Re: [RFC][PATCH][REGRESSION FIX] x86/ftrace: Add ORC annotation to 64 bit ftrace assembly

[Josh Poimboeuf]

On Fri, Dec 22, 2017 at 01:18:41PM -0500, Steven Rostedt wrote:
> From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> 
> While doing some debugging, I kicked off the stacktrace trigger, to see
> what was doing stacktraces in my code (it was causing a lot of noise in
> my function traces).
> 
> I ran:
> 
>   # trace-cmd record -p function -l save_stack_trace -l save_stack_trace:stacktrace
> 
> And took a look at the trace, but all I found was this:
> 
> trace-cm-1326    0d..2   107.791049: kernel_stack:         <stack trace>
> trace-cm-1326    0d..2   107.791077: kernel_stack:         <stack trace>
> trace-cm-1326    0...2   107.791084: kernel_stack:         <stack trace>
> trace-cm-1326    0d..2   107.791091: kernel_stack:         <stack trace>
> trace-cm-1326    0d..2   107.791097: kernel_stack:         <stack trace>
> trace-cm-1326    0...2   107.791102: kernel_stack:         <stack trace>
> trace-cm-1326    0d..4   107.791109: kernel_stack:         <stack trace>
> trace-cm-1326    0d..2   107.791116: kernel_stack:         <stack trace>
> trace-cm-1326    0d..2   107.791120: kernel_stack:         <stack trace>
> trace-cm-1326    0...2   107.791125: kernel_stack:         <stack trace>
> trace-cm-1326    0d..2   107.791133: kernel_stack:         <stack trace>
> trace-cm-1326    0d..2   107.791140: kernel_stack:         <stack trace>
> trace-cm-1326    0...2   107.791145: kernel_stack:         <stack trace>
> trace-cm-1326    0d..4   107.791157: kernel_stack:         <stack trace>
> [...]
> 
> This was not very useful. I kicked off a bisect to see where it broke,
> because it was working fine in 4.14, and this is a new regression in
> the 4.15 rc cycle.
> 
> The culprit ended up being:
> 
> fc72ae40e30 "x86/unwind: Make CONFIG_UNWINDER_ORC=y the default in
> kconfig for 64-bit"
> 
> As I find this to be a useful option, I don't want to revert it, so I
> looked at how to make ftrace work with this new unwinder. I came up
> with this solution. I'm not sure it is correct, but now my traces look
> like this (and objtool check doesn't complain):
> 
> 
> splice-t-1301    0d..1   203.199356: function:             save_stack_trace
> splice-t-1301    0d..2   203.199365: kernel_stack:         <stack trace>
> => ___slab_alloc (ffffffff81221c5d)
> => __slab_alloc.isra.81 (ffffffff81221d08)
> => kmem_cache_alloc (ffffffff812227ce)
> => getname_flags (ffffffff8124d49a)
> => SyS_execve (ffffffff812459f2)
> => do_syscall_64 (ffffffff8100394c)
> => return_from_SYSCALL_64 (ffffffff817c2dd8)
> splice-t-1301    0d..1   203.199379: function:             save_stack_trace
> splice-t-1301    0d..2   203.199382: kernel_stack:         <stack trace>
> => ___slab_alloc (ffffffff81221c5d)
> => __slab_alloc.isra.81 (ffffffff81221d08)
> => kmem_cache_alloc (ffffffff812227ce)
> => create_object (ffffffff81237593)
> => kmem_cache_alloc (ffffffff81222795)
> => getname_flags (ffffffff8124d49a)
> => SyS_execve (ffffffff812459f2)
> => do_syscall_64 (ffffffff8100394c)
> => return_from_SYSCALL_64 (ffffffff817c2dd8)
> splice-t-1301    0...1   203.199385: function:             save_stack_trace
> splice-t-1301    0...2   203.199387: kernel_stack:         <stack trace>
> => getname_flags (ffffffff8124d49a)
> => SyS_execve (ffffffff812459f2)
> => do_syscall_64 (ffffffff8100394c)
> => return_from_SYSCALL_64 (ffffffff817c2dd8)
> splice-t-1301    0d..1   203.199392: function:             save_stack_trace
> splice-t-1301    0d..2   203.199395: kernel_stack:         <stack trace>
> => ___slab_alloc (ffffffff81221c5d)
> => __slab_alloc.isra.81 (ffffffff81221d08)
> => kmem_cache_alloc_trace (ffffffff81222568)
> => do_execveat_common.isra.34 (ffffffff81245171)
> => SyS_execve (ffffffff81245a0c)
> => do_syscall_64 (ffffffff8100394c)
> => return_from_SYSCALL_64 (ffffffff817c2dd8)
> 
> P.S. the culprit was SLAB_DEBUG that was doing the stack traces
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 81bb565f4497..5d9eb722a04e 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -29,7 +29,6 @@ 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
>  
> diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
> index c832291d948a..6ab3308d632a 100644
> --- a/arch/x86/kernel/ftrace_64.S
> +++ b/arch/x86/kernel/ftrace_64.S
> @@ -4,6 +4,7 @@
>   */
>  
>  #include <linux/linkage.h>
> +#include <asm/unwind_hints.h>
>  #include <asm/ptrace.h>
>  #include <asm/ftrace.h>
>  #include <asm/export.h>
> @@ -148,9 +149,10 @@ EXPORT_SYMBOL(mcount)
>  
>  ENTRY(function_hook)
>  	retq
> -END(function_hook)
> +ENDPROC(function_hook)
>  
>  ENTRY(ftrace_caller)
> +	UNWIND_HINT_FUNC
>  	/* save_mcount_regs fills in first two parameters */
>  	save_mcount_regs
>  
> @@ -165,6 +167,7 @@ GLOBAL(ftrace_call)
>  	call ftrace_stub
>  
>  	restore_mcount_regs
> +	UNWIND_HINT_FUNC
>  
>  	/*
>  	 * The copied trampoline must call ftrace_epilogue as it
> @@ -187,11 +190,13 @@ WEAK(ftrace_stub)
>  END(ftrace_caller)
>  
>  ENTRY(ftrace_regs_caller)
> +	UNWIND_HINT_FUNC
>  	/* Save the current flags before any operations that can change them */
>  	pushfq
>  
>  	/* added 8 bytes to save flags */
>  	save_mcount_regs 8
> +	UNWIND_HINT_REGS
>  	/* save_mcount_regs fills in first two parameters */
>  
>  GLOBAL(ftrace_regs_caller_op_ptr)
> @@ -241,6 +246,7 @@ GLOBAL(ftrace_regs_call)
>  	movq RBX(%rsp), %rbx
>  
>  	restore_mcount_regs
> +	UNWIND_HINT_FUNC
>  
>  	/* Restore flags */
>  	popfq
> @@ -296,6 +302,7 @@ END(function_hook)
>  
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  ENTRY(ftrace_graph_caller)
> +	UNWIND_HINT_FUNC
>  	/* Saves rbp into %rdx and fills first parameter  */
>  	save_mcount_regs
>  
> @@ -311,11 +318,13 @@ ENTRY(ftrace_graph_caller)
>  	call	prepare_ftrace_return
>  
>  	restore_mcount_regs
> +	UNWIND_HINT_FUNC
>  
>  	retq
>  END(ftrace_graph_caller)
>  
>  GLOBAL(return_to_handler)
> +	UNWIND_HINT_EMPTY
>  	subq  $24, %rsp
>  
>  	/* Save the return values */

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.

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.

-- 
Josh