0

[PATCHv3 bpf-next 2/2] selftests/bpf: Add test for bpf_override_return helper

[Jiri Olsa]

We do not actually test the bpf_override_return helper functionality
itself at the moment, only the bpf program being able to attach it.

Adding test that override prctl syscall return value on top of
kprobe and kprobe.multi.

Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 .../bpf/prog_tests/kprobe_multi_test.c        | 44 +++++++++++++++++++
 .../bpf/progs/kprobe_multi_override.c         | 15 +++++++
 tools/testing/selftests/bpf/trace_helpers.h   | 12 +++++
 3 files changed, 71 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
index 6cfaa978bc9a..9caef222e528 100644
--- a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
+++ b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
@@ -1,4 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
+#include <errno.h>
+#include <sys/prctl.h>
 #include <test_progs.h>
 #include "kprobe_multi.skel.h"
 #include "trace_helpers.h"
@@ -540,6 +542,46 @@ static void test_attach_override(void)
 	kprobe_multi_override__destroy(skel);
 }
 
+static void test_override(void)
+{
+	struct kprobe_multi_override *skel = NULL;
+	int err;
+
+	skel = kprobe_multi_override__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "kprobe_multi_empty__open_and_load"))
+		goto cleanup;
+
+	skel->bss->pid = getpid();
+
+	/* no override */
+	err = prctl(0xffff, 0);
+	ASSERT_EQ(err, -1, "err");
+
+	/* kprobe.multi override */
+	skel->links.test_override = bpf_program__attach_kprobe_multi_opts(skel->progs.test_override,
+						SYS_PREFIX "sys_prctl", NULL);
+	if (!ASSERT_OK_PTR(skel->links.test_override, "bpf_program__attach_kprobe_multi_opts"))
+		goto cleanup;
+
+	err = prctl(0xffff, 0);
+	ASSERT_EQ(err, 123, "err");
+
+	bpf_link__destroy(skel->links.test_override);
+	skel->links.test_override = NULL;
+
+	/* kprobe override */
+	skel->links.test_kprobe_override = bpf_program__attach_kprobe(skel->progs.test_kprobe_override,
+							false, SYS_PREFIX "sys_prctl");
+	if (!ASSERT_OK_PTR(skel->links.test_kprobe_override, "bpf_program__attach_kprobe"))
+		goto cleanup;
+
+	err = prctl(0xffff, 0);
+	ASSERT_EQ(err, 123, "err");
+
+cleanup:
+	kprobe_multi_override__destroy(skel);
+}
+
 #ifdef __x86_64__
 static void test_attach_write_ctx(void)
 {
@@ -597,6 +639,8 @@ void test_kprobe_multi_test(void)
 		test_attach_api_fails();
 	if (test__start_subtest("attach_override"))
 		test_attach_override();
+	if (test__start_subtest("override"))
+		test_override();
 	if (test__start_subtest("session"))
 		test_session_skel_api();
 	if (test__start_subtest("session_cookie"))
diff --git a/tools/testing/selftests/bpf/progs/kprobe_multi_override.c b/tools/testing/selftests/bpf/progs/kprobe_multi_override.c
index 28f8487c9059..14f39fa6d515 100644
--- a/tools/testing/selftests/bpf/progs/kprobe_multi_override.c
+++ b/tools/testing/selftests/bpf/progs/kprobe_multi_override.c
@@ -5,9 +5,24 @@
 
 char _license[] SEC("license") = "GPL";
 
+int pid = 0;
+
 SEC("kprobe.multi")
 int test_override(struct pt_regs *ctx)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
+		return 0;
+
+	bpf_override_return(ctx, 123);
+	return 0;
+}
+
+SEC("kprobe")
+int test_kprobe_override(struct pt_regs *ctx)
+{
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
+		return 0;
+
 	bpf_override_return(ctx, 123);
 	return 0;
 }
diff --git a/tools/testing/selftests/bpf/trace_helpers.h b/tools/testing/selftests/bpf/trace_helpers.h
index 9437bdd4afa5..a5576b2dfc26 100644
--- a/tools/testing/selftests/bpf/trace_helpers.h
+++ b/tools/testing/selftests/bpf/trace_helpers.h
@@ -4,6 +4,18 @@
 
 #include <bpf/libbpf.h>
 
+#ifdef __x86_64__
+#define SYS_PREFIX "__x64_"
+#elif defined(__s390x__)
+#define SYS_PREFIX "__s390x_"
+#elif defined(__aarch64__)
+#define SYS_PREFIX "__arm64_"
+#elif defined(__riscv)
+#define SYS_PREFIX "__riscv_"
+#else
+#define SYS_PREFIX ""
+#endif
+
 #define __ALIGN_MASK(x, mask)	(((x)+(mask))&~(mask))
 #define ALIGN(x, a)		__ALIGN_MASK(x, (typeof(x))(a)-1)
 
-- 
2.52.0

Re: [PATCHv3 bpf-next 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Mon, 12 Jan 2026 13:11:56 +0100 you wrote:
> Mahe reported issue with bpf_override_return helper not working when
> executed from kprobe.multi bpf program on arm.
> 
> The problem is that on arm we use alternate storage for pt_regs object
> that is passed to bpf_prog_run and if any register is changed (which
> is the case of bpf_override_return) it's not propagated back to actual
> pt_regs object.
> 
> [...]

Here is the summary with links:
  - [PATCHv3,bpf-next,1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run
    https://git.kernel.org/bpf/bpf-next/c/276f3b6daf60
  - [PATCHv3,bpf-next,2/2] selftests/bpf: Add test for bpf_override_return helper
    https://git.kernel.org/bpf/bpf-next/c/934d9746ed02

You are awesome, thank you!
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