On Thu, 24 Aug 2023 00:16:37 +0900
"Masami Hiramatsu (Google)" [off-list ref] wrote:

+#if !defined(CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS) || \
+	defined(CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST)
+
+static __always_inline
+struct pt_regs *perf_fprobe_partial_regs(struct ftrace_regs *fregs)
+{
+	/* See include/linux/ftrace.h, this returns &fregs->regs */
+	return ftrace_partial_regs(fregs, NULL);
+}
+
+#define perf_fprobe_return_regs(regs) do {} while (0)
+
+#else /* CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS && !CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST */
+
+/* Since fprobe handlers can be nested, pt_regs buffer need to be a stack */
+#define PERF_FPROBE_REGS_MAX	4
+
+struct pt_regs_stack {
+	struct pt_regs regs[PERF_FPROBE_REGS_MAX];
+	int idx;
+};
+
+static DEFINE_PER_CPU(struct pt_regs_stack, perf_fprobe_regs);
+
+static __always_inline
+struct pt_regs *perf_fprobe_partial_regs(struct ftrace_regs *fregs)
+{
+	struct pt_regs_stack *stack = this_cpu_ptr(&perf_fprobe_regs);
+	struct pt_regs *regs;
+
+	if (stack->idx < PERF_FPROBE_REGS_MAX) {
+		regs = stack->regs[stack->idx++];
+		return ftrace_partial_regs(fregs, regs);
+	}
+	return NULL;
+}
+
+static __always_inline void perf_fprobe_return_regs(struct pt_regs *regs)
+{
+	struct pt_regs_stack *stack = this_cpu_ptr(&perf_fprobe_regs);
+
+	if (WARN_ON_ONCE(regs != stack->regs[stack->idx]))
+		return;
+
+	--stack->idx;
+}

Ah, I found that the perf_trace_buf_alloc() does the same thing. So

perf_trace_buf_alloc(size, &pt_regs, &rctx);

will give us the pt_regs at that point. Trace event does that so I think
it is OK to do that here.

Thank you,

+
+#endif /* !CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS || CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST */
+
 static int fentry_perf_func(struct trace_fprobe *tf, unsigned long entry_ip,
-			    struct pt_regs *regs)
+			    struct ftrace_regs *fregs)
 {
 	struct trace_event_call *call = trace_probe_event_call(&tf->tp);
 	struct fentry_trace_entry_head *entry;
 	struct hlist_head *head;
 	int size, __size, dsize;
+	struct pt_regs *regs;
 	int rctx;
 
+	regs = perf_fprobe_partial_regs(fregs);
+	if (!regs)
+		return -EINVAL;
+
 	head = this_cpu_ptr(call->perf_events);
 	if (hlist_empty(head))
-		return 0;
+		goto out;
 
-	dsize = __get_data_size(&tf->tp, regs);
+	dsize = __get_data_size(&tf->tp, fregs);
 	__size = sizeof(*entry) + tf->tp.size + dsize;
 	size = ALIGN(__size + sizeof(u32), sizeof(u64));
 	size -= sizeof(u32);
 
 	entry = perf_trace_buf_alloc(size, NULL, &rctx);

Here, we can borrow the pt_regs.

 	if (!entry)
-		return 0;
+		goto out;
 
 	entry->ip = entry_ip;
 	memset(&entry[1], 0, dsize);
-	store_trace_args(&entry[1], &tf->tp, regs, sizeof(*entry), dsize);
+	store_trace_args(&entry[1], &tf->tp, fregs, sizeof(*entry), dsize);
 	perf_trace_buf_submit(entry, size, rctx, call->event.type, 1, regs,
 			      head, NULL);
+out:
+	perf_fprobe_return_regs(regs);
 	return 0;
 }
 NOKPROBE_SYMBOL(fentry_perf_func);
 
 static void
 fexit_perf_func(struct trace_fprobe *tf, unsigned long entry_ip,
-		unsigned long ret_ip, struct pt_regs *regs)
+		unsigned long ret_ip, struct ftrace_regs *fregs)
 {
 	struct trace_event_call *call = trace_probe_event_call(&tf->tp);
 	struct fexit_trace_entry_head *entry;
 	struct hlist_head *head;
 	int size, __size, dsize;
+	struct pt_regs *regs;
 	int rctx;
 
+	regs = perf_fprobe_partial_regs(fregs);
+	if (!regs)
+		return;
+
 	head = this_cpu_ptr(call->perf_events);
 	if (hlist_empty(head))
-		return;
+		goto out;
 
-	dsize = __get_data_size(&tf->tp, regs);
+	dsize = __get_data_size(&tf->tp, fregs);
 	__size = sizeof(*entry) + tf->tp.size + dsize;
 	size = ALIGN(__size + sizeof(u32), sizeof(u64));
 	size -= sizeof(u32);
 
 	entry = perf_trace_buf_alloc(size, NULL, &rctx);

Ditto.

Thanks,
-- 
Masami Hiramatsu (Google) [off-list ref]