[PATCH 6.18 087/752] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

Sasha Levin
2026-02-28T17:49:22Z

From: Jiri Olsa <jolsa@kernel.org>

[ Upstream commit 276f3b6daf6024ae2742afd161e7418a5584a660 ]

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).

Reported-by: Mahe Tardy <mahe.tardy@gmail.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/bpf/20260112121157.854473-1-jolsa@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 15627ceea9bcc..386fa48c4a957 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 e7f1fe44352af..ae64b261de8e3 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:
--
2.51.0