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Date:   Fri, 22 Dec 2017 20:43:37 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH][REGRESSION FIX] x86/ftrace: Add ORC annotation to
 64 bit ftrace assembly

On Fri, Dec 22, 2017 at 01:38:01PM -0500, Steven Rostedt wrote:
> On Fri, 22 Dec 2017 12:30:24 -0600
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > I'm officially on vacation but I got a chance to look at this myself a
> > few minutes ago.  This looks mostly right.  In theory, you should able
> > to mark those as functions by changing END to ENDPROC.  Then you won't
> > need all the UNWIND_HINT_FUNCs.
> 
> Yep, that was my first solution, but as you found, objtool complained
> about it.
> 
> > 
> > I tried making that change, but objtool thinks the stack frame is still
> > modified when the functions return.  I didn't see anything obvious in
> > save_mcount_regs or restore_mcount_regs that should cause it to think
> > that.  I'll need to look into it a little more.
> 
> Thanks!
> 
> Worse comes to worse, we can keep this change, and you can enjoy your
> holidays. I just need this fixed before 4.15 is released.
> 
> I'll be jumping into objtool shortly, to see if I can merge the code
> from recordmcount.c with it too. I'm going to need to learn ORC and
> DWARF to start on extending the function tracer to act more like
> tracepoints (like I discussed with Linus in Prague).

Objtool doesn't like the fact that save_mcount_regs pushes rbp at the
beginning, but it's never restored (directly, at least).

How about something like this instead, where it skips the original rbp
push?  I didn't test it, but I think it should work.  It at least gets
rid of the warnings and doesn't need any unwind hints other than the
UNWIND_HINT_EMPTY in return_to_handler.


diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 81bb565f4497..7e2baf7304ae 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -29,10 +29,13 @@ KASAN_SANITIZE_stacktrace.o				:= n
 KASAN_SANITIZE_paravirt.o				:= n
 
 OBJECT_FILES_NON_STANDARD_relocate_kernel_$(BITS).o	:= y
-OBJECT_FILES_NON_STANDARD_ftrace_$(BITS).o		:= y
 OBJECT_FILES_NON_STANDARD_test_nx.o			:= y
 OBJECT_FILES_NON_STANDARD_paravirt_patch_$(BITS).o	:= y
 
+ifdef CONFIG_FRAME_POINTER
+OBJECT_FILES_NON_STANDARD_ftrace_$(BITS).o		:= y
+endif
+
 # If instrumentation of this dir is enabled, boot hangs during first second.
 # Probably could be more selective here, but note that files related to irqs,
 # boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index c832291d948a..38b079626018 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -7,6 +7,7 @@
 #include <asm/ptrace.h>
 #include <asm/ftrace.h>
 #include <asm/export.h>
+#include <asm/unwind_hints.h>
 
 
 	.code64
@@ -20,7 +21,6 @@ EXPORT_SYMBOL(__fentry__)
 EXPORT_SYMBOL(mcount)
 #endif
 
-/* All cases save the original rbp (8 bytes) */
 #ifdef CONFIG_FRAME_POINTER
 # ifdef CC_USING_FENTRY
 /* Save parent and function stack frames (rip and rbp) */
@@ -31,7 +31,7 @@ EXPORT_SYMBOL(mcount)
 # endif
 #else
 /* No need to save a stack frame */
-# define MCOUNT_FRAME_SIZE	8
+# define MCOUNT_FRAME_SIZE	0
 #endif /* CONFIG_FRAME_POINTER */
 
 /* Size of stack used to save mcount regs in save_mcount_regs */
@@ -64,10 +64,10 @@ EXPORT_SYMBOL(mcount)
  */
 .macro save_mcount_regs added=0
 
-	/* Always save the original rbp */
+#ifdef CONFIG_FRAME_POINTER
+	/* Save the original rbp */
 	pushq %rbp
 
-#ifdef CONFIG_FRAME_POINTER
 	/*
 	 * Stack traces will stop at the ftrace trampoline if the frame pointer
 	 * is not set up properly. If fentry is used, we need to save a frame
@@ -105,7 +105,11 @@ EXPORT_SYMBOL(mcount)
 	 * Save the original RBP. Even though the mcount ABI does not
 	 * require this, it helps out callers.
 	 */
+#ifdef CONFIG_FRAME_POINTER
 	movq MCOUNT_REG_SIZE-8(%rsp), %rdx
+#else
+	movq %rbp, %rdx
+#endif
 	movq %rdx, RBP(%rsp)
 
 	/* Copy the parent address into %rsi (second parameter) */
@@ -148,7 +152,7 @@ EXPORT_SYMBOL(mcount)
 
 ENTRY(function_hook)
 	retq
-END(function_hook)
+ENDPROC(function_hook)
 
 ENTRY(ftrace_caller)
 	/* save_mcount_regs fills in first two parameters */
@@ -184,7 +188,7 @@ GLOBAL(ftrace_graph_call)
 /* This is weak to keep gas from relaxing the jumps */
 WEAK(ftrace_stub)
 	retq
-END(ftrace_caller)
+ENDPROC(ftrace_caller)
 
 ENTRY(ftrace_regs_caller)
 	/* Save the current flags before any operations that can change them */
@@ -255,7 +259,7 @@ GLOBAL(ftrace_regs_caller_end)
 
 	jmp ftrace_epilogue
 
-END(ftrace_regs_caller)
+ENDPROC(ftrace_regs_caller)
 
 
 #else /* ! CONFIG_DYNAMIC_FTRACE */
@@ -313,9 +317,10 @@ ENTRY(ftrace_graph_caller)
 	restore_mcount_regs
 
 	retq
-END(ftrace_graph_caller)
+ENDPROC(ftrace_graph_caller)
 
-GLOBAL(return_to_handler)
+ENTRY(return_to_handler)
+	UNWIND_HINT_EMPTY
 	subq  $24, %rsp
 
 	/* Save the return values */
@@ -330,4 +335,5 @@ GLOBAL(return_to_handler)
 	movq (%rsp), %rax
 	addq $24, %rsp
 	jmp *%rdi
+END(return_to_handler)
 #endif

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