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

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).

Fixes: b9b55c8912ce ("tracing: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs")
Reported-by: Mahe Tardy <redacted>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
v1 changes:
 - used ftrace_partial_regs_update with comments from Steven

 arch/arm64/include/asm/ftrace.h | 24 ++++++++++++++++++++++++
 include/linux/ftrace.h          |  3 +++
 kernel/trace/bpf_trace.c        |  1 +
 3 files changed, 28 insertions(+)

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 1621c84f44b3..177c7bbf3b84 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h

@@ -157,6 +157,30 @@ ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
 	return regs;
 }
 
+/*
+ * 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(const struct ftrace_regs *fregs, struct pt_regs *regs)
+{
+	struct __arch_ftrace_regs *afregs = arch_ftrace_regs(fregs);
+
+	if (afregs->pc != regs->pc) {
+		afregs->pc = regs->pc;
+		afregs->regs[0] = regs->regs[0];
+	}
+}