linux-kernel - [PATCH 7/8] x86/dumpstack/ftrace: mark function graph handler function as unreliable

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Message-Id: <f8af15749c7d632d3e7f815995831d5b7f82950d.1471607358.git.jpoimboe@redhat.com>
Date:   Fri, 19 Aug 2016 06:53:01 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: [PATCH 7/8] x86/dumpstack/ftrace: mark function graph handler function as unreliable

When function graph tracing is enabled for a function, its return
address on the stack is replaced with the address of an ftrace handler
(return_to_handler).

Currently 'return_to_handler' can be reported as reliable.  That's not
ideal, and can actually be misleading.  When saving or dumping the
stack, you normally only care about what led up to that point (the call
path), rather than what will happen in the future (the return path).

That's especially true in the non-oops stack trace case, which isn't
used for debugging.  For example, in a perf profiling operation,
reporting return_to_handler() in the trace would just be confusing.

And in the oops case, where debugging is important, "unreliable" is also
more appropriate there because it serves as a hint that graph tracing
was involved, instead of trying to imply that return_to_handler() was
the real caller.

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/kernel/dumpstack.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 9bf3d02..6aad8d4 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -87,12 +87,21 @@ print_context_stack(struct task_struct *task,
 				bp = (unsigned long) frame;
 			}
 
-			ops->address(data, addr, reliable);
-
+			/*
+			 * When function graph tracing is enabled for a
+			 * function, its return address on the stack is
+			 * replaced with the address of an ftrace handler
+			 * (return_to_handler).  In that case, before printing
+			 * the "real" address, we want to print the handler
+			 * address as an "unreliable" hint that function graph
+			 * tracing was involved.
+			 */
 			real_addr = ftrace_graph_ret_addr(task, graph, addr,
 							  stack);
 			if (real_addr != addr)
-				ops->address(data, real_addr, 1);
+				ops->address(data, addr, 0);
+
+			ops->address(data, real_addr, reliable);
 		}
 		stack++;
 	}
@@ -116,12 +125,11 @@ print_context_stack_bp(struct task_struct *task,
 		if (!__kernel_text_address(addr))
 			break;
 
-		if (ops->address(data, addr, 1))
-			break;
-
 		real_addr = ftrace_graph_ret_addr(task, graph, addr, retp);
-		if (real_addr != addr)
-			ops->address(data, real_addr, 1);
+		if (real_addr != addr && ops->address(data, addr, 0))
+			break;
+		if (ops->address(data, real_addr, 1))
+			break;
 
 		frame = frame->next_frame;
 		retp = &frame->return_address;
-- 
2.7.4