Re: [PATCH 00/10] ftrace: Add register_ftrace_direct()

From: Steven Rostedt <rostedt@goodmis.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	X86 ML <x86@kernel.org>, Nadav Amit <nadav.amit@gmail.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Song Liu <songliubraving@fb.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>
Subject: Re: [PATCH 00/10] ftrace: Add register_ftrace_direct()
Date: Fri, 8 Nov 2019 20:00:37 -0500	[thread overview]
Message-ID: <20191108200037.5ee30af8@gandalf.local.home> (raw)
In-Reply-To: <20191108225100.ea3bhsbdf6oerj6g@treble>

On Fri, 8 Nov 2019 16:51:00 -0600
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> Here's the fix for the objtool warning:

Thanks, I applied it (will push it soon).

-- Steve

> 
> From: Josh Poimboeuf <jpoimboe@redhat.com>
> Subject: [PATCH] ftrace/x86: Tell objtool to ignore nondeterministic ftrace stack layout
> 
> Objtool complains about the new ftrace direct trampoline code:
> 
>   arch/x86/kernel/ftrace_64.o: warning: objtool: ftrace_regs_caller()+0x190: stack state mismatch: cfa1=7+16 cfa2=7+24
> 
> Typically, code has a deterministic stack layout, such that at a given
> instruction address, the stack frame size is always the same.
> 
> That's not the case for the new ftrace_regs_caller() code after it
> adjusts the stack for the direct case.  Just plead ignorance and assume
> it's always the non-direct path.  Note this creates a tiny window for
> ORC to get confused.
> 
> Reported-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
>  arch/x86/include/asm/unwind_hints.h |  8 ++++++++
>  arch/x86/kernel/ftrace_64.S         | 12 +++++++++++-
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/unwind_hints.h b/arch/x86/include/asm/unwind_hints.h
> index 0bcdb1279361..f5e2eb12cb71 100644
> --- a/arch/x86/include/asm/unwind_hints.h
> +++ b/arch/x86/include/asm/unwind_hints.h
> @@ -86,6 +86,14 @@
>  	UNWIND_HINT sp_offset=\sp_offset
>  .endm
>  
> +.macro UNWIND_HINT_SAVE
> +	UNWIND_HINT type=UNWIND_HINT_TYPE_SAVE
> +.endm
> +
> +.macro UNWIND_HINT_RESTORE
> +	UNWIND_HINT type=UNWIND_HINT_TYPE_RESTORE
> +.endm
> +
>  #else /* !__ASSEMBLY__ */
>  
>  #define UNWIND_HINT(sp_reg, sp_offset, type, end)		\
> diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
> index 5d946ab40b52..1c79624a36b2 100644
> --- a/arch/x86/kernel/ftrace_64.S
> +++ b/arch/x86/kernel/ftrace_64.S
> @@ -175,6 +175,8 @@ ENTRY(ftrace_regs_caller)
>  	/* Save the current flags before any operations that can change them */
>  	pushfq
>  
> +	UNWIND_HINT_SAVE
> +
>  	/* added 8 bytes to save flags */
>  	save_mcount_regs 8
>  	/* save_mcount_regs fills in first two parameters */
> @@ -249,8 +251,16 @@ GLOBAL(ftrace_regs_call)
>  1:	restore_mcount_regs
>  
>  
> +2:
> +	/*
> +	 * The stack layout is nondetermistic here, depending on which path was
> +	 * taken.  This confuses objtool and ORC, rightfully so.  For now,
> +	 * pretend the stack always looks like the non-direct case.
> +	 */
> +	UNWIND_HINT_RESTORE
> +
>  	/* Restore flags */
> -2:	popfq
> +	popfq
>  
>  	/*
>  	 * As this jmp to ftrace_epilogue can be a short jump