linux-kernel - Re: [PATCH] tracing: Stop FORTIFY_SOURCE complaining about stack trace caller

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Date:   Wed, 12 Jul 2023 23:08:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] tracing: Stop FORTIFY_SOURCE complaining about stack
 trace caller

On Wed, Jul 12, 2023 at 10:22:35PM -0700, Kees Cook wrote:
> But anyway, I think we can still do better. :)

Okay, what about this? I'm really not sure how to test this
appropriately, but it does appear to build. (famous last words)

This creates the union, but I think it ends up only being visible for
trace.c; everything else thinks it's just __array.

Anyway, maybe I'm closer, but trace internals are hurting my head right
now...

-Kees


diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 4529e264cb86..fba49d6c590c 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3157,7 +3157,7 @@ static void __ftrace_trace_stack(struct trace_buffer *buffer,
 
 	size = nr_entries * sizeof(unsigned long);
 	event = __trace_buffer_lock_reserve(buffer, TRACE_STACK,
-				    (sizeof(*entry) - sizeof(entry->caller)) + size,
+				    (sizeof(*entry) - sizeof(entry->__legacy_caller)) + size,
 				    trace_ctx);
 	if (!event)
 		goto out;
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index ed7906b13f09..29be88ad6227 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -83,6 +83,12 @@ enum trace_type {
 #undef __dynamic_array
 #define __dynamic_array(type, item)	type	item[];
 
+#define __min_array(type, item, size)		\
+	union {					\
+		type __legacy_ ## item[size];	\
+		DECLARE_FLEX_ARRAY(type, item);	\
+	};
+
 #undef __rel_dynamic_array
 #define __rel_dynamic_array(type, item)	type	item[];
 
diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
index 340b2fa98218..311a6c338385 100644
--- a/kernel/trace/trace_entries.h
+++ b/kernel/trace/trace_entries.h
@@ -190,7 +190,7 @@ FTRACE_ENTRY(kernel_stack, stack_entry,
 
 	F_STRUCT(
 		__field(	int,		size	)
-		__array(	unsigned long,	caller,	FTRACE_STACK_ENTRIES	)
+		__min_array(	unsigned long,	caller,	FTRACE_STACK_ENTRIES	)
 	),
 
 	F_printk("\t=> %ps\n\t=> %ps\n\t=> %ps\n"
diff --git a/kernel/trace/trace_export.c b/kernel/trace/trace_export.c
index 58f3946081e2..8f6dcd616d85 100644
--- a/kernel/trace/trace_export.c
+++ b/kernel/trace/trace_export.c
@@ -57,6 +57,9 @@ static int ftrace_event_register(struct trace_event_call *call,
 #undef __dynamic_array
 #define __dynamic_array(type, item)			type item[];
 
+#undef __min_array
+#define __min_array(type, item, size)			type item[size];
+
 #undef F_STRUCT
 #define F_STRUCT(args...)				args
 
@@ -123,6 +126,9 @@ static void __always_unused ____ftrace_check_##name(void)		\
 	.size = 0, .align = __alignof__(_type),				\
 	is_signed_type(_type), .filter_type = FILTER_OTHER },
 
+#undef __min_array
+#define __min_array(_type, _item, _len) __array(_type, _item, _len)
+
 #undef FTRACE_ENTRY
 #define FTRACE_ENTRY(name, struct_name, id, tstruct, print)		\
 static struct trace_event_fields ftrace_event_fields_##name[] = {	\
@@ -155,6 +161,9 @@ static struct trace_event_fields ftrace_event_fields_##name[] = {	\
 #undef __dynamic_array
 #define __dynamic_array(type, item)
 
+#undef __min_array
+#define __min_array(type, item, len)
+
 #undef F_printk
 #define F_printk(fmt, args...) __stringify(fmt) ", "  __stringify(args)
 

-- 
Kees Cook